# Miller's thorem and negative feedback

Discussion in 'Electronic Basics' started by Jean-Pierre Trolet, Jul 1, 2007.

1. ### Jean-Pierre TroletGuest

Hello,

Just a question of pure theory . Why does the Miller's theorem not
account for itself for the decreased output impedance when applying
negative feedback ?
To be clear :

suppose an inverting amp with a Rf / R feedback network with a gain of
- Rf/R. Rf is the feedback resistor joining the output to the
inverting input ( = very classical !). Suppose also Zout to be the
output impedance in open loop.
Suppose Av is the gain in open loop.

If we apply Miller's theorem on Rf it tells us that the effect on
the output is equivalent to putting a resistor in parallel to the
output that is worth = Rf * Av / ( Av -1 )
It does not account for the fact that Zout also decreases by a factor
1/ ( 1 + Av*beta).

So I think that applying Miller's theorem in other conditions than
calculating the parasite capacitors on the intput and output is

What do you think of it ?

Friendly
jptrol

2. ### neon

1,325
0
Oct 21, 2006
this is active miller is inactive capisce  