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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: AF Osc AGC alternatives to lamp/thermistor/FET
Date: Tue, 29 Oct 2024 16:11:48 +1100
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On 27/10/2024 3:18 am, Cursitor Doom wrote:
> On Tue, 22 Oct 2024 16:35:38 +0200, Arie de Muijnck wrote:
> 
>> On 2024-10-22 15:11, Liz Tuddenham wrote:
>>> piglet <erichpwagner@hotmail.com> wrote:
>>>
>>>> Yesterday in the thread "Random thoughts on sinewave oscillators" I
>>>> posted a LT Spice wirelist showing amplitude levelling without fet,
>>>> ntc thermistor or ptc filament instead using current steering long
>>>> tail pair - kinda poor mans multiplier.
>>>
>>> How about using a lamp as the gain-controlling element at a low signal
>>> level, so it doesn't self-heat, then heating it with superimposed D.C.
>>> from some sort of voltage comparator/rectifier which is separate from
>>> the signal path ?
>>>
>>> It might have to be in some sort of bridge circuit, so that any noise
>>> on the heating current doesn't appear in the signal.
>>>
>> How about the 'good old' lamp (LED) + LDR method?
>> But I cannot find any linearity data of the LDR resistance itself (at
>> constant illumination), important for harmonic distortion.
>> Most searches end up in the audiophoolery corner.
> 
> Optocoupler? There might be more detailed manufacturer data available for
> an appropriate device.

The photoFet seems like the obvious candidate.

The FET channel seems to be a pretty good resistor at low voltage drops 
though it moves towards being a constant current limiter as you pit more 
volts across it.

https://www.onsemi.com/pdf/datasheet/h11f3m-d.pdf

https://au.element14.com/on-semiconductor/h11aa1m/opto-cplr-phototransistor-7-5kv/dp/2322524

https://www.digikey.com.au/en/products/detail/onsemi/H11F1SM/1793946

There doesn't seem to be an LTSpice model for the part. A google search 
throws up tantalising hints, but nothiog that lead to a model that I 
could download.

-- 
Bill Sloman, Sydney