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How does this discrete differential FET amplifier followed by an opamp work?

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Palpurul

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I've been reading this post in analog dialogue, but I don't understand how the circuit in figure.1 works (basically all circuits in the post are the same as the circuit in figure.1). There is a JFET pair configured as differential follower and it is followed by a low noise opamp which has no feedback from its output to its input. How the gain of this circuit controlled? Isn't a minute difference between the opamp's input satures the opamp in the circuit? How the feedback network is implemented here or is there a feedback network at all?
 

basically all circuits in the post are the same as the circuit in figure.1
Nope, figure 1 is a differential amplifier with gain, not a differential follower. The difference matters for the amplifier loop gain and phase margin when closing the loop.

There is a JFET pair configured as differential follower and it is followed by a low noise opamp which has no feedback from its output to its input.
The circuit is intended to act as an OP, the feedback loop hasn't been closed around it yet. I agree that this point could be made clearer in the articles. The application examples shown down in the article are implementing specific closed loop gains (1,11,101), you have to imagine the respective feedback network.
 
Nope, figure 1 is a differential amplifier with gain, not a differential follower. The difference matters for the amplifier loop gain and phase margin when closing the loop.

Yes it was my fault. I wasn't careful enough when posting. It's obviously a differential stage with gain, but the rest of the FET stages are followers.

The circuit is intended to act as an OP, the feedback loop hasn't been closed around it yet. I agree that this point could be made clearer in the articles. The application examples shown down in the article are implementing specific closed loop gains (1,11,101), you have to imagine the respective feedback network.

I think by OP you mean operational amplifier. I guess the feedback network is just a regular resistor feedback network (from opamp's output to the inverting input).
 

Many opamps have Jfet inputs and a few have Mosfet inputs.
The gain of an opamp is so high that they are only used without negative feedback when they are a comparator that always has a saturated output.
 

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