raja.cedt wrote on Oct 25^th, 2011, 9:25am:


Yes.  The Vout(s)=0 at s=gm/CGD +j0.   This zero is at a complex frequency which has a non-zero real part and a zero imaginary part. If the circuit is responding to an excitation at this complex frequency, then the Vout would be zero.  

If the circuit is responding to a pure sinusoid with a radian frequency of w, i.e. s=0+jw, then Vout(jw) is not zero. If jw is varied from 0 to positive values, then the zero at s=gm/CGD +j0 contributes a +20dB/dec to the overall slope of the magnitude of Vout(jw)/Vin(jw) at frequencies higher than w=gm/CGD.  This value of w is often referred to loosely as a zero frequency in the magnitude versus w plot.  The actual zero at  s=gm/CGD +j0 causes a corner frequency at w=gm/CGD in the magnitude versus w plot.

raja.cedt wrote on Oct 25^th, 2011, 1:38pm:


Hi Raj,

s=gm/CGD +j0 is a point in the complex frequency plane.  Vout(s)=0 at this frequency in the complex frequency plane.

A circuit excited by pure sinusoids such as when sweeping the frequency in an ac simulation, is represented in the complex frequency plane as a sweep along the jw axis.

The value of Vout(jw)/Vin(jw) as w is swept is determined by the poles and zeros of Vout(s)/Vin(s) wherever the poles and zeros lie in the complex frequency plane.

That is, the poles and zeros of Vout(s)/Vin(s) effect the shape of  Vout(jw)/Vin(jw) versus w even when the poles and zeros do not lie on the jw axis.

I explained it as clearly as I am able in this posting type of communication.  Maybe someone else can explain in a way which would be more helpful to you.

Best Regards,
Garrett

@ Garrett.Neaves : thanks for your effort, i have last question, let us say if you have a TF=(s+1)/(s+2) now according to your theory at s=-1 you have zero (i also agree), but can you tell me in your magnitude plot will it go to 0, or let me ask you in another way can you give a sinusoidal signal which makes this TF magnitude 0.

Thanks,
raj.

hello buddypoor,
then why people says that @ zero frequency gain will be 0 and @ pole frequnecy gain will be inf, up to my knoledge if you have poles or zeros on iw axis then only you get .

But why this method is working in cases?

Thanks,
Raj.

hello Garrett.Neaves,

there is Contradiction between  first post and earlier post, so finally you agred that magnitude can't be zero, so why for finding Zero people are shorting o/p? for example take a simple CS amplifier there zero frequency is -gm/Cc, doesn't mean o/p at this frequency is 0 ....

Thnaks,
raj.

hello,
i donno how to document it. In your first post you said Vout will go to Zero and in one post you told it's won't go. Did you understand my Question? Any how i am decribing here for you

1.take a simple source follower, it gives left half zero @gm/Cgs, does it means o/p is zero at this frequency (just tell me yes/no)
2. In razaavi book he calculated this frequency by shorting the o/p.

Thanks,
Raj.

hi raja.cedt,

Fundamental things u have to bear in mind:-

1) Real frequency i.e jw and "s" (complex frequency) are two different things

2) T.F. must be zero or infinite at the ZEROs ( in s form) and at POLES  ( in s form)

3) If poles or zeros happens to be REAL frequency (i.e. s=jwz or jwp) then there exists sinusoids which makes T.F. infinity or zero

For example a LC parallel circuit:- in that case the denominator is LCs^2+1 . So poles are +/- j (1/(LC)^2). So real frequency poles.
So there exists a sinusoid at this frequency which makes the LC ckt to go to infinity. i.e. one get o/p at this frequency without any input (i.e it oscillate at the Resonance frequency).

Similarly you can find REAL zero for a series LC ckt.

4) Complex poles and ZEROS give  20 dB/decade roll off in bode plot.
   I hope this statement you can understand and prove urself why it is so .

Regards,
-Ricky