Re: Random thoughts on sinewave oscillators

On 10/18/24 23:19, Edward Rawde wrote:

"Cursitor Doom" <cd999666@notformail.com> wrote in message news:veuirv$3cmo3$10@dont-email.me...

On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote:
>

"Cursitor Doom" <cd999666@notformail.com> wrote in message
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On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote:
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"Cursitor Doom" <cd999666@notformail.com> wrote in message
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On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote:
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"Cursitor Doom" <cd999666@notformail.com> wrote in message
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On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote:
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"piglet" <erichpwagner@hotmail.com> wrote in message
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Edward Rawde <invalid@invalid.invalid> wrote:

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...

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Without doubt, it's the trickiest aspect of the design. Definitely do-able
though. Let us know how you get on.
>

 Ok. This simple circuit is based on the circuit you can find here.
 https://electronics.stackexchange.com/questions/697687/how-to-control-the-amplitude-of-a-wien-bridge-oscillator
 It produces a something wave.
I wouldn't call it sine but at least it's not clipping.
What's going on here?
 Version 4

[Snip...]
You're hitting the flat portion of the Id vs. Vds curve around the
top of the wave. In that region the dynamic resistance of the FET
is very large, and therefore the gain of the opamp drops to about
one. As a result, the positive tip of the output gets sort-of
squashed.
There are probably ways to fix this, for example by feeding a
portion of the output signal to the FET gate, but a quick
attempt I made didn't work very well. This is why FETs aren't so
great as gain setting elements.
Using a lightbulb --or more generally a PTC resistor-- for R7 is
really hard to beat.
Jeroen Belleman