Your welcome, I never learned electronics at scool so I gues you have a headstart
The oscillating is rather complex. Especcially with choppers while they oscillate inside by nature. Oscillating in opamp schematics has to do with phase delay and gain. 180 degrees delay and unity gain at a certain frequency makes that it starts oscillating. Every RC combination, for instance the Rout ot the opamp and the gate capacity form RC networks and so phaseshifts.
Your design must have enough Phase margin as it is called. If you want to know more about it look for laplace transfornmation, bodeplots and poles and zeros. But that is very, very complex stuff. I know the principle but my math is not good enough to do the transformation (i have done it recently for a smps compensation network and it took me hours and then I found out I started with the wrong value) but by playing with resistors and capacitors , and as Bob Pease calls it, banging the output with a squarewave, I most times find a way.
You can start to insert a resistor (10-100 Ohm between output and gate, and a speedup cap over it to compensate for gate capacity ) this will give a phaseshift and most times is enough to stop the oscillaions. Bob Pease gives some other solutions of RC compensation networks in his book troubleshouting analog circuits. on inputs, bewteen inputs, from in to output ect. You must shift the phase.
But what you are doing now is trying to use a Ferrari in the Paris-Dakar rally (or Baya race) or using a Mack truck as a golfcart. A chopper amp is a rather extreme device, a FET like this, with upto 20nF gate capacity too, but then on the opposite scale of extremes. Choppers are used to amplify micro signals, I? just using a few to make a voltage reference and because it must be able to drive shielded cables to a KV divider, and buffer the output of the KV I insert OPA277 and LM1010 as buffers. Choppers are not made deliver 4 to 20V to the current hungry gate a power mosfet.
Zero offset and drift are not important here. The tempco of the FET is probably many times bigger then the output drift of the worst opamp, the offset of the worst opamp will not make enough volts to drive the FET open. So even a 741 will do.
one thing you can try is inserting a normal opamp as follower between the chopper and the gate. The inverting input of the chopper stays at the source. the other opamp is just there to source the power the chopper can not deliver.
The drift, offset ect are not important but because the chopper still regulates that it will not become worse (if that makes you feel better
)
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