One advantage bipolar opamps have over FET types is that their input noise level is appreciably lower. However, when a bipolar opamp is used in a high-impedance circuit. its bias current often presents a problem. For instance, the bias current of the well-known NE5534 opamp is typically 0.5 μA with a variation of about 5 nA ∞C-1. Temperature variations of ± 10 ∞C Would thus cause current changes of almost 100%, which would make it impossible to hold the output voltage at zero volts d.c.
The circuit presented here compensates the bias current with a current source. Its design ensures a high input impedance. In practice, the current source is not entirely stable with temperature variations, but that is largely nullified by placing it in control loop.
A current source, T2, is connected to the non-inverting input of IC1 via a 10 MΩ resistor. The reference voltage for T2 is established by a Ma ancillary current source, T1, and D1. This arrangement ensures that noise on the supply lines hardly affects the circuit.
To keep the noise of T2 to a minimum the reference voltage’ of D1 is divided by R2-P1.
Opamp ICS measures the direct voltage at the output of 1Ci and regulates the current source via R3 in such a way that there is no direct voltage at the output of IC1. This means. of course, that IC1 cannot be used as a d.c. amplifier.
To ensure that the bias compensation is not nullified or affected by the output resistance or output offset of the preceding stage, a capacitor. C2 is necessary at the input. It might be thought that, in view of the high input impedance, this should have a small value. For a 20 Hz cut-off point and an input impedance of 25 Mc2. a value of 330 pF would be sufficient. But this is wrong, because, the high reactance of such a capacitor would cause a noise level of about 640 nV Hz-2 at 20 Hz. Because of this. the reactance of the capacitor must match the output impedance of the preceding signal source.
Preset P1 must be set so that the output voltage of IC2 to zero, or nearly so, at an ambient temperature at the center of the wanted temperature range.
