how to understand this curve?

[lhlbluesky]

i have designed a two stage opamp (used for a buffer, fck=15M, CL=12pF), the first stage is folded cascode, the second is common source, and i use cascode compensation to ensure frequency stability, now, i get the following simulation curve, when the input signal from 0.4 to 1.6, it settles well, but when the input signal from 1.6 to 0.4, it has a very big overshoot, why? the PM is 70 deg or so, and it is not like normal underdamped oscilation, what is the reason of this big overshoot? please give me some advice, thanks all.

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---------- Post added at 12:17 PM ---------- Previous post was at 12:16 PM ----------

the black curve is the input signal, the blue curve is the output curve, please help me, thanks.

 

[carporsche]

I think it may be related to the way the input signal is turning your transistors ON/OFF when the amplitude decreases from 1.6 to 0.4. I had faced a similar situation in a 2-stage amplifier design. It has been quite sometime and i do not remember exactly what i did. But if you are using virtuoso check for your transistors being cut off or in saturation from the ADE-XL checks.
Maybe someone else will be able to give you a better idea theoritically.

good luck!

 

[lhlbluesky]

thanks, any other ideas?

 

[AdvaRes]

It may be due to bad transistor sizing and huge parasitic capacitances.

 

[jiripolivka]

I do not see any time scale; the above advices are correct. I would try the smallest feedback resistor for a given gain as well as a small series resistor (100-500 Ohms) to the load . Many opamps behave similarly with large input signals; some gain distribution (adjustment) in each stage may also help.

 

[carporsche]

I forgot to see that the amplitude of input is large, I mean its 1.2 V p-p @ 15MHz.. Am thinking its because of slewing and the output load cap is also 12pF. Try reducing the fck and the amplitude to a lower value and check whether you experience the same result. That may give a better idea in determining the reason for the behavior. Hope this helps!

 

[eladla]

I agree with carporsche.
Looks like negative slewing to me too.
Exchange the load capacitor for a smaller one and check again.
You might need to turn up the gain on the output stage and turn it down at the input stage to compensate.
Higher gain = higher Gm = larger current = less slewing.