TL H 8499
A
Low-Noise
Precision
Op
Amp
LB-52
National Semiconductor
Linear Brief 52
Robert A Pease
December 1980
A Low-Noise Precision
Op Amp
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
I
e(optimum)
e
KT
q
0
h
FE
r
s
where r
s
is the output resistance of the signal source For
example in the circuit of
Figure 1 when r
s
e
1 kX and
h
FE
e
500 the l
e
optimum is about 500 mA or 560 mA
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 big-
ger than when l
e
is smaller In this example l
b
e
1 mA so
l
b
c
r
s
e
1 mV Even if the l
b
and r
s
are well matched at
each input it is not reasonable to expect the l
b
c
r
s
to track
better than 5 or 10 mV 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
l
e
e
500 mA and has very low noise Each half of Q1 is
operated at 11 mA
e
l
e
It will have a low base current
(20 nA to 40 nA typical) and the offset current of the com-
posite op amp I
b1
l
b2
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 mV C at r
s
e
1000X
The noise of Q1A and Q1B would normally be quite signifi-
cant about 6 nV
0
Hz but the 10 mF 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 nV
0
Hz which is quite
small compared to the Johnson noise of the 1 kX source
4 0 nV
0
Hz A noise figure of 0 7 dB is thus predicted and
has been measured and confirmed Note that for best DC
balance R6
e
976X is added into the feedback path so
that the total impedance seen by the op amp at its negative
input is 1 kX But the 976X is heavily bypassed and the
total Johnson noise contributed by the feedback network is
below
nV
0
Hz
To achieve lowest drift below 0 1 mV 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 mV C
and this has been confirmed in the range
a
10 C to
a
50 C
Overall we have designed a low-noise op amp which can
rival the noise of the best audio amplifiers and at the same
TL H 8499 1
V
OUT
j
(n
a
1) V
IN
a
V
OS
c
(n
a
1)
a
(l
b2
b
l
b1
)
c
r
s
c
(n
a
1)
a
V
noise
c
(n
a
1)
a
i
noise
c
(r
s
a
R
IN
)
c
(n
a
1)
FIGURE 1 Conventional Low-Noise Operational Amplifier
C1995 National Semiconductor Corporation
RRD-B30M115 Printed in U S A
LB-52
A
Low-Noise
Precision
Op
Amp
time exhibits drift characteristics of the best low-drift amplifi-
ers The amplifier has been used as a precision pre-amp
(gain
e
1000) and also as the output amplifier for a 20-bit
DAC where low drift and low noise are both important
To optimize the circuit for other r
s
levels the emitter current
for Q2 should be proportional to 1
0
r
s
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
Various formulae for noise
Voltage noise of a transistor per
0
Hz e
n
e
KT
0
2
ql
C
Current noise of a transistor per
0
Hz i
n
e
0
2ql
C
h
FE
Voltage noise of a resistor per
0
Hz e
n
e
0
4 KTR
s
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
Tracking TC
k
5 ppm C
Solid tantalum
Tracking TC
k
5 ppm C
Beckman 694-3-R100K-D or similar
TL H 8499 2
FIGURE 2 New Low-Noise Precision Operational Amplifier as Gain-of-1000 Pre-Amp
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