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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Fri, 18 Oct 2024 19:24:34 -0400
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"john larkin" <jl@glen--canyon.com> wrote in message news:qfo5hj5vqpi357bsads868ska3ap7k16rc@4ax.com...
> On Sat, 19 Oct 2024 00:19:35 +0200, Jeroen Belleman
> <jeroen@nospam.please> wrote:
>
>>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
>>>>> news:veucs2$3cmo3$9@dont-email.me...
>>>>>> On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote:
>>>>>>
>>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message
>>>>>>> news:veu7kt$3cmo3$8@dont-email.me...
>>>>>>>> On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote:
>>>>>>>>
>>>>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message
>>>>>>>>> news:veu45s$3cmo3$5@dont-email.me...
>>>>>>>>>> On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote:
>>>>>>>>>>
>>>>>>>>>>> "piglet" <erichpwagner@hotmail.com> wrote in message
>>>>>>>>>>> news:vetde5$38sbk$1@dont-email.me...
>>>>>>>>>>>> Edward Rawde <invalid@invalid.invalid> wrote:
>>>>>>>>>>>>>
>>> ...
>>>>
>>>> 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
>
> But tricky to Spice.
>
> And a thermal device of course makes the amplitude temperature
> sensitive, especially when the heating goes directly as the square of
> the sine amplitude.
>
> It would be better to have the native loop gain very close to 1.00 and
> give the AGC mechanism a small influence, like  +-  a few per cent
> maybe.

Attempting to reduce the gain control range of the FET seems to run into other issues which I've seen in other simulations.
The lower amplitue parts of the signal produced by this ciruit look nice and clean.
Now I just need a way to stop it generating an AM radio signal.
Manual adjustment of R8 is likely to be needed in any real circuit.

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