The test method of Figure 6-5 is appropriate for bench-testing bias current with reasonable accuracy. To understand the circuit operation, first close switch S2 momentarily. When S2 is opened, then:

Figure 6-5 Simple Input Bias Current Test Circuit

A recommended capacitor for C₂ is a 5pF or 10pF silver mica, NPO ceramic, or air-dielectric. When determining the magnitude of I_b−, take into account the leakage of the capacitor and socket. Leave switch S2 shorted most of the time, or else the dielectric absorption of the capacitor C₂ can cause errors.

Similarly, if S1 is shorted momentarily (while leaving S2 shorted):

where C_x is the stray capacitance at the noninverting input.