Re: Low distortion sinewave oscillator without big capacitor.

On 4/04/2025 2:21 am, JM wrote:

On Sun, 30 Mar 2025 14:54:56 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
 

Not long ago JM posted a 1KHz sinewave oscillator with very low distortion.
It used a 470uF non polarized capacitor which in practice would probably be made from two 1000uF capacitors.
There's nothing wrong with that but I wanted to see whether I could make a working circuit without needing such a large capacitor.
>
What I have so far is below.
>
Any comments?
What's the best way to control the output level?
Currently it's 5v pk-pk but I rather have half that.

<snip>

You'd be as well omitting Q1, Q2, Q11 and Q12.  Just tie R17 to
Q7e(Q8b) with other end to vee, and R28 to Q6e(Q9b) (adjust polarity
of gain vontrol voltage).  There will be no loss in performance.  Q6-9
are within an overall control loop, and there is no need for matched
transistors (within reason).
 If you splash out on a matched quad it would be better used to
construct a four quadrant multiplier, it would have lower harmonics
than this circuit if done correctly.
 Are the one ohm resistors used for some sort of simulation/measurement
purpose?

I made those changes to Edward Rawde's original circuit, and the revised circuit worked. I haven't yet got a clue how, but poking around in a working simulation should let me work out what's actually going on. It's 4.10am my time and I should have gone to bed four hours ago, so I'll leave that until tomorrow.
I can comfort myself with the idea that my reservations about the number of transistors in the original circuit turned out to be well founded, but John May clearly grasped how the circuit worked, and I've yet to get there.
--
Bill Sloman, Sydney