Thank you LvW,
I used the sc amplifying to test with dc input signal. Thus, the output is in dc form, too. The prob is that I can not see any oscillated signal in my amplifyer circuit although the opamp ac test shows unstability in 30% of Monte Carlo sim samples. The prob stands for itself for cases of less or higher number of Monte Carlo runs.
Do you think that there is a prob with my ac simulation configuration (I connect the single output to the negative input (negative feedback), and the other possitve input is connected to a reference level, by which the dc level of the output is fixed to the ref value. And I simulated the ac response of the buffer and derived the ac response of the opamp)?
Thank you in advance and best regards,
thikgaidep
Here are my questions:
1.) How is the opamp powered (single or double supply)?
2.) What is the role of Vref in the first picture?
3.) What is the purpose of Vref in the 2nd picture?
4.) Did you realize that the inv. input in the 2nd picture is floating? No dc path available!
5.) Did you check the dc operating point of the amplifier?
1) It is single powered opamp
which value?
2) There are 2 inputs and only 1 output of the opamp. Therefore, the positive input is assigned with a Vref voltage to bias the DC level, by which the DC level of the output is fixed to Vref.
Yes, this sounds logical - however, I cannot see that Vref is connected to the pos. input. Therefore my question
3) There are a reset phase in pic 2 to connect the output and the negative input in order to fix the DC level of the output. When the reset phase is over, the DC of the output is Vref (similar to pic 1), and the input is applied. Thus, the amplifier can work in desired DC config.
Thus, the dc level is fixed during the reset phase only, correct? For my opinion, an integrating device with single supply is rather critical.
4) The DC level of the negative input is fixed in the reset phase.
See my answer to 3)
5) Yes, I did. and the dc level is ok.
All the time? Or during reste phase only?
There are mis-matchings in monte carlo simulation of ac response in pic 1 with the transient response in the pic 2.
As indicated in my post#4 already, I do not understand the meaning of the pic you call "transient response".
The 1st sim shows that the opamp is unstable in 30% of monte carlo runs, but there is no oscillated in the transient monte carlo simulation. Would you plz give me some explains and tell me an appropriate method to test the stability of my circuit. I would be much appreciated.
Which parameter do you change/vary during the simulation runs?
it is 3.3Vdd. VREF is connected to the pos input. The dc level is fixed during the reset value and still be there in all operation (transient sim shows that). All the time the dc level is ok (transient sim shows that).
In my Monte carlo simulation, I put all parameters changing. I think I have no prob with the circuit, unstability is what I would like to test and verify.
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