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Opamp Question - Is this Stable ?

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Puppet1

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What if I connect an Opamp this way.

What happens ?

Thanks.
 

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Hi,

I doubt it is stable, because two Opamps are fighting each other at high frequencies.

To decide if an opamp circuit is stable you need to know the Opamp data like phase margin...
A simple circuit is not enough...

Klaus
 
I agree with KlausST. I don't think it is stable. At high frequencies, first opamp sees a local positive feed back making it unstable. However, simulating it would be a good test to arrive at any conclusion.
 
Hi Puppet1,


I visualize the given opamp configuration as "Two stage Opamp design with Miller Cap compensation".

I have attached the reference image.

Since the added Cc introduces a "Right hand side zero" the phase margin can be less than required.

So, it has a higher possibility to oscillate.

To make it stable try to place a "Resistor"in series with Cc. The value of resistor can be greater than the inverse of the input trans-conductance of second opamp. Getting this info may not be possible, so try to fix it through simulation.




Regards,
Jebas.
 

With or without the capacitor - the circuit is not stable.
For dc you have negative feedback - hence the whole system will find a stable quiescent operational point.
However, two opamps in a closed loop result in a loop gain magnitude function with a slope of at least -40dB/dec at the cross-over frequency (0 db gain).
This violates the stability criterion.
 

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