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Design >> Analog Design >> miller theorem
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Message started by raja.cedt on Nov 3^rd, 2012, 1:18am

Title: miller theorem
Post by raja.cedt on Nov 3^rd, 2012, 1:18am

Hello all,
i wanted to know miller theorem in bit clear manner. I am aware of miller theorem application to find input impedance, but how about output impedance. Because from output to input gain wont be 1/A (provided A being forward gain in many cases it will be around zero, for example consider common source amp which has good forward gain but very poor reverse gain and this is the essential requirement for an amplifier, then where this theorem at output can be used..

Thanks,
raj.

Title: Re: miller theorem
Post by buddypoor on Nov 3^rd, 2012, 5:34am

Hi Raj,

the Miller-Theorem applies to all networks with an element Z between two nodes, which are (or which should be) referenced to a common point (mostly ground).
This is well explained in Wikipedia and I shouldn`t repeat it here.
More than that, have a look at

http://paginas.fe.up.pt/~fff/eBook/MDA/Teo_Miller.html

Title: Re: miller theorem
Post by raja.cedt on Nov 3^rd, 2012, 6:03am

hello buddypoor,
thanks for the reply. I know that point, but what i mean is can you tel me some circuit where you can apply at the output? Give some example to me . For example in the link you gave, after splitting Z between V1 and V2 who will maintain V1/V2 relation?

Thanks,
Raj.

Title: Re: miller theorem
Post by buddypoor on Nov 3^rd, 2012, 6:46am

raja.cedt wrote on Nov 3^rd, 2012, 6:03am:

As you know, negative feedback drastically reduces the output resistance of opamp circuits.
The reduction factor is (1+T) with T=loop gain magnitude. This is the most illustrative example, for my opinion.