Hi Rakesh!

Is the plot for open loop gain or closed loop gain?

Ok, I suppose by GH you mean the loop gain without the inversion where you get the negative feedback. Then I also suppose you are worried by the -180deg at low freq, which points to a stability problem. I think the reason for the somewhat strange curves is that you have two feedback loops, the main one you know about and one other that you have not think about. Then those curves are typical. See also

http://www.designers-guide.org/Forum/YaBB.pl?num=1294178255

To convice yourself about the stability you can do the complete Nyquist criteria. The simplified one just checking if you not circle the -1 point is not enough. The reason is that it looks like you have two poles in the right half-plate for the open loop. This can be allowed, Nyquist will tell you. It also looks like you have one zero in the right half-plane at lower freq.

I'm not sure you can determine stability from the bode plot in such a case. Maybe some other knows a trick. When looking at phase margin and gain margin it is assumed that the loop behaves nice, which is not the case here. What is defining the stability is the lack of poles in the right half plane for the closed loop. Nyquist criteria is a way of finding that out.

Hi Rajesh,

My quick comments - i am assuming that this is the open loop gain phase plots. It seems to have a bandpass sort of a relationship and looks like there is a single zero and two poles (zero lower than the poles). I also dont think this is an unconditionally stable system - there are two unit gain crossover points - for the first one, there is no issue with phase margin of 120 as you say. For the second one, the phase reaches 180 while the loop gain is greater than one - this could be cause for instability

HI Rakesh,

Quote:


i completely agree with you for small signal analysis. the question is if this ensures stability over all input voltage magnitudes - even when the small signal gain is greater than 1, the large signal gain can be smaller and can be one (eg when the input peak to peak voltage is equal to the output voltage swing). this situation does not arise when the small signal UGC is less than when the phase is 180

Hi Rajesh,

Case 1. Small signal gain is greater than 1 at phase =180. As the input signal increases in magnitude, the gain can come down and even unity gain is possible (when input signal peak to peak value is equal to output voltage swing). Even thought the small signal gain is greater than one, here have a situation for large signals the gain becomes unity. So i think that gain crossover (gain going from greater than 1 to less than 1) happening before phase crossover may not ensure stability.

Case 2. Small signal gain is less than 1 at phase 180. in this case the large signal gain will also be less than 1, no issue here.

Hi Rakesh,

it does not really matter if your circuit has constant gm. In fact for most oscillators and VCO's the loop gain is chosen to be greater than 1 to ensure adequate startup.

Now lets say that your circuit has ideal components - you still have a finite supply voltage and your output swing cannot be greater than the supply. Your input signal cannot be greater than the supply - when the output swing becomes equal to input swing, you have the unity gain situation.

So i think gain crossover coming after phase crossever can cause instability. Feel free to correct me.