LB-52

A Low-Noise Precision Op Amp 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 0hFE q rs where rs is the output resistance of the signal source For example in the circuit of Figure 1 when rs e 1 kX and hFE e 500 the le 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 bigger than when le is smaller In this example lb e 1 mA so lb c rs e 1 mV Even if the lb and rs are well matched at each input it is not reasonable to expect the lb c rs 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 le e 500 mA and has very low noise Each half of Q1 is operated at 11 mA e le It will have a low base current (20 nA to 40 nA typical) and the offset current of the comIe(optimum) e National Semiconductor Linear Brief 52 Robert A Pease December 1980 posite 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 mV C at rs e 1000X The noise of Q1A and Q1B would normally be quite significant about 6 nV 0Hz 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 0Hz which is quite small compared to the Johnson noise of the 1 kX source 4 0 nV 0Hz 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 0Hz 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 VOUT j (n a 1) VIN a VOS c (n a 1) a (lb2 b lb1) c rs c (n a 1) a Vnoise c (n a 1) a inoise c (rs a RIN) c (n a 1) LB-52 FIGURE 1 Conventional Low-Noise Operational Amplifier C1995 National Semiconductor Corporation TL H 8499 RRD-B30M115 Printed in U S A A Low-Noise Precision Op Amp time exhibits drift characteristics of the best low-drift amplifiers 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 rs levels the emitter current for Q2 should be proportional to 1 0rs 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 0Hz en e KT 2qlC Current noise of a transistor per 0Hz in e hFE Voltage noise of a resistor per 0Hz en e 04 KTRs 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 0 0ql 2 C 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 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which (a) are intended for surgical implant into the body or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user National Semiconductor Corporation 1111 West Bardin Road Arlington TX 76017 Tel 1(800) 272-9959 Fax 1(800) 737-7018 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness LB-52 National Semiconductor Europe Fax (a49) 0-180-530 85 86 Email cnjwge tevm2 nsc com Deutsch Tel (a49) 0-180-530 85 85 English Tel (a49) 0-180-532 78 32 Fran ais Tel (a49) 0-180-532 93 58 Italiano Tel (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd 13th Floor Straight Block Ocean Centre 5 Canton Rd Tsimshatsui Kowloon Hong Kong Tel (852) 2737-1600 Fax (852) 2736-9960 National Semiconductor Japan Ltd Tel 81-043-299-2309 Fax 81-043-299-2408 National does not assume any responsibility for use of any circuitry described no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications