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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: AF Osc AGC alternatives to lamp/thermistor/FET
Date: Mon, 21 Oct 2024 12:22:00 -0700
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On Sun, 20 Oct 2024 19:25:08 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>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.

You still need a multiplier of some sort to twaek loop gain. And
multipliers are nonlinear!