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how can i adjust opamp

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Junior Member level 1
Dec 31, 2005
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as you can see from the following text the mosfet all in sat. but the opamp have a gain of -2db, how can i adjust them to make the gain become larger
element 0:mm7 0:mm5 0:mm4 0:mm6 0:mm11 0:mm10
model 0:p3v 0:p3v 0:p3v 0:p3v 0:n3v 0:n3v
region Saturati Saturati Saturati Saturati Saturati Saturati
id -5.9591u -11.0956u -11.0956u -5.9591u 5.9591u 5.9591u
ibs 2.4563f 3.214e-20 3.214e-20 2.4563f -2.005e-21 -2.005e-21
ibd 4.9951f 3.4532f 3.4532f 4.9951f -59.5594a -59.5594a
vgs -1.0718 -1.0500 -1.0500 -1.0718 850.0000m 850.0000m
vds -235.1159m -228.1767m -228.1767m -235.1159m 313.1664m 313.1664m
vbs 228.1767m 0. 0. 228.1767m 0. 0.
vth -885.9850m -840.6536m -840.6536m -885.9850m 622.6604m 622.6604m
vdsat -204.9377m -220.9339m -220.9339m -204.9377m 217.6919m 217.6919m
beta 334.0794u 524.9302u 524.9302u 334.0794u 234.0114u 234.0114u
gam eff 350.8455m 350.4868m 350.4868m 350.8455m 703.3935m 703.3935m
gm 50.8340u 84.1471u 84.1471u 50.8340u 44.0752u 44.0752u
gds 5.8549u 13.6127u 13.6127u 5.8549u 1.2973u 1.2973u
gmb 8.5821u 15.8838u 15.8838u 8.5821u 15.6347u 15.6347u
cdtot 11.5162f 19.6407f 19.6407f 11.5162f 2.7659f 2.7659f
cgtot 24.1660f 37.3839f 37.3839f 24.1660f 6.4646f 6.4646f
cstot 32.6768f 52.6794f 52.6794f 32.6768f 8.6611f 8.6611f
cbtot 25.7887f 42.4298f 42.4298f 25.7887f 8.4276f 8.4276f
cgs 20.8204f 32.3715f 32.3715f 20.8204f 4.5719f 4.5719f
cgd 1.1971f 2.2221f 2.2221f 1.1971f 242.7163a 242.7163a

element 0:mm9 0:mm8 0:mm3 0:mm2 0:mm1
model 0:n3v 0:n3v 0:n3v 0:n3v 0:n3v
region Saturati Saturati Saturati Saturati Saturati
id 5.9591u 5.9591u 10.2731u 5.1366u 5.1366u
ibs -294.8778a -294.8778a -2.551e-21 -35.9093a -35.9093a
ibd -1.4470f -1.4470f -59.6094a -289.2447a -289.2447a
vgs 786.8336m 786.8336m 850.0000m 1.2800 1.2800
vds 1.2235 1.2235 219.9544m 1.5519 1.5519
vbs -313.1664m -313.1664m 0. -219.9544m -219.9544m
vth 765.2300m 765.2300m 641.2358m 638.8488m 638.8488m
vdsat 85.3907m 85.3907m 205.1740m 553.8381m 553.8381m
beta 2.4804m 2.4804m 476.0728u 28.3763u 28.3763u
gam eff 703.9373m 703.9373m 703.3935m 703.7839m 703.7839m
gm 104.6919u 104.6919u 80.0055u 14.7931u 14.7931u
gds 244.2669n 244.2669n 4.6812u 37.5718n 37.5718n
gmb 33.4104u 33.4104u 28.9888u 4.5984u 4.5984u
cdtot 16.7024f 16.7024f 5.5850f 1.4659f 1.4659f
cgtot 45.8552f 45.8552f 12.3067f 20.8501f 20.8501f
cstot 61.1117f 61.1117f 16.9347f 19.6372f 19.6372f
cbtot 54.0396f 54.0396f 14.7215f 14.6562f 14.6562f
cgs 33.4331f 33.4331f 9.4549f 13.5439f 13.5439f
cgd 2.0605f 2.0605f 728.7084a 125.6004a 125.6004a

Without seeing the schematic, I can think of only a few things that might kill your gain.

1) You're running the AC analysis at a frequency that far exceeds your amplifier's bandwidth.

2) You're possibly trying to drive a resistive load and your output impedance is too high.

3) You're doing some funny feedback to attenuate the gain.

However, from looking at your mosfet dc operating points, the gm on all of them are very small. The biggest one I see is ~100uS. Even though they are all in saturation, there is probably some biasing problem that is preventing you from getting the value of gm you are looking for.

I guess that the AC analysis is being performed for improper bias condictions of the input terminals.

Assuming your amplifier is defined as a SUBCKT as follows

. SUBCKT opamp in_n in_p out

Implement the following feedback to perform a proper AC analysis

Xopamp in_n in_p out opamp

* DC Feedback with AC killer
Lfb out in_n 10e10
Cfb in_n 0 10e10

Vin in_p 0 vdd/2 AC 1

The DC feedback gaurantees you circuit is prepperly biased.

i have post the picture as an attachement but it didn't come out
am i have no right to attach attachement or i have done wrongly!
can someone help me ?

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