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

[synthdiy at adambaby.com]

That worked thx Ben

Adam

> On 27 Jan 2020, at 9:22 am, Ben Stuyts <ben at stuyts.nl> wrote:
> 
> I’m trying to track down the problem with certain images not showing on the list. Spiros has sent it to me separately and I’ve turned some knobs on the mailing list control panel. Hopefully this time it gets through:
> 
> <image.png>
> 
>> On 27 Jan 2020, at 00:01, synthdiy at adambaby.com <mailto:synthdiy at adambaby.com> wrote:
>> 
>> Spiros that image isn’t showing for me
>> 
>>> On 27 Jan 2020, at 6:43 am, Spiros Makris <spirosmakris92 at gmail.com <mailto:spirosmakris92 at gmail.com>> wrote:
>>> 
>>> 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
>>> 
>>> 
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>> 
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