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

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!