# Homework Help: Common collector

1. Oct 30, 2012

### erece

1. The problem statement, all variables and given/known data

for the common collector configuration shown i need to find the voltage gain.
The voltage gain formula is AV = (β+1)(roll RE)/(rπ+(β+1)(roll RE)
But in the circuit RE 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 ?
2. Relevant equations

3. The attempt at a solution

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Last edited: Oct 30, 2012
2. Oct 30, 2012

### CWatters

Yes the impedance of an ideal current source is infinite.

3. Oct 31, 2012

### aralbrec

Since resistance attached to the output of the transistor is infinite you can't ignore the output resistance of the transistor itself. You've no doubt been ignoring it in most of your circuits but this could only be done because Ro (of the transistor) >> than anything attached to the transistor output and it was connected in parallel to anything attached at the output.

Last edited: Oct 31, 2012