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I cannot understand how the clamping diodes prevent opamp from saturation when the switch/FET is off in the following sample-and-hold circuit:

enter image description here

I only know that if the negative feed back of an opamp is broken it easily saturates to one of its supply rails due to even a nV difference at the inputs.

But in this case diodes are preventing such saturation eventhough the feedback is cut when the FET is off.

Can this be explained in an easy step by step manner since I have found some very short info but dont get it.

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  • \$\begingroup\$ The diodes ensure that the maximum voltage difference between the (-) node and the op-amp output is limited to one diode drop. \$\endgroup\$ – Dwayne Reid Feb 20 '19 at 1:19
  • \$\begingroup\$ I understand that but why that prevents saturation? \$\endgroup\$ – user1999 Feb 20 '19 at 1:20
  • \$\begingroup\$ Im looking for a much more detailed answer since I cannot relate this to saturation. \$\endgroup\$ – user1999 Feb 20 '19 at 1:22
  • \$\begingroup\$ Also saturation of which opamp they are talking about? \$\endgroup\$ – user1999 Feb 20 '19 at 1:25
  • \$\begingroup\$ Without the diodes, when the FET is off, the op-amps have their feedback loop open so it has infinite gain so it will behave like a comparator and will try to set the output to be high or low (which is saturation). The diodes provide a closed feedback loop when the FET is open so that this does not happen \$\endgroup\$ – DKNguyen Feb 20 '19 at 1:30
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But in this case diodes are preventing such saturation even though the feedback is cut when the FET is off.

When one of the diodes is forward biased, then there is feedback through the diode.

So in the sample state, the feedback path is through the JFET and the 2nd op-amp.

In the hold state, the feedback path is through one of the diodes.

Since there's still feedback, the op-amp output will only go to ~0.6 V above the input voltage, rather than to its saturated value (near the supply rail).

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  • \$\begingroup\$ I understand %90 percent. Why are there two reverse diodes? Is that about different conditions? Can you also say couple of things about that? Thanks \$\endgroup\$ – user1999 Feb 20 '19 at 1:43

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