Hello Everybody,

For my fully differential fold-cascode OTA with SC-CMFB, according to the Ken's paper "Simulating Switched-Capacitor Filters with SpectreRF" and previous posts in 2003 and 2004, I use PSS and can achieve good common mode output voltage results when I let two input signals are disabled. I have used two "vsource" as my two differential inputs, and actually I set type=dc and leave others all blank.

But how can I get large signal transient plot from PSS, which just looks like the transient plot through conventional "tran" analysis. For example, from conventional "tran", normally if we set input with peak-to-peak 1mv sine wave, we can see large signal level in output sine wave, say peak-to-peak 2v~3v in some cases. But with PSS, right now, I only can get differential gain with 4 in rough from the PSS transient plot.

What I tried was to set vsource
type=sine,
and fill "frequency name 1",
"amplitude Vpk",
"phase for sinusoid 1",
and"sine dc level"=2.5v (which is common-mode input).

But It is expected to get 2000~3000 voltage gain.

So what I did wrong? Do I need to fill these specifications to get large signal transient plot in PSS?

Anybody can share his or her experience?

From then on, for PAC, after I set vsource "type=sine" with "display small signal params" and "PAC magnitude", "PAC phase", I got differential gain with 4 and about 10dB.

Any suggestion? Or my circuit indeed has only 10dB gain? I have replaced SC-CMFB with equavilent resistors, its gain goes up to 70dB.

Appreciate your help.

part of the netlist is attached:

// Cell name: fold_cascode_new_lowpower_sc_TG_PNOISE_invclk
// View name: schematic

//two input sources

V4 (Vin net082) vsource type=sine freq=30K ampl=500.0u sinephase=180 \
       sinedc=2.5 pacmag=500.0m pacphase=180 fundname="fin"
V1 (Vip net082) vsource dc=0 type=sine freq=30K ampl=500.0u sinedc=2.5 \
       pacmag=500.0m fundname="fin"

// two pairs of clock for SC_CMFB

V10 (ph2 0) vsource type=pulse val0=0 val1=5 period=5u delay=200.0n \
       rise=500p fall=500p width=2.2u fundname="clk2"
V36 (0 ph1_b) vsource type=pulse val0=5 val1=0 period=5u delay=0 rise=500p \
       fall=500p width=2.7u fundname="clk1_b"
V37 (0 ph2_b) vsource type=pulse val0=0 val1=5 period=5u delay=200.0n \
       rise=500p fall=500p width=2.2u fundname="clk2_b"
V8 (ph1 0) vsource type=pulse val0=5 val1=0 period=5u delay=0 rise=500p \
       fall=500p width=2.7u fundname="clk1"
.......

//PSS and PACsimulation setup
simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 \
   tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
   digits=5 cols=80 pivrel=1e-3 ckptclock=1800 \
   sensfile="../psf/sens.output"
pss  pss  fund=10K  harms=0  errpreset=moderate  tstab=1.5m
+    method=gear2only  maxacfreq=20M  annotate=status
pac  pac  start=1  stop=300M  maxsideband=0  annotate=status
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
saveOptions options save=allpub

[quote author=Ken Kundert  link=1120551427/0#1 date=1120598371]Why don't you leave the large signal inputs disabled and simply use PAC? It will be much faster.

-Ken [/quote]

Thank you for your suggestion, Dr. Kundert,

The reason that why I set the large signal inputs is that I would like to see the large signal transient plot through PSS. If I disable the large signal inputs and use PAC mag only, I only can get common-mode output voltage level for PSS as before.

Can you comment why I couldn't get large signal transient plot for output voltage in PSS even though I set large transient signal inputs? Thanks a lot.

by the way, I still have a quesiton regarding PAC. I read a post in Mixed-signal Design forum that
http://www.designers-guide.org/Forum/?board=ms_design;action=display;num=1101544...

basically, he said " PAC is considering the whole signal (over a complete clock cycle) when computing the gain. If your signal spends half its time at zero due to a reset phase, that will effectivly reduce your gain by half (6 dB). "

Is that the reason why I only got 10 dB AC gain with PAC, instead of expected 60~70dB gain?

Thank you for your time and help.

ethan