Bias Current Generators with Wide Dynamic Range

Bias Current Generators with Wide Dynamic Range
Analog Integrated Circuits and Signal Processing, 43, 247–268, 2005
c 2005 Springer Science + Business Media, Inc. Manufactured in The Netherlands.
Bias Current Generators with Wide Dynamic Range
TOBI DELBRÜCK AND ANDRÉ VAN SCHAIK
1
Institute of Neuroinformatics, ETH Zürich, and the University of Zürich, Winterthurerstr. 190, CH-8057, Zürich, Switzerland
2
School of Electrical and Information Engineering, The University of Sydney, NSW 2006 Australia
E-mail: tobi@ini.phys.ethz.ch, andre@ee.usyd.edu.au
Received September 24, 2004; Revised November 1, 2004; Accepted December 15, 2004
Abstract. Mixed-signal or analog chips often require a wide range of biasing currents that are independent
of process and supply voltage and that are proportional to absolute temperature. This paper describes CMOS
circuits that we use to generate a set of fixed bias currents typically spanning six decades at room temperature
down to a few times the transistor off-current. A bootstrapped current reference with a new startup and powercontrol mechanism generates a master current, which is successively divided by a current splitter to generate the
desired reference currents. These references are nondestructively copied to form the chip’s biases. Measurements
of behavior, including temperature effects from 1.6 and 0.35 µ implementations, are presented and nonidealities are
investigated. Temperature dependence of the transistor off-current is investigated because it determines the lower
limit for generated currents. Readers are directed to a design kit that allows easy generation of the complete layout
for a bias generator with a set of desired currents for scalable MOSIS CMOS processes.
Key Words:
1.
bias current generator, current reference, current splitter, current divider, PTAT
Introduction
Analog or mixed-signal CMOS chips usually require
a number of fixed reference currents for biasing amplifiers, determining time constants and pulse widths,
powering loads for static logic, and so on. Chips will
often have large pluralities of identical circuits (e.g.
pixels, column amplifiers, or cells) that require nominally identical biases. The required currents can extend
over many decades. For instance, consider a chip with
circuits that span timescales from ns to ms and that
uses subthreshold gm -C filters with 1 pF capacitors.
The rise time T —which we take as the timescale—of
simple gm -C circuits scales as C/gm . The transconductance gm of a transistor in subthreshold operation
scales as I /UT , where I is the bias current and UT is the
thermal voltage. Thus, such a chip would require bias
currents I = CUT /T from 10 uA to 10 pA—a range
of six decades.
Bias current references are often left out in experimental chips because designers assume that these
“standard” circuits can easily be added in later revisions when the chip’s design is productized. As a result, chips are designed that must later be individually
tuned for correct operation. These biases are often
specified by directly setting bias transistor gate voltages using off-chip components. However, the required
voltages depend on chip-to-chip variation in threshold
voltage. If these voltages are generated by potentiometers or supply-referenced digital-to-analog converters
(DACs), they are sensitive to supply ripple. The supply
currents depend in an exponential way on temperature. Potentiometers and DACs consume macroscopic
amounts of power and are expensive items for consumer goods. More importantly, each chip requires
individual tuning, which can be difficult and timeconsuming, especially if the space of tuning parameters has many dimensions. Furthermore, any drift in
off-chip components can be difficult to correct.
Some designers generate scaled versions of the desired currents using off-chip resistors that supply current to on-chip scaling current mirrors [1]. Although
this biasing scheme is straightforward, it requires a pin
for each independent bias, is sensitive to threshold voltage, requires a possibly wasteful regulated power supply, and can necessitate bulky on-chip current mirrors
when a small on-chip current is required. For example,
if we take a maximum feasible off-chip resistance of
248
Delbrück and van Schaik
1 M, then a 1 pA on-chip current requires a bulky
scaling current mirror—or series of mirrors—with the
ratio 106 :1.
Neither of the foregoing design choices guarantees
the feasibility of manufacturing the chip in quantity.
Here we show the architecture of the bias generator
circuits that we have been using regularly (e.g. [2–5])
to derive a wide-ranging set of fixed bias currents
from a single generated master current. This paper is
an expanded version of the work originally presented
in [6], with new analysis and measurements. Section
2 describes the circuits, Section 3 the measurements,
Section 4 the design kit, and Section 5 concludes this
paper.
2.
Biasing Circuits
The proposed circuit shown in Fig. 1 generates the master current Im and scaled copies of it. The master current is subdivided to form a set of smaller references,
which are copied by the circuits described in Section
2.3 to form the individual biases. The total supply current in the core bias generator circuit is 3Im , consisting
of 2Im in the master bias and Im copied to the splitter. In the following discussion, transistor “off-current”
means the saturation drain current of a transistor with
gate and source both tied to the bulk. This current is
also known as the subthreshold leakage current and is
not the diode or junction leakage current from active
region to bulk.
2.1. The Master Bias
The master current Im is generated by the familiar bootstrapped current reference attributed to Widler [8, 9]
and first reported in CMOS by Vittoz and Fellrath [10]
(see also textbooks such as [1, 11–13]). Transistors Mn1
and Mn2 have a gain ratio (Wn1 /L n1 )/(Wn2 /L n2 ) = M.
Since the currents in the two branches are forced to
be the same by the mirror Mp1 –Mp2 , the ratio in current density in the Mn ’s sets up a difference in their
gate-source voltage, which is expressed across the load
R. Resistance R and ratio M determine the current.
The master current Im that flows in the loop is computed by equating the currents in the two branches.
In subthreshold, this equality is expressed by Im =
eκ Vn /UT = Me(κ Vn −Im R)/UT , where κ is the back-gate or
body-effect coefficient (also known as κ = 1/n), resulting in the remarkably simple yet accurate formula
Im = log(M)
UT
,
R
UT =
kT
q
(2.1)
UT is the thermal voltage. The voltage VR across the
load resistor R does not depend on the resistance R
in subthreshold and provides a direct measurement of
temperature.
VR = log(M)UT
(2.2)
Above threshold, an analogous computation yields another formula that is not very accurate but still useful
Fig. 1. Bias generator core circuits. Im is the master current, and R is the external resistance. Transistor sizes are in units of λ (the scalable
length parameter) and are listed in Table 1. Ck1 and Ck2 are MOS capacitors. MR and M2R are identically sized unit transistors. The squares
represent bonding pads and recommended external connections.
Bias Current Generators with Wide Dynamic Range
Table 1. Transistor and capacitor sizing for the circuit in Fig. 1.
Transistor width to length ratios (W/L’s) are given in λ, the MOSIS
[7] scalable parameter, and are 24/6 unless listed differently. A
minimum length transistor is 2 λ long, so 2 λ is usually the process
technology dimension (e.g. λ = 0.4 µm for a 0.8 µm technology);
for submicron processes, λ is sometimes slightly larger than this
(e.g. a MOSIS 0.35 µ process has λ = 0.2 µ).
Transistor W/L
Mn2
24/6
Mn1
M ∗ 24/6
Mp1 , Mp2 , Mp3
76/65
Mc1 , Mc2
24/6
Ck1 ,Ck2
132/20
M
40
MR , M2R
24/12
Mpd , Mk1 , Mk2
6/6
Capacitance
Cn
∼10 pF
Ck1 , Ck2
∼1 pF
for estimating the required resistance.
1 2
Wn2
2
Im =
1− √
, β = µn Cox
(2.3)
βn R 2
L n2
M
Here µn is the electron-effective mobility, and Cox is
the unit-gate oxide capacitance. In strong inversion the
current decreases with R 2 , while in weak inversion it
decreases as R. Hence—and as shown later by the data
in Fig. 8—the estimated Im is approximately the sum
of Eqs. (2.1) and (2.3). With ideal transistors Im does
not depend on supply voltage or threshold voltage, but
is closely proportional to absolute temperature (PTAT)
in subthreshold. In reality it is slightly affected by the
supply voltage through drain conductance and also by
mismatch of the threshold voltage and β between the
transistors in the current mirrors.
