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I am unsure how a differential-input JFET amplifier in a closed-loop can be built. A very simple amplifier with differential inputs and differential outputs can look like this:

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As far as I understand, the disadvantage is a gain that depends on the transconductance of the transistors which is not well defined.

To form an amplifier with a well-defined gain one could form such a closed-loop structure:

enter image description here

However, in that case one of the differential inputs is used for the loop feedback such that an amplifier with a single-ended input results.

I am looking for a closed-loop amplifier structure based on a pair of two JFETs with a real differential input.

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Think about how this would be done with just an op-amp, you have the differential inputs and an output and when you want to close the loop you connect the output to one of the inputs through a network that determines the gain. The gain isn't well defined until you define it. The FET differential amp is the same as the input stage of an op-amp so you would close the loop the same way, add some gain after the differential stage and feed the output back to the inverting input.

schematic

simulate this circuit – Schematic created using CircuitLab

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