Question

Consider its input resistance to be R1. Now, the bypass capacitor is attached, so that the new input resistance is R2. Given that hie = 1000Ω and hfe = 50, find R1-R2.

(a) 112.2Ω

(b) 0Ω

(c) 110Ω

(d) 200Ω

I have been asked this question in an international level competition.

Enquiry is from Hybrid Equivalent Model in chapter Transistor Biasing and Low Frequencies of Analog Circuits

Answer 1

Right option is (a) 112.2Ω

Easiest explanation: For the circuit, CE amplifier without bypass capacitor, input resistance, R1=hie + (1+hfe)RE

R1 = 1000 + 51*2.2 = 1000 + 112.2 = 1112.2Ω

With a bypass capacitor attached, input resistance, R2 = hie = 1000Ω

Thus R1 – R2 = 112.2Ω.