bigworm
Member level 3
The oscillation of VGA
I met with a problem when I was designing the VGA with Gain -3 to -21
The Opamp is a 2-stage one with common-source amplifier as the 2nd stage and CMFB. CMFB uses large resistor to sense the output CM.
the openloop gain and PM meet my need.and GB is 45MHz.
the signal is 200KHz, quite low freqency.
I use resistor feedback to generate variable voltage gain. the resistors are in the order of 10K. I found the output of VGA oscillates when used in closeloop. the envelope is the low frequency signal and it has high frequency part.
I have done ac sim and tran, the openloop gain and PM is O.K.
why does it oscillate?
and can anyone give me some advice to eliminate the oscillation?
Thank you all in advance!
I met with a problem when I was designing the VGA with Gain -3 to -21
The Opamp is a 2-stage one with common-source amplifier as the 2nd stage and CMFB. CMFB uses large resistor to sense the output CM.
the openloop gain and PM meet my need.and GB is 45MHz.
the signal is 200KHz, quite low freqency.
I use resistor feedback to generate variable voltage gain. the resistors are in the order of 10K. I found the output of VGA oscillates when used in closeloop. the envelope is the low frequency signal and it has high frequency part.
I have done ac sim and tran, the openloop gain and PM is O.K.
why does it oscillate?
and can anyone give me some advice to eliminate the oscillation?
Thank you all in advance!