A question about DC gain of amplifer,thanx

[boooser]

dc gain from a graph

I test the DC gain by doing the .ac and .dc analysis
But the result is a little different.
The ac. analysis shows the DC gain is 61.7dB, about 1000
The dc. analysis shows the DC gain is 898.

Why the result is different ?
thanx

 

[elbadry]

calculating dc gain from dc analysis

Perhaps you need to increase the number of points in the DC sweep

 

[surianova]

I test the DC gain by doing the .ac and .dc analysis
But the result is a little different.
The ac. analysis shows the DC gain is 61.7dB, about 1000
The dc. analysis shows the DC gain is 898.

Why the result is different ?
thanx

What i see from the graph is 898 is not gain, the unit is in voltage and there might be something wrong because the voltage is too high , almost 1kV. The output graph also wrong as you sweep the input voltage. I guess something wrong the method you used.

 

[boooser]

I test the DC gain by doing the .ac and .dc analysis
But the result is a little different.
The ac. analysis shows the DC gain is 61.7dB, about 1000
The dc. analysis shows the DC gain is 898.

Why the result is different ?
thanx

What i see from the graph is 898 is not gain, the unit is in voltage and there might be something wrong because the voltage is too high , almost 1kV. The output graph also wrong as you sweep the input voltage. I guess something wrong the method you used.

898=dVout/dVin I think it is the DC gain.
That is not 1kV its is 898 V/V, it is a ratio not the absolute value.
Why the output graph is wrong?
I got the result by dc sweep the differential mode voltage of the input.

Can you give me a testbench for test the DC gain a amplifier? by .DC?
Thanks

Added after 2 minutes:

Perhaps you need to increase the number of points in the DC sweep

I decrease the .dc step which will increase the point of DC sweep.
But the result seems remain the same.

 

[surianova]

I test the DC gain by doing the .ac and .dc analysis
But the result is a little different.
The ac. analysis shows the DC gain is 61.7dB, about 1000
The dc. analysis shows the DC gain is 898.

Why the result is different ?
thanx

What i see from the graph is 898 is not gain, the unit is in voltage and there might be something wrong because the voltage is too high , almost 1kV. The output graph also wrong as you sweep the input voltage. I guess something wrong the method you used.

898=dVout/dVin I think it is the DC gain.
That is not 1kV its is 898 V/V, it is a ratio not the absolute value.
Why the output graph is wrong?
I got the result by dc sweep the differential mode voltage of the input.

Can you give me a testbench for test the DC gain a amplifier? by .DC?
Thanks

Added after 2 minutes:

Perhaps you need to increase the number of points in the DC sweep

I decrease the .dc step which will increase the point of DC sweep.
But the result seems remain the same.

actually what opamp u use ? differential amplifier? If so, u need ti fix a DC voltage at one of its input first, let say inp =1.2v and sweep the other input inn
from ard 1.1 to to 1.3 V, with a smaller DC step, you will see output switch from high to low , this i never see in your graph attach.

 

[boooser]

I test the DC gain by doing the .ac and .dc analysis
But the result is a little different.
The ac. analysis shows the DC gain is 61.7dB, about 1000
The dc. analysis shows the DC gain is 898.

Why the result is different ?
thanx

What i see from the graph is 898 is not gain, the unit is in voltage and there might be something wrong because the voltage is too high , almost 1kV. The output graph also wrong as you sweep the input voltage. I guess something wrong the method you used.

898=dVout/dVin I think it is the DC gain.
That is not 1kV its is 898 V/V, it is a ratio not the absolute value.
Why the output graph is wrong?
I got the result by dc sweep the differential mode voltage of the input.

Can you give me a testbench for test the DC gain a amplifier? by .DC?
Thanks

Added after 2 minutes:

Perhaps you need to increase the number of points in the DC sweep

I decrease the .dc step which will increase the point of DC sweep.
But the result seems remain the same.

actually what opamp u use ? differential amplifier? If so, u need ti fix a DC voltage at one of its input first, let say inp =1.2v and sweep the other input inn
from ard 1.1 to to 1.3 V, with a smaller DC step, you will see output switch from high to low , this i never see in your graph attach.

Yes, I used a differential amplifier.
You mentioned is one method.
Actually I use a balun for the input. I hold the common mode and sweep the differential by DC sweep.
Briefly speaking, I hold the common mode voltage,and let the voltage of the two input change at the opposite direction. And plot d Vout /d Vin

 

[surianova]

ok, i understand ur method now. May be you can check if input DC common mode when you doing AC and DC analysis is the same?