This master bias circuit is often called the constantgm circuit because the gm of a transistor biased with
current Im is independent of temperature for both weak
and strong inversion. The transconductance of a transistor with W/L the same as Mn2 biased with current
Im is given by
weak
κ log M
gm =
,
R
strong
√ 2(1 − 1/ M)
R
(2.4)
These gm depend only on R, M, and κ, and the βdependence of the strong inversion master current
249
has also disappeared [14]. Thus, gm does not depend on temperature in either weak or strong inversion if R and κ are independent of temperature.
As discussed in [15] in this issue, this temperatureindependence holds only if the transistor is of the
same type as Mn1 and running in the same operating
regime. Therefore, we expect that circuits that are biased from the splitter outputs will have some degree of
temperature-dependent gm . Temperature dependence
of the bias generator is discussed in more detail in
Section 3.6.
2.1.1. Power Supply Sensitivity To decrease the DC
power supply sensitivity of the master bias current, the
drain resistances of the transistors are increased by using long M p ’s and cascoding Mn1 with Mc1 and Mc2 .
This choice minimizes the size of the entire generator. We chose not to cascode the p-FETs to preserve
headroom. Razavi computes the power supply sensitivity of the master bias current as an exercise ([12],
example 11.1). The result of this small-signal analysis
is interesting and a bit surprising in that the sensitivity
vanishes if the Mp2 mirror output transistor in Fig. 1 has
infinite drain resistance. In other words, if the p-mirror
copies the current perfectly, the output resistance of
the Mn1 (or Mc1 ) transistor is irrelevant. Why is this
plausible? If the p-mirror copies perfectly, it is impossible for the n-mirror to have unequal output current.
Therefore, the original premise of the circuit that is expressed in Eqs. (2.1) and (2.3) is satisfied and power
supply variation has no effect. One might still think that
finite drain conductance in Mn1 increases the gain of the
Mn1 –Mn2 mirror, but this is not the case. Drain conductance does not increase incremental mirror gain; it only
increases output current, and incremental current gain
is what determines the master bias current. Simulations
of the master bias circuit that increase the length of the
Mn1 –Mn2 mirror (which decreases the mirror’s output
conductance) slightly increase the master bias current.
An additional interpretation of the result of Razavi’s
analysis is that increasing M reduces supply variation.
This interpretation is also reasonable because increasing gain in a feedback loop decreases the effects of
component imperfection. All of these effects are shown
in the simulation results of the supply sensitivity of the
low-voltage version of the master bias circuit (see Section 3.7.1). The results suggest that making transistors
Mp1 and Mp2 long, excluding the Mc1 –Mc2 cascode,
and using a large M are likely good alternative choices
for low-voltage operation.
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Delbrück and van Schaik
2.1.2. Stability The ratio M is not critical as long
as it is substantially larger than 1. We have used values from 20 to 120, and the measurements shown here
come from the design kit described in Section 4, which
uses M = 40. A very large ratio can destabilize the
circuit through the parasitic capacitance CR on VR . A
common error in this circuit (and one not mentioned
in any of the standard texts or original references) is
to have too much capacitance CR , which can cause
large-signal limit-cycle oscillations. The circuit can be
stabilized by making the compensation capacitor Cn
several times CR . In practice, we usually bring out Vn
to a bonding pad, where we can use an external capacitance to ensure that the master bias can be stabilized. Nicolson and Phang [15] show a new topology
for the master bias circuit that requires much less compensation capacitance. Lichtsteiner proposed another
compensation scheme [16] that places Cn between the
two legs of the master bias circuit so that destabilizing
swings in one branch are compensated by swings in
the other branch. For example, if the current increases
in the left branch, the downward movement of the leftbranch voltage causes a downward movement in the
right-branch voltage, which counteracts the increase
in current. He demonstrated in simulation that this arrangement is stable even when CR /Cn = 1, 000, but we
have not yet implemented this arrangement.
As an aside, this large-signal instability is not easy to
analyze, because a small-signal analysis shows that all
poles are almost always in the left-half plane regardless
of what values are chosen for capacitance and M [16].
The circuit is therefore nearly always small-signal stable. Even when the circuit is small-signal stable, it can
still easily be large-signal unstable. If one simulates
the circuit behaviorally with subthreshold dynamics
and infinite power supply rails, it is also large-signal
stable, and any oscillation eventually damps out. The
large-signal instability arises from the extremely nonlinear (exponential) large-signal characteristics of the
current mirror transistor Mn1 and the low impedance
of CR at high frequencies, allowing transient positive
feedback that approaches or even exceeds unity gain.
This can be understood by considering that in steady
state, Mn1 and Mn2 carry identical currents, but Mn1
is source-degenerated in DC by R, so that in DC the
current gain from Mn1 to Mn2 is less than 1. At high
frequencies, CR provides a virtual short to ground for
the source of Mn1 . In this condition the gain from Mn1
to Mn2 approaches 1, at least when Im is subthreshold
where gm = κ I /UT irrespective of the transistor geom-
etry. When Im is above threshold, the high frequency
gm of Mn1 will be greater than that of Mn2 when they
carry the same current because the Mn1 overdrive will
be lower. It is therefore possible that when Im is above
threshold, positive feedback can exceed unity gain; and
the larger the M ratio, the larger this effect. In any case,
this nonlinearity causes a response to a perturbation
that can easily cause the voltages to hit the power rails,
shutting off the current in the mirrors so that the oscillation cannot catch up with itself and limit-cycle oscillations continue forever. Coupled with the extremely
expansive nonlinearity of Mn1 , this positive feedback
makes limit-cycle oscillations simple to generate, e.g.
on startup. Slowing the other branch with Cn reduces
the positive feedback to the gate of Mn1 and prevents
the instability. Section 3.1 shows measurements of this
instability.
2.1.3. Startup and Power Control Very small
current—traditionally but incorrectly called “zero
current”—in both branches of the bootstrapped current mirror circuit can also be a stable or metastable
operating point [11]. Although this state can definitely
occur in implementations, why it does is not so easy to
see. A straightforward analysis shows that when a current mirror’s input transistor goes out of saturation, the
output of the mirror reduces to the off-current, but the
mirror’s incremental current gain is reduced only by
a factor of the back-gate coefficient κ. This situation
is illustrated in Fig. 2, which shows the degenerated
mirror over a wide range of currents. The output of
the mirror is the output transistor’s off-current when
the input current is zero, and the current gain is Mκ,
where κ = 1 in this example. Thus, the total current gain
around the loop when both mirrors are in their “off”
state, with both gate-source voltages zero, is Mκn κ p ,
where p and n refer to the n- and p-type back-gate coefficients, and this factor is certainly larger than one in
most implementations. Considering substrate leakage
from the drain junctions does not change this situation,
but a conductance from Vn to ground can produce a stable “off” state. Ordinarily there is no such intentional
conductance, but substrate leakage from the ESD protection structures in the Vn pad or across the drain of
Mpd can act as such a conductance. In addition, offcurrent or substrate leakage from Mk2 can supply some
of the current sunk by Mn1 , also reducing the gain. In
extensive behavioral as well as transient SPICE simulations, we have not been able to produce a true “off”
state. However, we have observed in practice that an
Bias Current Generators with Wide Dynamic Range
251
Fig. 3. Simulation of slow restart when off state is intentionally
produced by clamping Vn low. A resistor Rn was connected between
Vn and ground, and a large capacitance Cn = 10 nF was used to
demonstrate slow self-restart.
Fig. 2. Detailed action of degenerated mirror showing entire range
of operating currents, from “off” to intended currents. Current is
normalized to the Mn2 off-current, and voltages are in units of UT .
R is in units UT /I0 . Bottom left shows the currents on a linear scale
whereas bottom right shows them on a log scale.
intentionally produced “off” state (produced by using
an extra transistor to tie Vn momentarily to ground)
can be stable at room temperature for many seconds.
