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Design >> RF Design >> amplifier stability
https://designers-guide.org/forum/YaBB.pl?num=1143071352

Message started by boong on Mar 22^nd, 2006, 3:49pm

Title: amplifier stability
Post by boong on Mar 22^nd, 2006, 3:49pm

hello
how important is it to have unconditionally stabilized (K>1) amplifier from dc to freq.>>operating freq.? my narrow band amplifier is stable at the operating frequency (20 GHz) and higher but is unstable at frequencies below 6 GHz down to dc. my question is how important is it for me to stablize the amp. for those low frequencies. the simulation is saying that it might oscillate (K<1) if the condition for oscillation is matched at those frequencies and if that happens then i know my amp. won't work but let's say i take a chance and don't stablize it at lower frequencies. i've done some simulation where i'm comfortable and optimistic that the condition for oscillation won't be met (-R won't see an equal +R). is there any other issue i have to take into account. by experience, does any amp. designed in industry unconditionally stable at all low frequencies down to dc? i mean, i know it sounds stupid, but what are the chances that the condition for stability will be met at frequencies much lower than my operating frequency?
boon

Title: Re: amplifier stability
Post by Paul on Mar 23^rd, 2006, 12:18pm

Hi Boon,

oscillations of an unstable oscillator may start from noise, not only from the input stimulus. You certainly have some wideband noise in your circuit which may trigger such oscillations, but that also depends on how much residual gain you have at these frequencies.

Providing a mag/phase Bode plot here might be of help in the discussion.

If you are interested in bandpass operation only, would it be possible in your case to implement a low-frequency cancellation loop which reduces/kills the low-frequency gain. With such a structure you might be able to achieve gain <0dB at the frequencies where your phase shift is in the order of 180deg.

Paul

Title: Re: amplifier stability
Post by naren on Jun 16^th, 2006, 7:23pm

Hi,

Yes, your amplifier will oscillate at those freuencies where |S_11| and/or |S_22| are >1.

If you run a transient analysis by first removing the "PSIN" and not attaching any "VSIN", you will notice that there is a finite multi-harmonic sinusoidal signal present on your output.

This multi-harmonic sinusoidal output will be the superposition of sinusoidal signals whose frequency will be in the range where |S_11| and/or |S_22| are >1.

In simple terms, it will be a sum of sine waves from DC to 6GHz!!!

Naren
:)