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

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.