Increasing the temperature, which increases the transistor off-current and the junction leakage current, decreases the duration of this metastable operating point.
Whatever the cause, the simulation in Fig. 3 shows that
escape from an “off” state can be very slow, even when
it is unstable, so a startup circuit is necessary to escape
this parasitic operating point quickly when power is
applied.
A large number of startup mechanisms are currently
in use [12, 17, 18]. In the present circuit we use a new 4transistor startup circuit that transiently injects current
into the current mirror loop on power-up and then shuts
itself off completely. Unlike many other startup circuits, this mechanism is process-independent because
it does not depend on threshold or supply voltage and
does not require any special devices. The inventors of
this startup circuit request anonymity, although they
have agreed to its description here. It is used on a commercial product that has shipped over 100 million units.
Fig. 4.
Close-up of startup and power control circuits.
To make the explanation of this startup circuit clearer,
part of Fig. 1 is reproduced as Fig. 4.
Transistors Mk1 , Mk2 , and Mpd , and MOS capacitors Ck1 and Ck2 enable the startup and power-control
functionality. The loop is kick-started on power-up by
the current flowing from Mk2 , which is “on” until Vk is
charged to Vdd by Mk1 , which then shuts off. Ck2 holds
Vk low on power-up (Vpd is at ground), while Ck1 ensures that V p is initially held near Vdd, holding Mk1
“off” so that the kick-start can occur. Ck1 and Ck2 must
be large enough so that sufficient charge flows into the
loop to get it going; we usually use about 1 pF. Ck1
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Delbrück and van Schaik
and Ck2 are MOS capacitors to avoid the necessity of
a special capacitor layer, such as a second polysilicon
layer. The polarity of the MOS capacitors is arranged
so that they operate in inversion (Ck2 ) or accumulation
(Ck1 ) when they need to. (Ck1 has another important
role that is discussed later.) While the bias generator is
operating, essentially zero current flows in this startup
circuit. The charge injected by Mk2 is a complex function of circuit parameters, but the essential point is that
Mk2 is not shut off until the master bias has current
flowing in it.
If the master bias circuit ever falls into a metastable
low-current state, there is no rapid automatic recovery.
We have not experienced this problem or been able to
produce it in simulations or experiments by manipulation of the power supply, but it is possible that such a
circumstance could arise under, for instance, very deep
brown-out conditions with slow recovery of the supply voltage. Capacitor Ck1 is important here because
it tends to hold the gate-source voltage of Mp1 —and
hence its current—constant when Vdd changes. If Ck1
is not included, a sudden drop in Vdd can transiently
turn off Mp1 , possibly leading to an unintended and
extended shutdown of the master bias.
In some systems the ability to completely shut off all
bias currents and then restart them is desirable, such as
for a sensor chip that needs only periodic activation by
an external periodic wakeup signal. We have included a
method to enable this “soft” power control by input Vpd ,
which is grounded for normal operation. Raising Vpd to
Vdd turns off the master bias and the derived biases by
pulling Vc to ground through Mpd and shutting off the
current in the loop. Yanking Vpd back to ground yanks
Vk low, through Ck2 (Mk1 is “off”), and the start-up
circuit restarts the current as before. While Vpd is high,
no current flows in Mpd, because Vk is at Vdd and
Mk2 is off. A conductive path to ground from Vn (say,
through a leaky Cn ) could require pumping Vpd for a
few cycles at a sufficient rate to move the current mirror
loop to a regime of positive feedback. But if, as usual,
there is no DC path other than through Mn2 , a single
downward transition on Vpd is sufficient for restart, as
demonstrated in the measurement in Fig. 10.
2.2. The Current Splitter
The master current is copied to a Bult and Geelen [19]
current splitter, which successively divides it to form
a geometrically spaced series of smaller currents. At
each branch, half of the current is split off, and the
rest continues to later stages. The last stage is sized to
terminate the line as though it were infinitely long. In
Fig. 1, MR and the two M2R transistors (which each
have the same W/L as the MR transistor) form the R2R network; the octave splitter is terminated with a
single MR transistor. The splitter has N stages, and the
current flowing transversely out from the splitter at the
kth stage is Im /2k . The final current is the same as the
penultimate current. Our transistor sizing for the octave
splitter is given in Table 1. The reference voltage for
the p-FET gates in the splitter should be a low voltage to minimize splitter supply- voltage requirements,
but it needs to be high enough to saturate the diodeconnected n-type output transistors. We use the master
bias voltage Vn , which conveniently scales correctly
with master bias current. We chose p-FET devices for
the splitter because they are built in an n-well implanted
in a p-substrate and can be protected from the effects of
parasitic photocurrents simply by covering them with
metal.
The current splitter principle accurately splits currents over all operating ranges, from weak to strong
inversion, independent of everything but the effective
device geometry. In this R-2R splitter, behavioral independence from the operating regime is most easily understood by following each transistor’s operation back
from the termination stage and observing that the transverse M2R and lateral MR transistors share the same
source and gate voltage and that the transverse transistor is in saturation. It can be easily observed that combining series and parallel paths causes half the current
to flow into each branch at each stage without any assumption about channel operating conditions. It is also
easy to see that, looking from the input terminal, the
entire splitter forms a “compound transistor” that has
an effective W/L equal to one of its MR or M2R unit
transistors.
That the transverse transistor is in saturation can
be observed as follows. Assuming that n- and p-type
transistors have comparable threshold voltages and ignoring back-gate effect, the source of this compound
splitter transistor will be at approximately 2 Vn . The
drain will be at Vn because this is the gate voltage of
the compound Mro diode-connected readout transistor.
Therefore, the splitter will have about Vn across its
“drain-source”, ensuring that it is in saturation. The
same will hold for the individual transverse transistors
in the splitter because they and the corresponding Mro
will carry only scaled copies of Im .
Bias Current Generators with Wide Dynamic Range
Figure 1 shows an R-2R splitter—built from unit
transistors—that splits by octaves, but we have also
built decade splitters by using MR and M2R with different aspect ratios. However, we strongly recommend
the use of unit transistors in an R-2R configuration. We
have discovered subtle effects that act differentially on
transistors with differing aspect ratios. These effects
cause non-ideal splitter behavior, especially in deep
subthreshold, and are discussed in Section 3.3.1.
The diode-connected Mro transistors read out the currents to make copies for individual biases. This arrangement allows for non-destructive readout at the cost of
mismatch in the Mro transistors.
2.3. Generating Individual Biases
An individual bias (Vbn , Vbp ) is generated by copying
a splitter current using one of the cells shown in Fig. 5.
A p-type bias is generated by (a). First, the splitter
current is copied using a cascoded transistor for better
accuracy. This current is drawn from a diode-connected
p-MOS transistor with the desired W/L ratio. The W/L
ratio of the transistor is the same as the W/L ratio used
in the user’s circuit. The resulting gate voltage is then
used as the bias voltage and is wired to other parts of
the chip. We have used this “voltage routing” distribution method exclusively, although “current routing”—
where the splitter current is copied and routed to the
place it is needed—is, of course, also feasible if the
bias is required in only a small number of places [1].
An n-type bias is generated by (b). The p-type mirror
used for the n-type bias is cascoded for better accuracy,
and the bulk of the cascode transistors is tied to the gate
253
voltage of the mirror. This arrangement provides a bit
more headroom because the back-gate bias is reduced,
which reduces the required gate-source voltage. A differential pair is used by (c) to enable fine-tuning of the
programmed bias in a controlled manner by external input Vtune . Tying Vtune to Vc programs half of the splitter
current, and the actual value can be varied from zero to
the full splitter current. An additional diode-connected
copy of the W/L transistor could be used at the drain
of the Vc transistor to improve circuit symmetry, but
because the bias is tunable this extra transistor might
be superfluous.
