Texas Instruments | Simplify Transimpedance Applications with High-bandwidth, Precision JFET Op Amps | Application notes | Texas Instruments Simplify Transimpedance Applications with High-bandwidth, Precision JFET Op Amps Application notes

Texas Instruments Simplify Transimpedance Applications with High-bandwidth, Precision JFET Op Amps Application notes
Simplify Transimpedance Applications with Highbandwidth, Precision JFET Op Amps
Raphael Puzio, Luis Chioye
Modern JFET-input operational amplifiers (op amps)
offer a combination of high input impedance, excellent
DC and AC performance, low noise, high bandwidth,
and wide voltage supply range, making them a natural
choice for transimpedance op amp (TIA) applications.
A TIA is a current-to-voltage converter, typically used
as a front-end for optical sensors such as photodiodes,
commonly found in Optical Line Cards, Light Level
Sensors, PM 2.5 Detectors, and many more. Silicon
photodiodes produce a current output that changes
linearly with incident light, where the typical photo
current ranges from a few pico-amps to a few
milliamps. The combination of high input impedance,
with low input bias currents of a few pico-amps at
room temperature, and very low current and voltage
noise, allow JFET op amps to be used in precision,
high-resolution photodiode applications. Furthermore,
JFET op amps operate over a wide voltage range and
are able to cover a broad spectrum of photodiode
output current, which is beneficial in TIA applications
as this increases the overall resolution and accuracy of
the system. Figure 1 shows a typical photodiode
transimpedance application.
Total parasitic capacitance:
CT = CJ + CDIFF +CCM2
GND
Photodiode Current (I)
CCM2
CDIFF
±
When selecting an op amp for a photodiode TIA
application, it is important to carefully consider the
three factors that dictate the minimum gain bandwidth
product of the op amp (fGBW) necessary for circuit
stability: the TIA required V/I gain, the required closedloop TIA bandwidth, and the parasitic junction
capacitance of the photodiode (CJ).
The stability of an op amp is related to its closed-loop
gain and phase response over frequency. The closedloop gain is defined as the product of the open-loop
gain of the op amp (AOL) and the feedback factor of
the op amp (β), defined as AOL × β. Figure 2 shows
the bode plots of the open loop gain (AOL) and 1/β
plot of a typical TIA. See the TI Precision Labs - Op
Amps: Stability – Lab Video Series for more
information about stability analysis and simulation.
AOL: High BW OPA
AOL: Lower BW OPA
1/t
Gain (dB)
CF
RF
Transimpedance Op Amp Gain, Bandwidth, and
Stability
fP
Rate of closure = 20dB
STABLE
Rate of closure = 40dB
UNSTABLE
fz
VOUT = I x RF
OPA145
1
2S CFRF
1
2S (CF CT ) RF
fC
+
CJ
fz
fP fC
CF
fGBW
CF CT
fGBW
Frequency
CCM1
GND
GND
Photodiode
parasitic junction capacitance
Figure 2. AOL and 1/β Plot for Transimpedance Op
Amp Circuit
2S DPS¶V SDUDVLWLF LQSXW
Capacitance: Neglect CCM1 since
non-inverting input is grounded
Figure 1. Photodiode Transimpedance Amplifier
The feedback resistor (RF) across the op amp
converts the photodiode current (I) to a voltage
(VOUT) using Ohm’s law, shown in Equation 1:
VOUT = I × RF
(1)
The feedback resistor (RF) determines the gain of the
transimpedance op amp, and the feedback capacitor
(CF) defines the closed-loop bandwidth of the circuit. In
addition, the feedback capacitor (CF) is required for
stability and is used to compensate for the total
parasitic capacitance (CT) at the inverting input of the
op amp: the photodiode junction capacitance (CJ) and
the input capacitance of the op amp (CDIFF + CCM2).
SPACER
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The 1/β curve of the Figure 2 presents a zero (fZ) and
a pole (fP) on its frequency response. Above the zero
(fZ), the 1/β curve increases a rate of +20 dB per
decade. At frequencies above the pole (fP), the 1/β
curve remains flat. The 1/β curve intersects the AOL
curve at frequency fC is shown in Equation 2:
(2)
In Equation 2, fGBW is the unity gain bandwidth of the
op amp. By analyzing the rate of closure of AOL and
1/β when the curves intersect, you can determine the
stability of the circuit. A handy rule of thumb for this
method is that the rate of closure must equal 20-dB for
optimal stability. Therefore, to maintain stability, the
AOL curve must intersect the 1/β curve when the 1/β
curve is flat (assuming a unity gain stable op amp). If
the AOL curve intersects the 1/β curve when the 1/β
Simplify Transimpedance Applications with High-bandwidth, Precision JFET
Op Amps Raphael Puzio, Luis Chioye
Copyright © 2019, Texas Instruments Incorporated
1
www.ti.com
curve is rising, as shown by the lower bandwidth op
amp AOL curve in Figure 2, the circuit may be
unstable, leading to many unfavorable circuit
behaviors. Equation 3 gives us the necessary
condition to avoid these problems:
(3)
Substituting the equations for fC and fP into the
inequality provided on equation 3, and solving for the
amplifiers unity gain bandwidth (fGBW), provides a
useful equation.
Equation 4 determines the minimum required
bandwidth of the amplifier to guarantee stability for a
TIA design. Therefore, higher bandwidth amplifiers
support higher gain and bandwidth TIA circuits, and
tolerate a higher parasitic photodiode capacitance
while remaining stable. Consider the case of a
photodiode application with the following
specifications: Transimpedance Amplifier Gain (50-K
V/A), Transimpedance Amplifier Bandwidth, 1-MHz,
and Photodiode Junction Capacitance (Cj) (100 pF).
In contrast, the higher fGBW OPA828 is able to support
the TIA gain of 50 k V/A with 1-MHz bandwidth while
offering 55 degrees of phase margin using a singlestage as shown on Figure 5. A single stage TIA
implemented with a precision, high bandwidth op amp
offers better noise performance and accuracy than a
dual stage version built with two lower bandwidth
amplifiers of similar accuracy, since only one amplifier
contributes to noise, offset, and drift errors in the
system. This greatly simplifies your design while
offering lower component count, simpler routing and a
smaller solution size.
3.2 pF
T 140
OPA828 Single-Stage TIA
50 k
TIA Gain = 50kV/A, BW = 1MHz
OPA828 fGBW = 45-MHz
120
-15V
100
AOL
AOL
100nF
GND
±
Vout
OPA828
+
Gain (dB)
80
Photodiode
60
Rateof
of Closure
Closure
Rate
20-dB
==20-dB
Stable!
Stable!
40
100nF
1/Beta
1/Beta
GND
20
GND
+15V
0
OPA828 Single Stage:
Transimpedance G=50k V/A
-20
1
10
100
(4)
For comparison, consider two implementations with op
amps with different bandwidths: the OPA140 offers 11MHz of gain bandwidth product, while the OPA828
offers 45-MHz. Using the previously derived equation
for minimum bandwidth, the minimum fGBW for the
transimpedance op amp is ~37-MHz. Using a single
stage, the OPA140 is unstable as shown on Figure 3.
T
140
3.2 pF
OPA140 Single-Stage TIA
TIA Gain = 50kV/A, BW = 1MHz
120
1k
10k
Frequency (Hz)
100k
1M
10M
Figure 5. Single-stage OPA828 (fGBW = 45-MHz) TIA
Stability Analysis (Stable)
Summary
Modern JFET op amps combine high input impedance,
excellent noise performance, high bandwidth, and wide
output voltage range, making JFET amplifiers an
optimal choice in the use of high gain, and high
resolution transimpedance photodiode circuits.
OPA140 fGBW = 11-MHz
50 k
100
-15V
100nF
GND
±
Vout
OPA140
+
Table 1. TIA Op Amps
AOL
AOL
80
Gain (dB)
Photodiode
60
Rateof
of Closure
Closure
Rate
40-dB
==40-dB
Unstable!!
Unstable!!
40
100nF
20
1/Beta
1/Beta
GND
GND
0
+15V
-20
OPA140 Single Stage:
Transimpedance G=50k V/A
1
10
100
1k
10k
Frequency (Hz)
100k
1M
10M
DESCRIPTION
OP AMP
36-Volt, High-speed (45 MHz GBW and 150 V/µs SR),
low-noise (4 nV/√Hz) RRO JFET Op Amplifier
OPA828
5.5 MHz, High Slew Rate, Low-Noise, Low-power, RRO
Precision JFET Op Amplifier
OPA145
Low-Offset, Low-Drift, Low-Noise, 11-MHz, 36-V JFETInput, RRO Op Amplifier
OPA140
Figure 3. Single-stage OPA140 (fGBW = 11-MHz)
Stability Analysis (Unstable)
Table 2. Related Documentation
To meet the TIA requirements, two cascaded stages of
the OPA140 are required. A transimpedance stage
with a lower gain of 10-kΩ cascaded a with a noninverting gain stage of 5 V/V as shown on Figure 4.
TYPE
TITLE
Application Brief
Green-Williams-Lis: Improved Op Amp Spice
Model
Application Report
Cookbook Circuit: Transimpedance Amplifier
16 pF
1k
4.02 k
T
10 k
-15V
GND
OPA2140
+
Vout
OPA2140
+
100nF
Gain (dB)
GND
±
GND
±
OPA2140 Dual Stage TIA
First TIA Stage Stability
TIA Gain = 10kV/A, BW=1-MHz
OPA2140 fGBW = 11-MHz
100
100nF
Photodiode
140
120
100nF
-15V
80
AOL
AOL
60
40
1/Beta
1/Beta
100nF
20
Rateof
of Closure
Closure
Rate
20-dB
== 20-dB
Stable !!
Stable
GND
0
GND
GND
+15V
st
1 Stage: Transimpedance
G=10k V/A
+15V
nd
2 Stage: Non-Inverting Gain
G=+5 V/V
-20
10
100
1k
10k
Frequency (Hz)
100
k
1M
10M
Figure 4. Alternate Two-stage OPA140 TIA
Amplifier (Stable)
2
Simplify Transimpedance Applications with High-bandwidth, Precision JFET
Op Amps Raphael Puzio, Luis Chioye
Copyright © 2019, Texas Instruments Incorporated
SBOA354 – May 2019
Submit Documentation Feedback
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