[OPamp testing] HELP!

[ee484]

Hi, all.

I'm currently testing a opamp with unity gain feedback.

With small DC input (<30mV), it seems that it does not have any problem.
However, as DC input increases, the output of the opamp (unity gain feedback) has oscillation and it is proportional to the input amplitude.

Since it does oscillate with DC input, I don't think it comes from lack of phase margin..

What's the typicall senario that cause this oscillation in a testing? The oscillation frequency is about 1.2MHz.

If I put 1MHz cutoff freq. low pass filter, it does it's job.

Thanks...


Best.

B

 

[pfd001]

The gain is associated with your DC input level. Also the phase margin is changed when the DC is changed. You'd better check the phase margin

 

[ee484]

DC input changes within allowed input range so that there is not much gain change - especially when it is used with feedback structure.

Also, why does DC input matter with the phase margin???
Can you please explain it for me?? the phase margin can be discussed at unity gain, which is about 4MHz in my case.

 

[Ghitani]

What is the circuit u r working in ? Is it a simple buffer ?

 

[ee484]

Yes, negative feedback unity gain buffer which has PMOS input so that quite large input range.

 

[renwl]

the phase margin ,gain and bandwidth may vary in different input DC.

it depends on your design.