2.3.1. Bypass Decoupling of Individual Biases A
diode-connected transistor sinking current Ib and operating in subthreshold has a gate or drain conductance
g ≈ Ib /UT . This means that the bias voltage for a small
bias current will have a high impedance and can easily be disturbed by other signals on the chip that are
capacitively coupled to it, e.g. by crossing wires or by
drain-gate parasitic capacitance. The simplest remedy
is to bypass the bias with a large capacitance to the
appropriate power rail (Vdd for p-type and ground for
n-type; see Fig. 5), which is easy to do if the bias is
brought off-chip. Bypassing the bias has the additional
benefit of greatly reducing the effect of power-supply
ripple on the bias current. It is important to bypass to the
appropriate power rail so that the bias voltage is better
stabilized relative to the appropriate transistor source
voltage. The parasitic capacitance to the other rail will
then have much less effect on the gate-source voltage.
In a production chip, a pad may not be economically
justifiable, but in a prototype chip we strongly advise
bringing all biases out to pads anyway.
Fig. 5. Generating individual biases from the current splitter outputs. M1 has same W/L as Mro in Fig. 1. The square attached to each capacitor
represents a bonding pad.
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Delbrück and van Schaik
If the chip will be exposed to light, care must be
taken when bringing these generated bias voltages offchip because ESD protection structures in the bonding pads can produce significant currents under illumination (e.g. several nA under 1 klux). When the programmed bias current is very small, these parasitic photocurrents in the bonding pads can significantly perturb
the bias currents. In addition, parasitic conductance between package pins can significantly affect the generated biases when bias currents are in the sub-nA range,
especially under humid conditions.
We have also investigated active buffering of the
generated bias voltages to reduce the effects of coupling. The total capacitance on the bias voltage is often
large when a large number of identical circuits (e.g. pixels) are biased. On one chip, we tried using a sourcefollower arranged in a current conveyer structure, as
shown in Fig. 6(a), to buffer the low- current biases, but
we observed that even with a huge current ratio Ibuf /Ib
of 104 (1 µA/100 pA), transient capacitive coupling
to the bias produced a systematic, activity-dependent
shift of the bias current. The source-follower has very
unsymmetrical large-signal characteristics and acts as
a peak detector for transients coupling to the bias line,
so the net result is a systematic shift in the bias current
that depends on the frequency and amplitude of transient coupling. If the individual capacitive coupling is
small, then this effect is probably insignificant unless
it is synchronous. We have not experimentally investigated the use of a linear amplifier as the buffering
element, as shown in Fig. 6(b), but simulations suggest that it would not exhibit a systematic bias current
shift. However, the resonance frequencies of this circuit must be considered because the amplifier may be
driving a large capacitance, so the time constants at the
input and output nodes of the amplifier can be comparable. If the resonance frequencies are comparable
to the disturbance frequencies, the buffer can amplify
the disturbance rather than suppress it. The circuit in
Fig. 6(c) is more stable (assuming that the amplifier
can stably drive its load capacitance) because the amplifier’s internal time constants will generally be much
smaller, but it introduces random and systematic offset
to the bias voltage.
We can calculate the condition that avoids resonance
in Fig. 6(b) using the equivalent circuit in Fig. 6(d).
The response to a sinusoidal disturbance Vx coupled
through capacitance C x is given by (2.5), where the
parameters are shown in the figure.
Vbn
τx s(τi s + 1)
≈
Vx
(τi s + 1)(τo s + 1) + Ai
(2.5)
To achieve critical damping, condition (2.6) must apply
to the buffer amplifier time constant.
τo <
Fig. 6. Active bias-voltage buffering circuits. The desired bias voltage Vbn for a bias current Ib is actively buffered to other parts of the
chip. (a) is simple but has large signal asymmetry that can systematically shift bias value in response to coupled disturbances. (b) is
linear and does not introduce significant mismatch but must be biased correctly, as must (a), to avoid resonance amplification. (c) is
straightforward but can introduce additional random and systematic
mismatch. (d) shows the circuit analyzed for biasing conditions.
τi
4Ai
(2.6)
Here τo is the “time constant” of the output of the unitygain buffer amplifier, which drives (usually) a large
capacitance. τi is the time constant of the amplifier input
node, which consists of the small input capacitance
Ci and the large input resistance ri ≈ VE /Ib , where VE
is the Early voltage at the input node. Ai is the gain of
the input node looking from the gate of Mi .
If τo = τi , we √
have the condition of maximum resonance, and Q = Ai /2, which will typically be about
10. When the circuit is properly biased according to
(2.6), however, the equivalent time constant of the high
pass filter shrinks to (2.7):
τ=
τi
ri C i
Ci
Ci UT
=
=
≈
4Ai
4ri gi
4gi
Ib
(2.7)
Bias Current Generators with Wide Dynamic Range
Fig. 7.
255
Measured bias generator circuit.
Compared with the diode-connected schemes of 2.3.1,
which have a high-pass time constant of τ = Co UT /Ib ,
the actively buffered bias high-pass time constant
is reduced by a factor Co /Ci , which can be many
decades.
3.
Measured Characteristics
We have used the bias generator circuits described in
this paper in several generations of CMOS process
technology (1.6, 0.8, and 0.35 µ) with no striking differences in performance. Here we show measurements
and discuss the limits of operation. Most of the results
shown here come from bias generators with 20-stage
octave splitters built in two different 3.3 V, 0.35 µ processes. These layouts were generated by the design kit
described in Section 4.
While writing this paper, we realized that the fabricated circuits require more voltage headroom than they
should. Figure 7 shows the measured circuit. Compared
with the proposed circuit of Fig. 1, it copies the current from the master bias to the splitter using a doubly
cascoded mirror and includes an extra p-FET on the
front of the splitter. These additional transistors reduce
possible headroom and provide no advantage; their effect is visible, e.g. in the measurements shown later in
Fig. 17. (These shortcomings have not been corrected
in the design kit.)
3.1. Master Bias
The variation with resistance R of the master current Im
and the voltage VR across the load resistor R are shown
Fig. 8.
Master current Im and voltage V R versus resistance R.
in Fig. 8, along with the theoretical values given by
Eqs. (2.1) and (2.3) and SPICE BSIM3v3 simulation
results. The theory gives a reasonable estimate for the
measured values in the subthreshold range of operation, and the SPICE simulation does even better. The
above-threshold model is not very accurate. The exact
behavior is usually not important for practical purposes
256
Delbrück and van Schaik
Fig. 9.
Measurements of master bias stability. VX is an external signal capacitively coupled into the V R node.
because R is generally external and is selected for the
desired operating point. The maximum possible current is determined by the power supply voltage and the
headroom required by the current mirrors.
Measurements that demonstrate possible master bias
instability are shown in Fig. 9. We used large external
CR and Cn to intentionally produce the various regimes
of stability and coupled an external perturbation VX
to the VR node using a 220 pF C X capacitor with a
200 mV square wave input. CR was fixed at 1 nF, and
we varied Cn between 20 pF (the oscilloscope probe),
1 nF, and 8.2 nF. When Cn /C R = 1, the master bias
is marginally stable; Cn /CR = 8.2 makes the master
bias circuit unconditionally stable. The master bias oscillated when Cn consisted of only the 20 pF oscilloscope probe, corresponding to an undesirable situation where excess capacitance is allowed on the VR
node.
Fig. 10. Power control with Vpd (Vpd is rescaled). There was a 30second delay since the last cycle. A high-impedance analog buffer
was used to prevent a conductive path from Vn to ground.
3.2. Power Control Circuits
The power control behavior is shown in Fig. 10. Vpd
is initially high (the powered “off” state) and is then
brought low. Vn first jumps upward when Mk2 injects a
packet of charge and then increases exponentially until
it reaches its stable value. An unusually large Cn = 10
nF was used to demonstrate that a large Cn does not
affect restart except to delay it.
