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How to force 2-stage OpAmp to find DC point for AC analysis?

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tyanata

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OpAmp AC Anqalysis

Can any one tell how to force Two stage operational amplifier two to find its DC operational point to make accurete AC analysis because, now I try to simulate two stage Op Amp (first stage is folded cascode) but the output Invertor can't find its operational points. :?:
 

Re: OpAmp AC Anqalysis

Make sure to bias the Opamp.

Also, you should put it in DC closed loop. This is done by connecting a huge inductor 10e10) between its output and the minus input with a huge (capacitor 10e10) between the minus input and ground.

Then, you can apply the AC signal (with a proper DC value) to the plus input.

Feel free to ask again
 

Re: OpAmp AC Anqalysis

Alternate approach (offset measurment):

Assuming you are decoupling your common mode an differential input:

1) make a fine input DC sweep (e.g., from -5m to +5m with 100uV step) of the differential signal and measure the input value corresponding to the desired output value.

2) inlcude the measured value in your differential input signal and run AC analysis.

nathan
 

Another question about AC analysis:

When you simulate closed-loop feature, which point you choose to break the loop? and why?
 

hardings said:
Another question about AC analysis:

When you simulate closed-loop feature, which point you choose to break the loop? and why?

The input of op amp connected to the feedback circuit (negative input terminal). Because this is the summing node. Its response affects the closed-loop stability.
 

Are sure that these big inductur and capacitor wouldnt influence some characteistics of the circuit?!
 

Hi, Hughes ,thanks a lot.But if the feedback circuit is a resistor,you break feedback circuit and negative input terminal, then you cancel one pole(resistor and the input capacitance of the input terminal).

Anyway wherever you break , the AC characteristics will change, do you have any better way?

tyanata
I always use the method to get open_loop characteristics. The inductor and the capacitor only
give a operation point.You can read Allen's book to get more information.
 

hardings said:
But if the feedback circuit is a resistor,you break feedback circuit and negative input terminal, then you cancel one pole(resistor and the input capacitance of the input terminal).

Anyway wherever you break , the AC characteristics will change, do you have any better way?

I don't have any other way to tackle this problem. But I think the pole should be in high freuqency since the input capacitance is usually small compared with that of output. Any one has some good idea?
 

Hardings

I learned a new method similar to yours, to connect output to the input with voltage controled voltage source in series to output after it big resistor in series to the source and big capacitor to ground and after that the AC source is connected between the resitor and the input.

What do you think is it proper method?
 

Oh,tyanata

Can you explain more what you said? Or with a picture?
 

Re: OpAmp AC Anqalysis

Hi, That huge resistor and big cap will take care of the offset voltage of the opam. (As it act like a unity feedback at low frequency and open circuit at high frequency..so open circuit AC analysis at high frequency)..
But inorder to take the effect of the load capacitance at the feedback node due to cap of the input device there are two method..
1) Either connect dummy mosfet of same size as input mosfet at the output of the opamp.
2) Connect the same opam at the output of the first opamp with positive input same as that of the first opamp and negative input as the output of the first opamp.

I feel that second method will give more realistic result as it take into consideration the miller multiplication effect of the input cap..
What you people feel..
Amit
 

Hi, hardings the explanation of amitbhaiji is right.
 

Re: OpAmp AC Anqalysis

amitbhaiji said:
Hi, That huge resistor and big cap will take care of the offset voltage of the opam. (As it act like a unity feedback at low frequency and open circuit at high frequency..so open circuit AC analysis at high frequency)..
But inorder to take the effect of the load capacitance at the feedback node due to cap of the input device there are two method..
1) Either connect dummy mosfet of same size as input mosfet at the output of the opamp.
2) Connect the same opam at the output of the first opamp with positive input same as that of the first opamp and negative input as the output of the first opamp.

I feel that second method will give more realistic result as it take into consideration the miller multiplication effect of the input cap..
What you people feel..
Amit
Hi, can you provide some pictures, I can't catch it very clearly, cuz I am just a newbie in the field, thank you in advance!
 

OpAmp AC Anqalysis

I think you must know your DC point and let your circuit work in proper situation. To get that you should use dc loop config
 

Re: OpAmp AC Anqalysis

on the same question...
i would also like to ask how to do the frequency response analysis in cadence for op-amp???
 

Re: OpAmp AC Anqalysis

Humungus said:
Make sure to bias the Opamp.

Also, you should put it in DC closed loop. This is done by connecting a huge inductor 10e10) between its output and the minus input with a huge (capacitor 10e10) between the minus input and ground.

Then, you can apply the AC signal (with a proper DC value) to the plus input.

Feel free to ask again

thanx for your explain, i have seen that in baker book (CMOS mixed-Signal circuit Design)P-292, but i dont now if we have to remove the input source of mins input or not.
 

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