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Design >> Analog Design >> common collector
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Message started by aman gupta on Oct 30^th, 2012, 3:08am

Title: common collector
Post by aman gupta on Oct 30^th, 2012, 3:08am

for the common collector configuration shown i need to find the voltage gain.
The voltage gain formula is AV = (β+1)(r_o ll R_E)/(r_Π+(β+1)(r_o ll R_E)
But in the circuit R_E is absent . So how do we consider it ??
Nothing is given about the current source.

from the circuit theory i know that the internal impedance of a current source is very high (am i correct ?).

So in this case how do we relate it ?

Title: Re: common collector
Post by raja.cedt on Oct 30^th, 2012, 5:18pm

hello,
yes you are correct, ideal current source is infinite resister. So please substitute Re=inf and you get gain=1.

By the way you can explain intutively, due to current source incremental current will be zero, so gm*(Vbe)=0 hence Vbe=0 Vb=Veso unity gain.

Thanks,
Raj.

Title: Re: common collector
Post by aman gupta on Oct 30^th, 2012, 9:47pm

thanks

Title: Re: common collector
Post by Dan Clement on Oct 31^st, 2012, 7:41pm

This is a homework problem from Sedra and smith.