A conductive path to ground from Vn (e.g. from an
oscilloscope probe) can make it harder to restart the
master bias current because the transient current injected by the transient drop in Vpd is leaked away by
the resistive load faster than the positive feedback can
restore it. In this case, the master current can still be
Fig. 11. Restarting the master bias by using multiple pulses on Vpd
when there is a conductive path from Vn to ground.
restarted by a series of low-going pulses on Vpd , as illustrated in the measured data in Fig. 11. In this setup,
the resistive load was a 10 M oscilloscope probe,
and three to four pulses were necessary to restart the
Bias Current Generators with Wide Dynamic Range
257
Fig. 13. Measurements of the splitter currents using a variety of
master bias resistances R. The measured nFET off-current is shown
as I0 .
Fig. 12.
Octave splitter behavior.
master bias. The capacitance Cn was the oscilloscope
probe capacitance of about 20 pF.
3.3. Current Splitter
The behavior of the octave splitter is shown in Fig. 12.
A separate n-type transistor with W/L = 24/6 was used
to measure the splitter currents. We connected the splitter output voltages successively to the transistor’s gate
while holding its drain in saturation and measuring its
drain current using a Keithley 6430 source measure unit
(www.keithley.com). To ensure that this test transistor,
which is located far distant from the bias generator on
the chip, had the same threshold voltage and body effect
as the Mro transistors reading out the splitter current,
we measured them each in a diode-connected arrangement.
The splitter behavior is amazingly ideal over 20 octaves (6 decades) spanning strong to weak inversion.
A current of 10 pA is reliably generated from a master
current of 10 uA. Each splitter current is within 10% of
the ideal predicted value. The measured n-type transistor off-current (Vg = Vs = 0, Vd in saturation) of the
test transistor used to measure the splitter currents is 3
pA at room temperature. Figure 13 shows more measurements of the current splitter, with a range of master
currents determined by varying the external resistance
R. Each set of measurements is ideal to within about
10% down to a few times the transistor off-current I0 .
Imagers and focal plane arrays generally require biases that are not affected by illumination. However,
illumination creates parasitic photocurrents in all uncovered transistor source and drain regions as well as
in covered, native-type transistors that can collect diffusing minority carriers. These parasitic currents have
particularly significant effects on transistors with low
currents or with large areas of affected junctions. The
effect of parasitic photocurrent can be greatly reduced
by covering transistors with opaque metal and ensuring
that native transistors (ones that are built in the wafer
substrate) are surrounded by guard rings. These guard
rings are generally built using well implants and help to
absorb diffusing minority carriers that could otherwise
create parasitic photocurrents in the native transistors.
In the design kit layout, the splitter and individual
bias devices are covered with metal and surrounded by
n-well guard bars to protect against parasitic photocurrents. Immunity to illumination is illustrated by the
data in Fig. 14, which compares the master and splitter
currents with and without chip illumination. The measured drain parasitic substrate photocurrent induced in
the test transistor is also shown in this plot. We used
chip illumination of 460 lux by uncovering the chip in
our office, which is lit by fluorescent illumination. (In
conjunction with f /3 optics, this chip illumination level
corresponds to scene illumination of about 15 klux—
about the ambient light on a cloudy day [20].) Direct
258
Delbrück and van Schaik
rents is limited to the off-current because the bias transistors are presumably in saturation. Generation of still
smaller currents would require techniques such as those
outlined in [21, 22], where the splitter output current
is copied by a source-biased current mirror whose gate
input is shifted downward relative to the drain by a
source-follower voltage shifter.
Fig. 14.
Effect of illumination on generated currents.
illumination with 460 lux increases Im by about 20%,
and it has a maximal effect of a 50% increase in the
splitter output currents that peaks for the middle splitter outputs. Therefore, this layout is suitable for use in
imagers or focal plane arrays as long as slight changes
in all currents can be tolerated. The n-well guard bars
are at least 20 λ wide. We did not cover the master
bias, thinking that the effect of parasitic photocurrent
would be insignificant because the master current is
much larger than the parasitic photocurrent from any
one junction. We also neglected to place a guard bar at
the start of the splitter where it abuts the master bias
circuit. We believe that the 20% effect on the master
bias arises from a direct effect of parasitic photocurrent,
mostly on its large Mn1 transistor, and that some of the
remaining effect in the splitter comes from the lack of
a guard bar at its starting side. Covering the master
bias with metal would most likely greatly reduce these
remaining effects.
The smallest current that can be generated is limited by the off-current of transistors. In the case of the
measured 0.35 µ chip, this I0 is about 3 pA. The minimum possible generated current is a few times I0 . The
currents in the last stages of the splitter can approach
the junction leakage currents, which are typically much
smaller, but a bias generated from a copy of these cur-
3.3.1. Splitter Nonidealities When not Using Unit
Transistors In Section 2.2 we advised the use of unit
transistors in the current splitter because we have observed nonidealities in non-R-2R splitters built without
using unit transistors. The data in Fig. 15 illustrate these
nonidealities, which were measured from a bias generator predating the design kit and built in a 1.6 µm process. This splitter used transistor W/L ratios of 24/81
for the lateral MR and 24/10 for the transverse M2R,
and was terminated with a 24/9 MR . By following back
from the termination of the splitter, it is easy to see
that this should generate decade steps in splitter current
(the ratio of the last two stages in the decade splitter
is 9:1). Some measurements have two values because
there were two separate biases (n-type and p-type) with
the same current level. The measurements show that as
the currents enter weak inversion, they are larger than
predicted by the theory but that the nonideality is well
modeled by the SPICE simulation. The following discussion is presented with the caveat that when we use
unit transistors in an R-2R configuration, these nonidealities disappear both in simulation and in reality,
and we are still not certain of their underlying device
physics cause.
Fig. 15. Nonidealities of decade splitter built not using unit transistors. Points show ratio of measured current to ideal current and
measured current to SPICE-simulation current for all instrumented
splitter taps and individual biases. Error bars show variation over five
chips. Inset shows current splitter transistor sizing.
Bias Current Generators with Wide Dynamic Range
The nonideality is predicted by SPICE BSIM3v3
simulations of the circuit, but it was hard to trace
down its cause. The effect appears only in the parts
of the current splitter operating in weak inversion and
is not a channel-length modulation effect that only
appears in subthreshold. Channel-length modulation
would tend to have a greater effect on the shorter
transverse M2R transistors, which would have the opposite effect than what is observed. Any nonideality that increases current through a shorter transistor
more than through a longer one has the wrong sign—
it would make too much current split off in the early
stages, leaving too little for later ones. In other words,
such a nonideality would increase the “slope” of the
splitter as viewed, like the curve in the top half of
Fig. 12. What we actually observe is that this “slope” is
decreased.
The nonideality turns out to be a complex mixture
of threshold shift and short-channel transistor effects.
It is illustrated in the SPICE simulation results shown
in Fig. 16 of the final two stages of the decade splitter.
Ideally the currents should be in the ratio 9:1. The plot
shows the ratio as a function of injected current Iin .
With long transistors the size of the ones we built, the
ratio drops to about 6:1 in subthreshold, indicating that
too much of the current is going into the branch with
the longer transistors. This is a surprising result and
a huge effect, representing a deviation of 50% from
ideality. When we reduce the length of both transistors
by a factor of 4, the nonideality flips over and we see the
259
more familiar short-channel effect. In this case, the ratio
is much larger than expected, about 20:1. In summary,
these measurements and simulations suggest that it is
dangerous to rely on length scaling even for very long
transistors that are substantially wider than minimum
width, and especially in subthreshold operation. The
octave splitter does not have this problem, because it is
built from unit transistors.
3.4. Matching
Generated bias currents will be mismatched owing
to inherent transistor mismatch. Although we have
not extensively characterized mismatch, the data in
Fig. 15 offers guidance. It was measured from a set
of five chips, and the error bars indicate the chip-tochip variation in measured currents. In the strong inversion region the variations are under 5%, while in
the weak inversion region they grow to about 20%.
