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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 = 2kΩThis 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»

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’ = ∞.



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