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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: AF Osc AGC alternatives to lamp/thermistor/FET
Date: Sun, 20 Oct 2024 19:25:08 -0000 (UTC)
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On Sun, 20 Oct 2024 11:42:27 -0700, john larkin wrote:

> On Sun, 20 Oct 2024 14:25:33 -0400, bitrex <user@example.net> wrote:
> 
>>On 10/20/2024 11:38 AM, john larkin wrote:
>>> On Sun, 20 Oct 2024 14:15:19 +0100, 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.
>>>>
>>>> Here is another alternative, using diode conductance bridge
>>>> "variolosser".
>>>>
>>>> For the full 1960s vibe replace U2 U3 with bjt current sources.
>>> 
>>> That's still limiting a Wein bridge, essentially a single-element
>>> resonator.
>>> 
>>> If you do the double-integrator type oscillator, you can do gain
>>> control on the first integrator, and then the second one adds 6
>>> dB/octave harmonic reduction.
>>> 
>>> I think you can even get 12 dB/octave harmonic reduction if you do the
>>> amplitude limiter right. 12*3 = 36!
>>> 
>>> 
>>> 
>>There's probably a way to finagle the 4th section of an LM13700 to apply
>>a bias current inversely proportional to the output level into pin IB1,
>>here:
>>
>><https://imgur.com/a/xRVx2PC>
> 
> At some point, opamp distortion and slew rate will usually dominate THD.
> I'd guess that the LM13700 isn't very good.
> 
> It might be OK as the small-influence AGC gain tweak, ahead of two good
> integrating opamps.

At the cost of adding another component, how about adding another opamp 
solely to regulate the output signal level? Say you want 2V p-p output, 
sample some of the output, rectify it and compare it to a reference level 
which is applied to the other opamp input.
Just my 2c.