These variations are probably acceptable in many applications. From another design fabricated in a 0.35 µ
process, we have anecdotally observed that final bias
currents match specified values with a variability of
about 10% in strong inversion to 50% in weak inversion using transistor sizing as given in Table 1. Thus, it
can be expected that matching is possible to within
the resolution of the current splitter over the entire
range.
3.5. Power Supply Sensitivity
Fig. 16. SPICE simulations of the terminating decade splitter stage
with two transistor length scales, as illustrated in the insets. Transistor
width W = 24 λ.
Figure 17 shows measured sensitivity of the 0.35 µ bias
generator to power supply voltage along with SPICE
simulations. The master current was 5 µA, and the splitter outputs were measured by using the on-chip test
transistor with a fixed drain-source voltage of 0.3 V.
Each curve is normalized by its ideal value. There is a
rather poor qualitative match between measured results
and simulation except with regards to the power supply
requirements. In this 3.3 V process, where the threshold
voltages are VTn = 0.49 V and VTp = −0.71 V, the master bias requires a power supply voltage of about 1.75 V,
and the splitter requires about 2.25 V to operate so that
all transistors are correctly saturated. An additional 0.5
V is required to operate the splitter because of the unfortunate choice of splitter input current shown in the
measured circuit (Fig. 7). Simulations of the proposed
circuit, such as those discussed in Section 3.7, show
that removing this mirror and directly supplying the
260
Delbrück and van Schaik
Fig. 17. Measured power supply sensitivity of the master bias current and splitter output currents. Im = 1 µA, and each curve is normalized by its ideal value. The test transistor used to monitor the
current was held at 0.3 V drain voltage, and R = 30 k was used to
make a master current of about 5 µA.
splitter from the master bias significantly reduces the
bias generator supply voltage requirements. We think
that the poor match between simulation and measurement in Fig. 17 with regards to the supply sensitivity
arises from leakage pathways in the bonding pads.
3.6. Temperature Dependence
Temperature dependence of biasing circuits is clearly
important for real-world applications. Sometimes it is
desirable to have a bias current that results in a constant
gm ; at other times it may be desirable to have a constant
current—for instance, when that current determines a
slew rate or pulse width. The current generators presented here will act as PTATs when the master bias is
operated in subthreshold, where Im = log(M)UT /R, so
that they fit well with circuits requiring constant gm , but
applications requiring temperature-independent constant current will have to employ different techniques.
If a bias current determines the level of a current pulse
and the pulse width is determined by the reciprocal of
another bias current (e.g. as in a silicon model of a
synapse), then using a PTAT generator will make the
product of pulse height and width constant and result
in a fixed-size charge packet. It should be kept in mind
that PTAT sources vary their output by a factor of only
Fig. 18. Using static gate-voltage biases leads to exponential temperature dependence of current, as illustrated in these measurements
from the system reported in [3]. Each plot shows the spike rate (rate
of current-to-frequency converter) for different cells on the chip as a
function of temperature. The top measurements were collected with
static gate-voltage bias, whereas the bottom measurements used the
bias generator circuits reported here. Using the bias generator leads to
a much more stable operation. In fact, the cells slow down and finally
stop firing as temperature increases; this arises from the increase in
substrate leakage current on an oversized critical transistor.
1.5 over a temperature range of −20 to 100◦ C, which
would be acceptable for some applications.
To clearly show that constant gate-voltage biasing
has very poor temperature sensitivity, the example data
in Fig. 18 collected from the system described in [3]
compares constant gate-voltage and constant-gm biasing using the circuits described here. Over a range
of 15◦ C to 50◦ C, the constant gate-voltage behavior
changes by a factor of more than 5, whereas using the
bias generator circuits results in a variation of only
about 20%, most of which is due to parasitic substrate
leakage.
We measured temperature dependence using a thermal wand (Temptronic Thermostream TP04100A;
www.temptronic.com) to control the chip temperature.
This thermal wand (or “elephant”) is a benchtop device
that blows heated or chilled air from a small tip that can
be directed at a packaged chip. A thermocouple under
the chip package measures the package temperature,
and the thermal wand uses this measured temperature
in a feedback loop to accurately set the package temperature. We found it difficult to explore temperatures
near 0◦ C because water condensation from our standard (nondried) compressed air source created conductive paths that corrupted the low-current measurements;
Bias Current Generators with Wide Dynamic Range
Fig. 19. Temperature measurements of bias generator core circuit.
Plot shows master bias and splitter behavior at four temperatures.
The straight lines show theoretical Im /2(Octave+1) predictions of the
current.
therefore, we varied the temperature from 100 to 15◦ C,
which is a factor of 1.3 in absolute temperature. We
controlled only the temperature of the chip, leaving the
external resistor R at room temperature to make interpreting the data more straightforward.
The results of the measurements of the bias generator
are shown in Fig. 19. The main observation is that increasing temperature slightly increases the master bias
current but does not affect the splitter except to increase
the minimum possible current. This increase in minimum current is consistent with the expected increase
of the transistor off-current with temperature.
3.6.1. Influence of Temperature on Minimum Current
Increasing temperature will increase transistor offcurrent through the increase of carrier density in the
channel, thus increasing the minimum possible current
that can be generated from the current splitter and decreasing the range of currents that can be generated.
It is of interest to understand this phenomenon. We
first discuss how this effect arises and how it is related
to measured parameters such as the threshold voltage.
We then show measurements of temperature effects and
compare them with the theory.
The value of the off-current in (3.1) comes from
a commonly accepted expression (e.g. [23–25])
for subthreshold current that includes the threshold
voltage VT :
−κ VT
2
I0 = Ns UT β(T ) exp
(3.1)
UT
261
Here Ns is a dimensionless preexponential that accounts in part for the concentration of carriers in the
source. It is dimensionless because the rest of the expression has the familiar units βV 2 . One factor of UT
accounts for the effective density of states in the channel at the source end. It depends on temperature because
it arises from integration of a Fermi distribution over
the (unknown) energy density of states in the channel, and higher temperature spreads the electrons over
more energy states, increasing the effective number of
states in the channel [26]. The other factor of UT is part
of the diffusion coefficient, which is given by the Einstein relation to the mobility (kT /q)µ(T ). The mobility
is a weak function of temperature µ(T ) = µr (Tr /T )k
where Tr is a reference temperature and k ranges from
1 to 2 [27]. Increasing temperature increases density
of channel states and the diffusion coefficient, but
these increases are nearly compensated by reduction of
mobility.
The use of constant κ in (3.1) is inaccurate. When
the channel is near flat-band, κ changes significantly
with gate voltage and is also different than its value
at threshold because the depletion capacitor is just
starting to form and changes rapidly with surface potential. Nonetheless, for this analysis we will simply use the value of κ at threshold, which is very
close to the value over most of the subthreshold
range. In (3.1) we have ignored additional parameters, such as Voff , that appear in SPICE BSIM3v3
[24, 25] and that connect weak and strong inversion
operation in a sensible way, but that are rendered
meaningless for physical interpretation because they
are subverted for curve fitting in automatic parameter
extraction.
