# CE Amplifier Input Impedance question

Discussion in 'Homework Help' started by ECE101, Nov 26, 2011.

1. ### ECE101 Thread Starter New Member

Nov 25, 2011
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I'm trying would the input impedance would be for the circuit shown below..

I understand the input signal will see the Beta*Re resistance, but if I assume the constant current source is a resistor, would it just be:

Rb || Beta*Re?

I've seen it defined as Re + (1 + Beta)*RE

where RE is an emitter resistor, as there is no RE in this diagram I'm not really sure how to derive the input impedance.

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The input resistance Rin=Vin/Iin.

Find Vin and Iin and divide them.

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3. ### ECE101 Thread Starter New Member

Nov 25, 2011
29
0

I've just found the answer and

Rin = Beta/Gm (or Beta*Re as I suspected)

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
The input resistance

Rin = RB|| ( Beta + 1) * re

re = 26mV/Ic ---> dynamic emitter resistance ( looking into the emitter to base).
And this intrinsic resistance (re) acts as if is is in series with the emitter.

gm = 1/re

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