SC-CMFB problem regarding output waveform

[ljy4468]

hi every body~
i designed fully differential opamp with sc-cmfb circuit.
and simulate it.....
but problem occur.
it seems that output common is right...
but output waveform is like this

the clock is nonoverlapping, and cap size is 400f 600f.
And variation in cap size..it is not useless~
help me

thanks

 

[eng_Semi]

Re: sc-cmfb problem

Try to check the stability of the CMFB circuit

 

[ljy4468]

sc-cmfb problem

I solved this problem.
I reduced capacitor value(25f, 10f)
then, operate well

But in corner simulation (ss,ff)
ac sweep...
Av is too low and output common is too high or too low.(hspice output file) (e.g. some mos is in linear region)

But In transient simulation(10mV sine input), output waveform is almost exactly same with tt (typical typical) simulation. (same amplification, output cm correct)
And looks like working well

How can I interpret it??
Could I think that my circuit operate well??

I think spice got data before my circuit's stable.
(of course, my amp doesn't work in first 3~4 clock cycle)

anybody help me please
thank you

 

[sunking]

sc-cmfb problem

The Vcc ?

It seems that the swing of the output is two large.
Try a small output swing.

 

[nxing]

sc-cmfb problem

you can not work with cap in the order of 25f or 10f if you considering the parasitic effect during layout.

 

[dingjingfeng]

sc-cmfb problem

yeah ,25f and 10 f capacitor is too smalle to be acheived accurately, even in GaAs technology.
so when choosing on-chip passive elements,
you should first consider whether they can be really fablicated

 

[amitbhaiji]

Re: sc-cmfb problem

have you check the gain reducton of the amplifier with loading effect of the sc cmfb. Higher the cap lower will be the resistance and lower will be the gain of opamp.
What clk freqecy you are using for sc cmfb.