Question

For the given circuit, input resistance RI = 20Ω, hfe = 50. Output resistance = ∞. Find the new values of input and output resistance, if a base resistance of 2kΩ is added to the circuit.

(a) RI = 20Ω, RO = ∞

(b) RI = 20Ω, RO = 2kΩ

(c) RI = 59Ω, RO = ∞

(d) RI = 59Ω, RO = 2kΩ

This question was posed to me in an online quiz.

My doubt stems from Hybrid Equivalent Model topic in chapter Transistor Biasing and Low Frequencies of Analog Circuits

Answer 1

The correct answer is (c) RI = 59Ω, RO = ∞

Explanation: RI = 20k = hie/(1+hfe) = hie/51

hie =1020 Ω

Hence, after adding base resistance, RI’= (hie+RB)/(1+hfe) = (1020+2000)/51 ≅ 59Ω

There is no change in output resistance or current gain due to an extra base resistance. RO’ = ∞.