error amplfier ac simulation

[paraon00]

Hi,
I designed TYPE-3 error amplifier.
How can I ac-simulate my error amp?

At first, I used macro opamp model(with 80dB DC gain, 10kHz 1pole system) and ac-simulated.

My problem is
when input common mode voltage varys, vout(s)/vin(s) are different!

For example,
in+ = 0V
in - = 0V
vout(s)/vin(s) is same as what i desired.

But,
in+ and in- are not equal to 0V
Vout(s)/Vin(s) is different.

I've posted the schematic and simulation results.
(I thougth there is no DC feedback loop! Thus, there is no virtual ground. Is it right?
If I use real op-amp, I wonder the method to make DC operating point.)

I guessed that operating point of output node voltage is important to ac-simulation.
I used the command .nodeset V(out)=xxxV ....
But, It doesn't work!

How can I simulate my error amplifier to obtain right gain and phase curves?

Thanks,
J. Lee

 

[LvW]

DC feedback is an absolute "must" in order to get a fixed operating point.
Otherwise a real unit with an unavoidable offset will not work as an amplifier.

 

[paraon00]

!^O^!

Oh~
Thank you LvW
so much~!

I'll do that!

Added after 19 minutes:

My DC feedback doesn't work.

The schematic has been attached.

LvW, tell me how to construct the DC feedback loop, please.... ^^

 

[LvW]

The feedback resistor is too large (nearly infinite !).

The ratio Rfeedbck-to-Rinput should be not larger than app. 1E3.
(Rinput is the R in front of the opamp neg. input). In this case the offset is multiplied by 1000 what seems to be acceptable in many cases.

 

[integ52002]

May I know what kind of Amplifier did you use for this Error Amplifier? What kind of Operational Transconductance amplifier is it? or is it an Op amp?

Does it has a gain/BW requirement?