Temperature effects in the exponential last term in
(3.1) are dominant; the term expresses the concentration of carriers at the source end of the channel as a
function of the barrier potential, or “activation potential,” Va = κ VT between source and channel. This form
can be misleading because if one assumes that Va is a
temperature-independent constant equal to κ VT , then
the fit to reality is very poor. The threshold voltage
decreases with temperature increase because the carriers are hotter and lower gate voltages are required for
the same channel concentration. We will approach the
problem of understanding temperature variation of the
off-current by including the variation of VT with temperature. We can compute the temperature sensitivity
of (3.1) as follows, where all temperature dependence
262
Delbrück and van Schaik
that is not exponential is ignored:
κ VT
UT
d I0 /dT
d log I0
=
dT
I0
d VT
κ VT
−
=
UT T
dT
log Io = const−
(3.2)
The first term in (3.2) represents the effect of average
carrier energy for the barrier, while the second term
represents the change in barrier height. Temperature
sensitivity of the threshold voltage is well known [1,
23, 28] and is given by
1
1 (2φ F − VBG )
−
κ
2
T
(2φ F − VBG )
=
when κ = 0.66
T
d VT
=
dT
(3.3)
where 2φ F = 2UT log(N A /n i (T )) is the surface potential at threshold, and VBG = 1.206V is the band gap of
silicon at 300◦ K [29]. The value of d VT /dT ranges
from −3 mV/◦ K to −1 mV/◦ K as depending on channel doping and oxide thickness [27]. 2φ F is the surface
potential that brings the channel to a state of inversion
that equals the channel doping N A . The intrinsic concentration at T = 300◦ K is n i = 1.0·1010 /cm3 (and not
the commonly accepted value of 1.45 · 1010 /cm3 that
has clearly been shown to be incorrect by 45% [30]).
Using (3.3) in (3.2), we can numerically evaluate (3.2)
for representative values of threshold voltage and channel doping to obtain
d log I0
0.66
=
dT
25 mV
500 mV −2mV
− ◦
300◦ K
K
≈
9%
◦K
(3.4)
The two terms are comparable in (3.4), so both change
in carrier energy and barrier height are significant. Measurements of current versus gate-source voltage at various temperatures for single transistors are shown in
Fig. 20. Separate measurements of just the off-current
are shown in the insets. The off-current very closely
follows the classical form where log I0 is linear in
1/T . The extracted activation potential is Va = 0.66 V
Fig. 20. Measured transistor temperature effects. The main plots shows Ids vs. Vgs with temperature as a parameter. The inset plots show
the log off-current log(I0 ) as a function of 1/T along with the fitted activation potential Va in (3.1). For the nFET the off-current was directly
measured, whereas for the pFET it was inferred from the intercept of the fits to the Ids vs. Vgs curves. SPICE BSIM3v3 simulation results for
the off-current using vendor process parameters are also shown in the insets.
Bias Current Generators with Wide Dynamic Range
263
for the n-FET and Va = 1.01 V for the p-FET. The
value for Va for the n-FET means that, at room temperature, the off-current increases about 9%/◦ K, or a
doubling every 8◦ K. The n-FET off-current grows as
large as 100 pA at T = 100◦ C, but at room temperature is about 1 pA in this process. A higher threshold
voltage implies a smaller minimum current, but (3.2)
further implies that a higher threshold voltage (larger
Va , smaller I0 ) will result in a larger fractional variation of I0 with temperature. We can see that, generally,
the deeper in subthreshold the transistor operates, the
larger the temperature sensitivity.
As the insets in Fig. 20 show, BSIM3v3 qualitatively
models the temperature sensitivity of the off-current,
but the quantitative correspondence is not very good:
the magnitude of off-current differs by about a decade
from the measured values, the activation potential differs by about 10%, and the discrepancies are in opposite
directions for the two types of transistors.
3.6.2. PTAT Behavior of Master Bias Current It is
also of interest to measure whether the master current
Im is truly a PTAT current. A measurement of a master
bias circuit built in a 0.35 µ process is shown in Fig. 21
as the master bias current Im and the voltage VR versus
temperature for weak and strong inversion operation.
R was left at room temperature to simplify interpretation. The theory claims that in weak inversion operation, the master current is a PTAT current. Therefore,
the line fitted to the measured data should intersect the
origin. It almost intersects the origin, and VR comes
even closer. Simulations of temperature dependence,
however, say that even for an above-threshold master
bias current, the current should still be approximately
PTAT. This is demonstrated in the lower part of Fig. 21,
which shows the same measurements for strong inversion operation of the master bias circuit. In this case,
the current rises more steeply with temperature, which
is expected, since according to (2.3), Im ∝ 1/βn ∝ T k
with k ranging from 1 to 2.
3.6.3. Influence of Temperature Dependence on Choice
of Resistance Of course, temperature also affects passive components in a system. A fully integrated chip
would include the resistor R on-chip. We would advise
against the use of a diffused resistor when building
an imager chip because it would collect diffusing minority carriers generated by light in the local substrate
unless it was well protected. It is sometimes benefi-
Fig. 21. Measurements of Im and V R plotted versus temperature to
show absolute temperature dependence.
cial for several reasons to put part of the resistance
on-chip and the rest off-chip. Putting part of R onchip increases the stability of the master bias circuit
because it degenerates the gain of Mn1 as seen from the
bonding pad. On-chip and off-chip resistance behavior
with temperature can also sometimes be made to cancel each other. Figure 22 shows measurements of two
resistors: a standard carbon axial through-hole resistor
of 120 k and an on-chip unsilicided polysilicon resistor of 40 k with aspect ratio 2,500 λ/2λ built in a
1.6 µ process. They were measured to see how their
temperature coefficients compare with each other and
with the temperature coefficient of the PTAT master
bias current. First, the magnitudes of the temperature
coefficients of both types of resistors are much lower
than PTAT. Second, the two types of resistors behave
264
Delbrück and van Schaik
Fig. 22. Comparing temperature effects on a carbon resistor,
polysilicon resistor, and master bias PTAT current. The inset shows
on an absolute temperature scale the relative influence of temperature
coefficient on the measured resistors and the PTAT master current.
oppositely: The carbon resistance goes down with temperature (as though the carbon grains come closer),
whereas the polysilicon resistance goes up with temperature (as though the mobility decreases). The silicon
and carbon resistors have roughly equal but opposite
temperature coefficients, so they could in principle be
balanced, but the exact values of temperature coefficient matter and may not be known ahead of time.
In a contemporary submicron process, the temperature coefficients of polysilicon and all diffused resistors are usually positive, probably because mobility
decreases with temperature, although dedicated highresistance polysilicon can have a negative temperature
coefficient [31, 32]. For off-chip components, the temperature coefficients of surface-mount thin-film chip
resistors are typically less than ±200 ppm/◦ C [33]. Not
worrying about it and just using a standard off-chip carbon resistor with a temperature coefficient of −1,000
ppm/◦ C will decrease the bias current away from PTAT
only by about 10% over 100◦ C, and using a metal
film off-chip resistor will decrease this effect even
more.
of increasing importance. Since we have built these circuits only in 3.3 V or 5 V processes where low voltage
is not a great concern, we cannot provide experimental results from a low-voltage process. The simplest
modification to reduce the required supply voltage requirements is to remove all cascodes from the circuit.
Removing cascodes could increase supply sensitivity,
but as discussed in Section 3.4, if the master bias pmirror can be built with high output resistance, this
sensitivity can be minimized.
To study low-voltage operation limits of the present
circuit, we removed all cascodes, leaving all remaining transistor scaling identical to the values in Table 1.
We ran SPICE simulations of the bias generator using
BSIM3v3 model parameters publicly available from
MOSIS for a contemporary 0.18 µm non-epitaxial substrate mixed-signal RF process with a maximum power
supply of 1.8 V (MOSIS-run T44E). In the master bias
circuit we removed transistors Mc1 and Mc2 and tied
the drain of Mpd to Vn . In the individual bias circuits
we removed the n- and p-type cascodes. As mentioned
in Section 2.2, the entire current splitter has an effective equivalent transistor W/L equal to a single one
of its unit transistors. Hence, we expect that the bias
generator requires slightly more supply voltage than
|VTn | + |VTp | to operate so that transistors that should
be in saturation are saturated.
We modeled a bias generator that generates a master
current of 1 µA and that has an octave current splitter of 20 stages. In Fig. 23 the top set of traces show
the results of a DC sweep of the power supply Vdd.
