adjusting the gain margin of a system

[bharathr87]

the images of the buffer amplifier ckt and its freq response(gain and phase plots) are below....it is unstable right? wat modifications do i have to do for the circuit if i hav to make it stable? the load cap. can be >=3.5pF.

 

[ravirajdv]

Hi,
There seems to be some problem with the simulation setup. If you look at the mag and the phase plots, they don't go hand-in-hand. Near to 0Hz, the mag plot shows a +x dB roll-off, whereas the phase rolls off by -x deg. At 1K the phase plot flattens out, but the mag plot rolls off.
Please check the setup and the measurements.
I think in Gray meyer book there is a mention of the a 3-amp structure for AC analysis, instead of opening the loop using the elements.

Regards,
RDV

 

[haff99]

It is clear from your sim results that something is wrong. Your DC gain is negative, which is indicitive of something is not biased right or something. You should be seeing a gain of at least 40-50db with that amp. You shouldn't be seeing -50dB. Something is wrong with your setup.