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Design >> Analog Design >> which one is correct for miller equivalent
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Message started by dandelion on Jan 31^st, 2007, 5:53pm

Title: which one is correct for miller equivalent
Post by dandelion on Jan 31^st, 2007, 5:53pm

Hi,
This question is about the miller equivalent.

Pls. see the attached two digrams. The C1,C2 is the miller equivalent of C for the 1st amp, while C3 and C4 is the miller equivalent of C for the 2nd amp.

I wonder which one is correct for the two diagrams? The cap C can be miller equivalented simultaneously for the two ampifiers?

Thanks

Title: Re: which one is correct for miller equivalent
Post by mg777 on Feb 1^st, 2007, 8:58pm

You've got positive feedback for either sign of A, so I'd be cautious about a linear anaysis. Is this a memory cell?

As far as linear analysis goes: if the amplifiers are ideal then the time constant is likely to come from the r₀, so I'd go with your second circuit. BTW, you may want to write the A = -g_mr₀.

M.G.Rajan
www.eecalc.com

Title: Re: which one is correct for miller equivalent
Post by dandelion on Feb 12^th, 2007, 5:46pm

mg777 wrote on Feb 1^st, 2007, 8:58pm:

hi mg777,
Thanks for the reply.

Would u pls. elaberate your explaination a bit more?

It is just a demonstration diagram, it would not be a positive feedback. And it is not a memory cell also.

Because the cap C is tied to the two amplifiers both, I was puzzled how to break the loop to do the stabilty analysis. I have concerns that the miller effect will disapear for one amp if breaking the loop. So my puzzles is in fact if the cap is miller equivalent for one amp, it would be miller equivalent to another amp simultaneously? Or put another words, the same cap can be miller equvalented twice simultaneously in one block?

Thanks in advance

Title: Re: which one is correct for miller equivalent
Post by avlsi on Mar 1^st, 2007, 8:41am

wht i feel is the same,since it is +ve feedback, is it miller approximation? the sign of A is positive. that means, there is no error signal developing.

Title: Re: which one is correct for miller equivalent
Post by dinojr on Jan 8^th, 2009, 7:15pm

I've had a concern on this as well, I've search around for an answer but couldn't find anything. My concern was that why can't the first gain stage be non-inverting and the second stage inverting. This would present a negative feedback situation and be no different then a miller compensated differential amplifier driving a common source fet. If I open the loop at the input of the differential amplifier aren't I missing an effect of gmR0 of the diif amp multiplied by the miller cap?? This is very puzzling for me to a seemingly simple circuit.