Question

The input resistance of the base of a voltage-divider biased transistor can be neglected _________

(a) at all times

(b) only if the base current is much smaller than the current through R2 (the lower bias resistor)

(c) at no time

(d) only if the base current is much larger than the current through R2 (the lower bias resistor

This question was posed to me during an interview for a job.

I'd like to ask this question from Input Bias and Offset Currents of the Bipolar Differential Amplifier topic in section Differential and Multistage Amplifiers of Electronic Devices & Circuits

Answer 1

The correct answer is (b) only if the base current is much smaller than the current through R2 (the lower bias resistor)

Explanation: The input resistance can be neglected if the values of R1 and R2 are very large. Therefore the base current becomes very small.