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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 18:58:43 -0400
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"Jeroen Belleman" <jeroen@nospam.please> wrote in message news:veumn5$3fbqu$1@dont-email.me...
> 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.

Ok thanks Jeroen.

It looks like the best approach for the gain control is either a filament or something like that used in the document Bill Sloman 
posted.

https://www.analog.com/media/en/technical-documentation/application-notes/AN132f.pdf

Either that or an analog multiplier of some sort.


>
> Jeroen Belleman
>