VFB operational amplifier is the right choice for precision low-frequency applications requiring high open-loop gain, low offset voltage and low bias current. The input offset voltage of high-speed bipolar input VFB operational amplifier is rarely fine tuned, because the offset voltage matching of the input stage is very excellent, generally 1 to 3 MV, and the offset temperature coefficient is 5 to 15 µ V / ° C. After fine tuning, the input offset voltage below 20 µ V can be realized. The operational amplifier with self stabilizing zero architecture can provide offset voltage lower than 5 µ V, but we will not consider it here. For details of self stabilizing zero operational amplifier, refer to guide mt-055.

    The input bias current (no input bias current compensation circuit) on the VFB operational amplifier is roughly equal at the (+) input and (–) input, ranging from 1 to 5 µ a. Some FET input operational amplifiers have an input bias current of less than 200 FA, which is suitable for electrometer and other applications. (e.g. AD549).

    The output offset voltage caused by the input bias current can be reset to zero by equal effective source resistance in the inverting and in-phase inputs. This method is not effective for bias current compensated VFB operational amplifiers because there is an additional current error source at the input of such amplifiers. In this case, the net input bias current is not necessarily equal or of the same polarity.