TLH8499

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

KT

q

example in the circuit of

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-

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

(20 nA to 40 nA typical) and the offset current of the com-

Thus errors caused by bias current and offset current drift

e

1000X

The noise of Q1A and Q1B would normally be quite signifi-

cant about 6 nV

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 14 nV

small compared to the Johnson noise of the 1 kX source

40 nV

has been measured and confirmed Note that for best DC

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

To achieve lowest drift below 01 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 05 mV C

Overall we have designed a low-noise op amp which can

rival the noise of the best audio amplifiers and at the same

TLH8499 – 1

FIGURE 1 Conventional Low-Noise Operational Amplifier

C1995 National Semiconductor Corporation

RRD-B30M115Printed in U S A