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Consider a Darlington amplifier. In the self bias network, the biasing resistances are 220kΩ and 400 kΩ. What can be the correct value of input resistance if hfe = 50 and emitter resistance = 10kΩ.

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Consider a Darlington amplifier. In the self bias network, the biasing resistances are 220kΩ and 400 kΩ. What can be the correct value of input resistance if hfe = 50 and emitter resistance = 10kΩ.

(a) 141 kΩ

(b) 15 MΩ

(c) 20 MΩ

(d) 200 kΩ

I had been asked this question during an online exam.

The query is from Cascade and Darlington Amplifier topic in chapter General and Feedback Amplifiers of Analog Circuits

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Correct option is (a) 141 kΩ

To elaborate: R’ = 220k||400k = 142 kΩ

RI = (1+hfe)^2RE = 26MΩ

RI’ = 26M||142k = 141.22 K.

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