[sdiy] How is the feedback loop of the exponential converter analysed?

[Spiros Makris]

Hello!
Attached is a barebones version of an expo converter realised with PNP
transistors. Thermal circuitry is omitted for clarity. The type of HF
compensation shown is not important(?).
I know from my experiments that a TL082 (usually?) needs a feedback
capacitor to ensure it does not oscillate in this configuration. I have
built it before and arrived to 47p for the stability capacitor, which keeps
the loop stable but allows a bandwidth of ~15kHz (Iout/CV), if memory
serves me right.
How could someone estimate that value using pen and paper calculations? Or,
at the very least, some slight assistance from spice, yet not just brute
force it with trial and error? What qualities of the opamp matter in this
application and what are the tradeoffs?

Any resources or explanations are very welcome.

Regards,
Spiros

[image: image.png]
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