I need Help in two stage amplifier desgin!

[kongququ]

I design a two stage amplifer
The first stage is telescopic
The second stage is common source.
I use trans command to see the waveform of second stage output.
The waveform seems pretty good.
But when I observe the output of the first stage.
The waveform is so bad。
I realy do not know the reason why the first stage is so bad.
All the transistor is work in Sat region.
And I use two cmfb. one for first stage, one for second stage.

Thank you very muckvsdafdsf

 

[nxing]

Can you post your circuit as well? Maybe that will be a little bit easy to find the reason.

 

[mcjackson_hxw]

I guess u should check the phase margin of the CMFB circuit to make sure that the cm voltage stable.
You can try first.

 

[reltol1]

I guess the CMFB for the first stage does not have enough phase margin.

 

[jiangwp]

What do you mean the first output is bad ?
I think that if the common mode voltage of first stage is not within the second stage input common mode range , the second stage will not work normally . So some transistors will be out of saturate region . In the mean while , the first stage is telescope topolog , the output range is narrow , so the signal amplitude shoud is small , it shoud look good if the amplifier work normally.

May be you simulate it with .tran with close loop feedback .

In another way , you shoud simulate it with unit gain feedbak, then look the waveform .

 

[xwcwc1234]

To reltol1,
How to measure gain and phase margin of CMFB ? Would you post a spice example file here ?

 

[kongququ]

I will post my schematic.
Who can tell what's wrong.
Many thanks!
My main amplifer.

 

[kongququ]

Bias network.

 

[kongququ]

commom mode feedback

 

[reltol1]

To reltol1,
How to measure gain and phase margin of CMFB ? Would you post a spice example file here ?

simply cut the feedback and input ac signal to the CMFB at the close bias point. The load is directly connect the main amp. I don't have an example..... Try it.

 

[xwcwc1234]

Is this method can apply to all CMFB type , no matter SC or continuous ?

 

[jiangwp]

The open loop generate the nonlinear when simluating.

This is generly named "gain nonlinear" or present low distortion.

If you simlulate it with close loop , the distortion will be disappear because of the feedback effect. So i suggest u simulate it with close loop.

 

[OMEsystem]

You use two cmfb in one opamp, is it ok? Will cmfb beating or conflicting happen?

 

[youyang]

hi, as your schematic shown
i guess the compensation cap between the two stages introduces a RHP zero in both CM and DM situations, which should make the phase margin worse. so i suggest you do a pole-zero analysis and check this zero.

youyang

 

[windman]

Can you post your simulation result? What does the bad waveform exactly mean?