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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 23:03:20 -0400
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"john larkin" <JL@gct.com> wrote in message news:j656hjp1rq659uh61k3q75bipaf386qqh1@4ax.com...
> On Fri, 18 Oct 2024 18:58:43 -0400, "Edward Rawde"
> <invalid@invalid.invalid> wrote:
>
>>"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
>
> Note that the LDR has a very small influence range on the loop gain.
>

That's why I added R3 in this circuit.
It does not seem to be safe to reduce R3 below 1k.

R4 helps a lot too for reasons I don't fully understand.
It may be moving the FET to a better part of its operating characteristics.

A single rail version also works with another op amp producing 6V for R4 and two 20k resistors for R2 between 12V and 0V.
As expected, this produces twice the output voltage and I've not found a way to reduce it.

This will probably be my final offering for a 1KHz sinewave oscillator unless anyone can suggest improvements without using light 
dependent resistors.
From the LTSpice plot, I can't discern any impurity in the signal this circuit produces.
It would be interesting to see what a real circuit and a spectrum analyzer says but I probably won't be building it.

I haven't used an LDR since playing with an ORP12 around age 10.
I seem to remember that they can degrade over time but maybe that only happens in sunlight.

Version 4
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SYMBOL voltage -512 64 R0
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