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LB-52 Scheda tecnica(PDF) 1 Page - National Semiconductor (TI) |
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LB-52 Scheda tecnica(HTML) 1 Page - National Semiconductor (TI) |
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1 / 2 page  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 Ie(optimum) e KT q 0hFE rs where rs is the output resistance of the signal source For example in the circuit of Figure 1 when rs e 1kX and hFE e 500 the le optimum is about 500 mAor560 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 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 com- 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 01 mV Catrs e 1000X The noise of Q1A and Q1B would normally be quite signifi- cant 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 14 nV 0Hz which is quite small compared to the Johnson noise of the 1 kX source 40 nV 0Hz A noise figure of 07 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 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 and this has been confirmed in the range a10 Cto a50 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 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) FIGURE 1 Conventional Low-Noise Operational Amplifier C1995 National Semiconductor Corporation RRD-B30M115Printed in U S A |
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