Texas Instruments | Building an Op Amp With Bipolar Transistors, A Historical Application Note (Rev. A) | Application notes | Texas Instruments Building an Op Amp With Bipolar Transistors, A Historical Application Note (Rev. A) Application notes

Texas Instruments Building an Op Amp With Bipolar Transistors, A Historical Application Note (Rev. A) Application notes
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Literature Number: S1/$$
December 1980–Revised September 2016
A Historical Application Note
It is well known that the voltage noise of an operational
amplifier can be decreased by increasing the emitter current
of the input stage. The signal-to-noise ratio will be improved
by the increase of bias, until the base current noise begins to
dominate. The optimum is found at:
where rs is the output resistance of the signal source. For
example, in the circuit of Figure 1, when rs = 1 kΩ and hFE =
500, the le optimum is about 500 µA or 560 µA. However, at
this rich current level, the DC base current will cause a
significant voltage error in the base resistance, and even
after cancellation, the DC drift will be significantly bigger than
when le is smaller. In this example, lb = 1 µA, so lb x rs = 1
mV. Even if the lb and rs are well matched at each input, it is
not reasonable to expect the lb x rs to track better than 5 or
10 µV/˚C versus temperature.
A new amplifier, shown in Figure 2, operates one transistor
pair at a rich current, for low noise, and a second pair at a
much leaner current, for low base current. Although this
looks like the familiar Darlington connection, capacitors are
added so that the noise will be very low, and the DC drift is
very good, too. In the example of Figure 2, Q2 runs at
le = 500 µA and has very low noise. Each half of Q1 is
National Semiconductor
Linear Brief 52
Robert A. Pease
December 1980
operated at 11 µA = le. It will have a low base current (20 nA
to 40 nA typical), and the offset current of the composite op
amp, Ib1–lb2, will be very small, 1 nA or 2 nA. Thus, errors
caused by bias current and offset current drift vs. temperature can be quite small, less than 0.1 µV/˚C at rs = 1000Ω.
The noise of Q1A and Q1B would normally be quite significant, about 6
but the 10 µF capacitors completely
filter out the noise. At all frequencies above 10 Hz, Q2A and
Q2B act as the input transistors, while Q1A and Q1B merely
buffer the lowest frequency and DC signals.
For audio frequencies (20 Hz to 20 kHz) the voltage noise of
this amplifier is predicted to be 1.4
which is quite
small compared to the Johnson noise of the 1 kΩ source,
4.0
. A noise figure of 0.7 dB is thus predicted, and
has been measured and confirmed. Note that for best DC
balance R6 = 976Ω is added into the feedback path, so that
the total impedance seen by the op amp at its negative input
is 1 kΩ. But the 976Ω is heavily bypassed, and the total
Johnson noise contributed by the feedback network is below
1⁄2
.
To achieve lowest drift, below 0.1 µV/˚C, R1 and R2 should,
of course, be chosen to have good tracking tempco, below 5
ppm/˚C, and so should R3 and R4. When this is done, the
drift referred to input will be well below 0.5 µV/˚C, and this
has been confirmed, in the range +10˚C to +50˚C.
00849901
VOUT . (n + 1) VIN + VOS x (n + 1) + (lb2 − lb1) x rs x (n + 1) + Vnoise x (n + 1) + inoise x (rs + RIN) x (n + 1)
FIGURE 1. Conventional Low-Noise Operational Amplifier
Overall, we have designed a low-noise op amp which can
rival the noise of the best audio amplifiers, and at the same
time exhibits drift characteristics of the best low-drift ampli-
© 2002 National Semiconductor Corporation
AN008499
fiers. The amplifier has been used as a precision pre-amp
(gain = 1000), and also as the output amplifier for a 20-bit
DAC, where low drift and low noise are both important.
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Building an Op Amp With
Bipolar Transistors
www.national.com
LB-52
To optimize the circuit for other rs levels, the emitter current
for Q2 should be proportional to
The emitter current of
Q1A should be about ten times the base current of Q2A. The
base current of the output op amp should be no more than
1/1000 of the emitter current of Q2. The values of R1 and R2
should be the same as R7.
Current noise of a transistor,
Voltage noise of a resistor, per
For a more complete analysis of low-noise amplifiers, see
AN-222, “Super Matched Bipolar Transistor Pair Sets New
Standards for Drift and Noise”, Carl T. Nelson.
Various formulae for noise:
Voltage noise of a transistor,
00849902
*Tracking TC
< 5 ppm/˚C
**Solid tantalum
***Tracking TC
< 5 ppm/˚C, Beckman 694-3-R100K-D or similar
FIGURE 2. New Low-Noise Precision Operational Amplifier as Gain-of-1000 Pre-Amp
www.national.com
2
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