The master bias current and selected splitter currents
are normalized by their ideal values and are plotted on
a log scale. The data show that in this process, with
approximately equal threshold voltages of 0.5 V for
n- and p-type transistors, the bias generator is usable
down to about 1.25 V. The last splitter output is about 10
times higher than it should be because the off-current
in this process is substantially higher than the desired
1 pA splitter current. The two lower sets of traces in
Fig. 23 show the results of a transient simulation with
the supply voltage Vdd and the power-down input Vpd
varied as shown.
3.7. Low-Voltage Bias Current Generator
Many designers are presently concerned with lowvoltage operation because, as process technologies
scale down, maximum supply voltages are becoming
lower, and battery-powered low-power applications are
3.7.1. Supply Sensitivity We also studied in simulation the power supply sensitivity of the master bias
current, as discussed in Section 2.1.1, to understand the
constraints on transistor sizing. Using the low-voltage
version of the master bias circuit, we programmed
Bias Current Generators with Wide Dynamic Range
265
Fig. 23. Simulation results for a master bias and 20-stage current splitter from a 0.18 µ 1.8 V process. The top traces are from a DC sweep of
the power supply voltage and the lower traces show a transient simulation where Vdd and power control Vpd were both varied.
a master current of about 400 nA and measured the
supply sensitivity while varying the transistor lengths
LP for the Mp1 –Mp2 mirror, LN for the Mn1 –Mn2 mirror, and the gain multiplier M. These simulation results are shown in Fig. 24. We assume that transistor drain resistance scales with transistor length, which
is valid for a range of transistor lengths that are not
too short (where short-channel effects dominate) or too
long (where impact ionization dominates). These plots
are useful for estimating the supply sensitivity. Using
the transistor sizing in Table 1, supply sensitivity of the
low-voltage master bias circuit in the 0.18 µ process is
about 8%/volt. The benefit of using a large M is clearly
visible in the lower plot. One might think that doubling
the n-mirror output resistance would halve the supply
sensitivity, but here it has only a small effect. Only when
the n-mirror transistor lengths are made comparable to
the p-mirror transistor lengths does the n-mirror transistor length begin to be significant. These observations
are consistent with the discussion in Section 2.1.1.
4.
Fig. 24. Scaling of power supply sensitivity of the low-voltage
version of the master bias current with circuit parameters. The dashed
lines indicate default values, with LN = 6 λ, LP = 65 λ, and M = 40.
Design Kit
One of the authors (T.D.) has developed a design kit that makes it simple to construct a complete bias generator when using Tanner design tools
(www.tanner.com/eda). A prerequisite for successful
use of this kit is knowledge of the necessary bias currents. The process parameters and desired bias currents
are specified in the schematic using parameter cells
266
Delbrück and van Schaik
Fig. 25. Design kit use and generated layout. The user uses parameter cells to specify the bias currents and their types, as shown by the example
cell, defines the process parameters, and then runs the compiler to generate the layout. The final layout has an area of 0.02 mm2 in a 0.35 µ
process.
such as the one shown in Fig. 25. A layout compiler
parses the netlist from the schematic and computes the
range of biases, the number of required splitter cells,
and the master bias current. The required resistance R
is estimated and reported. Bias current values can be
chosen arbitrarily but can be programmed to only 2−k
of the master current; therefore, an arbitrarily chosen
current will always be programmed to within 33% of
the desired value. The compiler then builds the layout
of the complete generator using a set of predefined layout and routing cells. The layout is clearly labeled for
connection to the user’s circuits, and a log file is written
to show what has been generated. A variety of SPICE
test bench files are provided to assist circuit simulation.
The final generated layout has dimensions similar to the
example shown in Fig. 25, which generates nine biases
and occupies an area of about 0.02 mm2 in a 0.35 µ
process. Including 10 pF MOS bypass capacitors on
each individual bias would approximately double the
area.
The design kit layout cells are based on MOSIS [7]
(www.mosis.org) scalable λ-based design rules, with
double-metal, single-poly processes; thus the layout is
compatible with any MOSIS CMOS process, including deep submicron processes. The cells are shielded
by metal and have guard rings, so they are suitable
for use in imager or focal plane arrays. No special
techniques are used to minimize device mismatch except for the use of rather large geometry and the
regularity of the current splitter. Two compiled bias
generators built in a 0.35 µ process were the source
of most of the data presented here. Readers are referred to www.ini.unizh.ch/∼tobi/biasgen for the free
kit.
Many industrial productions hold back wafers at various process steps during fabrication. With modifications, this design kit could generate layout so that a
current for each bias could be determined by a metal
mask. This capability would be useful if values for necessary bias currents are not known at the time the chip is
first produced. Modifications that would allow desired
currents to be programmed from a serial shift register
are also possible but require more chip area and a digital interface to the chip. We have not developed this
technology because we have been interested mostly in
completely integrated chips that do not require external
components.
5.
Conclusion
The biasing circuits described in this paper enable designers with requirements for a wide range of bias currents to generate them systematically. Currents can be
generated ranging from strong inversion to a few times
the transistor off-current. Chips with known requirements for a wide range of bias currents can benefit
significantly from the use of these circuits. Use of experimental chips by naı̈ve users can be much easier to
support if the parameters are truly fixed and the chips
are not dependent on fine tuning of external parameters. The chip designers themselves will have the satisfaction of understanding the operation of the chip
and knowing that it could probably be manufactured in
quantity. The design kit described in Section 4 makes it
simple to add these biasing circuits to any chip (especially a chip designed with MOSIS scalable rules) and
provides reasonable assurance that they will function
correctly the first time.
Bias Current Generators with Wide Dynamic Range
Acknowledgments
Oliver Landolt told us about current splitters and compound transistors. Discussions with Michael Godfrey
about device physics were helpful, as were reviewers’ detailed suggestions. The authors collaborated on
this work at the Telluride Neuromorphic Engineering
Workshop from 1999 to the present. This work was
supported by the University of Zürich, ETH Zürich,
the Institute of Neuromorphic Engineering and EC 5th
framework project CAVIAR (IST-2001-34124).
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Delbrück and van Schaik
Tobi Delbrück is a group leader at the Inst. of Neuuroinformatics (INI), part of ETH Zurich and the University of Zurich, Switzerland. His main interest is in
developing application-specific low-power vision sensor chips. In 1993 he graduated with a PhD in Computation and Neural Systems from Caltech, where he
worked in Carver Mead’s laboratory. He co-invented
with Mead a widely-used adaptive photoreceptor circuit and invented the bump circuit. Subsequently he
worked for several years for Arithmos, Synaptics, National Semiconductor, and Foveon, where he was one of
the founding employees. In 1998 he moved to Switzerland to join INI. In 2002 he was lead developer of a
tactile luminous floor used in INI’s exhibit ”Ada: Playful Intelligent Space” experienced by more than a half
million visitors to the Swiss National Exhibition. He
has been awarded 7 patents, and has written or coauthored 8 journal papers, 14 peer-reviewed conference
papers, 4 book chapters, and 1 book.
André van Schaik obtained his M.Sc. in electronics
from the University of Twente in 1990. In 1991–1994,
he worked at CSEM, Neuchâtel, Switzerland, in the
Advanced Research group of prof. Eric Vittoz. In this
period he designed several analogue VLSI chips for
perceptive tasks, some of which have been industrialised. A good example of such a chip is the artificial, motion detecting, retina in Logitech’s Trackman
Marble TM.
From 1994 until 1998, he was a research assistant and
Ph.D. student with Prof. Vittoz at the Swiss Federal Institute of Technology in Lausanne (EPFL). The subject
of his Ph.D. research was the development of analogue
VLSI models of the auditory pathway. In 1998 he was
a post-doctorate research fellow at the Auditory Neuroscience Laboratory at the University of Sydney.
In April 1999 he became a Senior Lecturer in Computer Engineering for the School of Electrical & Information Engineering at the University of Sydney. He
is now a Reader in the same School and Head of the
Computing and Audio Research Laboratory. His research interests include analogue VLSI, neuromorphic
systems, wireless sensor networks, human sound localisation, and virtual reality audio systems.
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