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Opamp + capacitor gain circuit

electronicsLearner77

Jul 2, 2015
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Trying the impact of capacitor in the opamp circuit as below
1690462655152.png

The equations of the opamp are
V1 - i1R2 -i1R1=Vout -> 1
The voltage at inverting input is same as non inverting input
V1 - i1R2 = 1/C1690462789473.pngic dt -> 2
But there is no current in positive terminal as high impedance of the opamp, so i assume ic=0. What will be the behavior of the circuit?
 

crutschow

May 7, 2021
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But there is no current in positive terminal as high impedance of the opamp, so i assume ic=0. What will be the behavior of the circuit?
Since there is no path for the op amp input bias current, the output will likely go to one of the supply rails and stay there.
The capacitor will have no other effect on the circuit response, even if you did provide a DC path.
 

danadak

Feb 19, 2021
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Its not a "find" per se, but related to input bias current. If you cannot supply
the bias current with a DC path the input stage, bipolar, mosfet, jfet, whatever,
because its very hi Z, will take leakage current and Hi Z x inputleakage = drive the
input to a rail. Not only that, being very hi Z, the input will pickup stray signals,
like power line AC, so no telling exactly what its going to do. Also if the input is
driven outside its common mode range the input stage is unpredictable.

Regards
 

Harald Kapp

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But there is no current in positive terminal as high impedance of the opamp, so i assume ic=0.
Here's your error.
For example MAX4478:
1690530621573.png
OP177:
1690530668111.png

Even a few nA of input current will over time charge the capacitor. So definitely Iin <> 0 A!
 

danadak

Feb 19, 2021
797
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Not to complicate the discussion but the cap leakage current characteristics will also
determine if cap takes on a charge from bias current. Electrolytics, Tants, Ceramic pretty
leaky, film caps generally very low leakage. Complicating that Ceramics have a highly non-
linear relationship of V versus leakage.

Point of this is in high Z circuits one must pay attention to passive usage and
characteristics. even PCB track to track leakage can cause problems in high
performance DC applications.

Interesting ap notes when you have some time (your application not really applicable) :






Regards, Dana.
 
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