Re: AF Osc AGC alternatives to lamp/thermistor/FET

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