Modern Physics 6 Question 5

5. An npn transistor operates as a common emitter amplifier, with a power gain of 60dB. The input circuit resistance is 100 Ω and the output load resistance is 10kΩ. The common emitter current gain β is

(Main 2019, 10 April I)

(a) 102

(b) 6×102

(c) 104

(d) 60

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Solution:

  1. Given, AP=60dB (in decibel)

Power gain in decibel can be given as

AP=10log10 Output power  Input power 60=10log10Pout Pin log10Pout Pin =6Pout Pin =106=AP

Also, given Rout =10kΩ,Rin =100Ω

Power gain of a transistor is given by

AP=β2Rout Rin 

where, β is current gain.

β2=AP×Rin Rout =106×10010×103β2=104 or β=102



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