# Emitter follower question (transistors)

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## Homework Statement

3. An emitter-follower is built using a transistor with β = 100 and R_E =
1000 Ω . (a) Whatʼs the input impedance with unloaded output? Now
suppose the input is connected to a source having output impedance
500 Ω . Whatʼs the output impedance of the circuit? (c) Remove the
source. Now, if a load 100 Ω is at the output, whatʼs the input
impedance?

Here, R_E/I_E refers to the output part of the transistor, and R_B/I_E refers to the input part.

(a)
β = I_C/I_B, so I_C = 100I_B.

So with unloaded output, should the input impedance be R_E*100?

(b)
If input is connected to a source with output impedance 500Ω, does that imply that the input impedance is 500+R_E*100? And that the output impedance is, thus, (500+R_E*100)/100?

(c)
100Ω output, so does that imply that the input impedance is 100(1000||100)? I believe that 1000||100 should be the total output impedance of R_E (since the load means that they're in parallel), and the input impedance is 100 times that, right?

Have I done things correctly? Am I missing anything?

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gneill
Mentor
The http://en.wikipedia.org/wiki/Common_collector" [Broken] has a nice summary.

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Gold Member
Yeah, I've already read it. I'm just not sure if I'm missing anything here.