MT-038| Application Note

MT-038 TUTORIAL
Op Amp Input Bias Current

DEFINITION OF INPUT BIAS CURRENT Ideally, no current flows into the input terminals of an op amp. In practice, there are always two input bias currents, IB+ and IB- (see Figure 1).

+
IB+ IB-

A very variable parameter! IB can vary from 60 fA (1 electron every 3 s) to many A, depending on the device. Some structures have well-matched IB, others do not. Some structures' IB varies little with temperature, but a FET op amp's IB doubles with every 10 C rise in temperature. Some structures have IB which may flow in either direction.

Figure 1: Op Amp Input Bias Current Values of IB range from 60 fA (about one electron every three microseconds) in the AD549 electrometer, to tens of microamperes in some high speed op amps. Op amps with simple input structures using bipolar junction transistors (BJT) or FET long-tailed pair have bias currents that flow in one direction. More complex input structures (bias-compensated and current feedback op amps) may have bias currents that are the difference between two or more internal current sources, and may flow in either direction. Bias current is a problem to the op amp user because it flows in external impedances and produces voltages, which add to system errors. Consider a non-inverting unity gain buffer driven from a source impedance of 1 M. If IB is 10 nA, it will introduce an additional 10 mV of error. This degree of error is not trivial in any system. Or, if the designer simply forgets about IB and uses capacitive coupling, the circuit won't work-- at all! Or, if IB is low enough, it may work m
omentarily while the capacitor charges, giving even

Rev.0, 10/08, WK

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