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Collector feedback and output impedance

Thread starter #1
Hi there.

I found a website that uses Miller's Theorem to analyze a simple collector biased transistor (voltage shunt feedback). There's something about the ouput Z that I don't understand.

https://wiki.analog.com/university/.../chapter-9#example_972_using_miller_s_theorem

By substituting Rf using Miller’s Theorem, Rf appears much smaller at the input, and the input impedance is very much reduced. This is expected. So far, so good.

But Rf appears almost the same at the output (voltage gain is supposed much bigger than one).
According to this, it seems that the output impedance is just this miller-transformed-Rf in parallel with collector resistor.
This way, output impedance gets reduced just a little bit. From 10k to 8,6k or so.

But, the output impedance is supposed to be reduced by a factor of the gain loop.

A simple spice simulation shows this clearly. I get around 600 ohm output Z for a 2n2222.

So, what is this Miller's Theorem based analysis missing?

Thanks.
 

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