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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: AF Osc AGC alternatives to lamp/thermistor/FET
Date: Mon, 21 Oct 2024 21:13:52 -0000 (UTC)
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Joe Gwinn <joegwinn@comcast.net> wrote:
> On Mon, 21 Oct 2024 19:45:28 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> john larkin <jl@glen--canyon.com> wrote:
>>> 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!
>>> 
>>> 
>> 
>> My vote would go to a comparator sensing the peak of the AC waveform, and
>> reducing the DC bias current a smidge each time it fires. 
>> 
>> Changing the current on the peaks causes minimal change to the phase,
>> assuming that you do it right, and the tiny nonlinearity goes away in
>> between, rather than causing distortion all through the cycle. 
>> 
>> Some months back, we talked about an audio amp design of Jim T’s that set
>> it’s quiescent bias that way, except switching near the zero crossing.
>> Turns out to be a startlingly good approach. 
> 
> .<https://people.engr.tamu.edu/spalermo/ecen620/general_pn_theory_hajimiri_jssc_1998.pdf>
> 
> People don't much use Hajimiri for phase-noise estimates (Leeson being
> adequate and simpler), but this is relevant to the audio-amp question.
> 
> Joe Gwinn
> 

Looks like a good read, thanks. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics