Question

A transistor is connected in CB configuration. The emitter voltage is changed by 200mV, the emitter by 5mA. During this transition the collector base voltage is kept constant. What is the input dynamic resistance?

(a) 30Ω

(b) 60Ω

(c) 40Ω

(d) 50Ω

I got this question during an internship interview.

Origin of the question is Biasing Parameters topic in section Small-Signal Low-Frequency AC models of Transistors of Electronic Devices & Circuits

Answer 1

Right choice is (c) 40Ω

The explanation is: The ratio of change in emitter base voltage (∆VEB) to resulting change in emitter current (∆IE) at constant collector base voltage (VCB) is defined as input resistance. This is denoted by ri.

 We know, ∆VEB/∆IE=ri

=200/5=40Ω.