Hi guys,

What would be the frequency and amplitude of oscillation of an uncompensated opamp?

I've attached a zip file with three waveforms. it is the simulation result of an uncompensated two stage opamp (PMOS input pair) in unity negative feedback configuration. I've shown the frequency response of STB analysis, transient response with VIN_P pulled slowly to 2V and then a fast jump to 3V and back. Third waveform is the zoomed version of the transient to show the frequency of oscillation.

As you can see the oscillation is at 153MHz with amplitude of 805mV. From frequency response, the frequency at which phase margin becomes zero is around 130Mhz,

1. so why is the oscillation is at 153Mhz instead of 130Mhz?

2. which factor decides the amplitude of oscillations?

Thanks to all the guys who post on this forum, i've learnt a lot from their posts.
especially the posts from VIVKR, raja.cedt, loose-electron, RobG where very informative and helpfull. Thank you guys  :)

Praveen  

hello pravin,
Always try to post figs rather than giving some zip files, people may not have time to open.

Coming to your opamp, it has 5 deg PM so it will give damped oscilations and frequency of the opamp keep on changing because of damping. Aproximatly  you can say frequency of oscilations is wn*sqrt(1-g^2) where g is damping factor. By the way have you verified how frequency is varing with time, i guess it will.

Idealy oscilations occur only at the frequency where gain=1 and Phase=180, but here it is not happening, eventually some non-linerities makes your gain lower or some thing, finally it will get a point where it can oscilate.

Please refere pg 491 in razaavi to get more info.

Thanks,
Raj.

hello boe,
yes you are correct. It is very difficult to estimate every thing through bode because phase is no monotonic.

Thanks,
Raj.

@BOE : you are right to note that at ~153Mhz the phase margin has already crossed 0deg and it has a margin of -5deg, why do you say that since its is -5deg, its amplitude should be increasing, could you please give me a reference for this reasoning?
If things are not deducible from PM plot, then nyquist plot is only other one, but how do i find my oscillating freq. from it? all i know is that it goes in circles around that (-1,0)  :)

@raja.cedt : there was no provision for me to attach more than one file in a single post, that's why i had no choice but zip it.
how do i find this natural freq. Wn of this system?
I checked the transient (conservative) simulation, the freq doesn't change much it hovers around that 153MHz through out, in fact the amplitude is also the same through out. thanks for you reference at razavi, i'm reading it.

@harpoon: b) you are right, the osc freq hovers around 153Mhz
c) my loop break point is correct and i'm doing an STB analysis which actually does middlebrook's, so loading issues on both side taken care off.

As you all pointed out its a large signal behaviour, can i reason like this, when the sudden jump happens at VIN_P, the VIN_N tries to catch up but over shoots, once overshot little bit, that is amplified by the gain and the reponse comes after the delay of the first stage, then the second stage amplifies and the reponse comes after the delay of the second stage, by this time it has overshot in the other direction
so can i say the amplitude will depend on the delay in the two stages and its gain?

thank you Boe, Raja and harpoon for your precious time in replying,
thanks,
Praveen

@BOE : you are right to note that at ~153Mhz the phase margin has already crossed 0deg and it has a margin of -5deg, why do you say that since its is -5deg, its amplitude should be increasing, could you please give me a reference for this reasoning?
If things are not deducible from PM plot, then nyquist plot is only other one, but how do i find my oscillating freq. from it? all i know is that it goes in circles around that (-1,0)  :)

When the PM is negative, the closed-loop pole is in the RHP of the s-plane. This has two effects:
* the factor "sigma" is positive leading to a rising sinusoidal signal ,
* the frequency of this signal is NOT identical to the frequency that can be observed for the case PM=0 (loop gain= 0 dB).

@buddypoor: Thanks for your explanation, i understand the idea of increasing exponential in time when in negative phase margin.

@Harpoon: simulating the system at different DC points changes the gain a little bit, the oscillation freq. remains the same around 153Mhz.

@raja.cedt: why should the oscillations change with time? why should the oscillations be damped?

after some middling around with the output pole and the output stage, my conclusions are these,

1. The oscillation frequency: is around the freq where PM goes to 0 (and gain > 1dB) ( brokewson's criteria)  

2. The amplitude of oscillation: ideally speaking it the supply limits, as its a growing exponential. But it depends on our output stage current capability, in the simulation shown in my first post, the output stage is just current source with common source stage (class A type), which obviously has very less current capability to pull the output to supply rails in that oscillating frequency, so the oscillations appeared damped but it is not. So if i had a class AB with high current output, then output amplitude is just the supply rails. The oscillations are never damped!!!

thanks,
Praveen

hi buddypoor,


sorry about the spelling, its 'Barkhausen criteria' . you can google it.

typo in my reply---- it should be gain>1 not 1dB.

regards,
praveen

hello buddypoor,
What i feel is if any system has clossed loop gain>1 and phase is 0, then it start blowing up and if there any non-linerity eventually it will make your gain =1 and this what i feel is criterion for oscilations. Please corect me if am wrong.

i didn't understand your post regaring some thing missing in Barkhausen criterion. Could you please explain  this.

Thnaks,
Raj.