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−, the leakage of the capacitor and socket must be taken into account. 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), then:

where C_x is the stray capacitance at the + input.