Capacitor at output of opamp

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cupoftea

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Hi,
Isnt it strange that you cant have a capacitor of >~100n at an opamp output, but if its an opamp clamp (as attached), then you can have a cap as big as you like.
Why is this?

LTspice also here...
Click to expand...

 

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Hi,

with a usual circuit the capacitor is pushed and pulled by the OPAMP and the OPAMP tries to stay in regulation all the time. Every input noise is also driven to the capacitor, causing current.

On the clamp circuit, the capacitor os pulled only and the OPAMP goes out of regulation (at least) half of the time.
Usually you avoid "out of regulation" condition with OPAMPs because of
* distortion
* violating differential input voltage range.
* latch up time
* phase reversal
.. and all the resulting drawbacks.

You need careful OPAMP selection for this clamp circuit .. not to damage the OPAMP and to get the desired results.

Usa a random OPAMP and get disastrous output.

Klaus
 
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Thanks, better to use say a 330R resistor from D10 cathode to opamp U25 output?
 

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Hi,

The resistor may limit the current, but does not prevent a single of the problems written in post#2.

Klaus
 
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Last edited:
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Thanks, page 12 of the article kindly given by Dana shows also a cap at opamp output, just with a diode inbetween

there is a fair chance that ckt will latch low at start up ...
Click to expand...
...thanks, and we are wondering whether it will latch low and then oscillate?...as you know, an opamp isnt generally allowed to have a cap at its output......and the opamp probably cant "see" the diode between its output and the cap?
 

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Hi,
cupoftea said:
...thanks, and we are wondering whether it will latch low and then oscillate?...as you know, an opamp isnt generally allowed to have a cap at its output......and the opamp probably cant "see" the diode between its output and the cap?
Click to expand...
But understand: the capacitor isn´t connected to the output.

Oscillations means "up and down".
When using the circuit of post#1 the capacitor is "conneted" only when the OPAMP output is smaller than the capacitor (diode voltage drop), then the capacitor get´s discharged.
Due to the nature of the capacitor ot will keep it´s voltage, even when the opamp output becomes higher. Then there is no current flow. No current flow, no problem. So due to the "up and down" of the oscillation the oscillation stops itself.

Usually an oscillation means current flow in both directions. This is avoided by the diode.

With a forced AC we just see short current peaks at the negative peaks of the voltage. The most of the time there is no current flow at all.

Klaus
 

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There are OpAmps specifically compensated to handle large C loads. This situation
a little unusual, so I simed it, with an LM324 which is not comped for large Cload.
It does not seem to oscillate. That may be the fact that when loop is closed in your
circuit its unity G and that means Rout of OpAmp at its lowest, so its pole/phase
shift occurs at a freq >> GBW.

Regards, Dana.
 

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