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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Five transistor version of the low distortion sine-wave oscillator
Date: Tue, 15 Apr 2025 09:12:46 -0400
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vtl0bc$364bt$1@dont-email.me...
> On 15/04/2025 1:56 am, Edward Rawde wrote:
>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vtihob$sfdm$1@dont-email.me...
>>> On 12/04/2025 6:27 pm, Bill Sloman wrote:
>>>> Edward Rawde posted an eight-transistor low distortion sine wave oscillator circuit recently, and John May pointed out that you
>>>> could leave out half the transistors.
>>>>
>>>> I couldn't immediately see exactly how either of the circuits worked, though I could get the simulations to run under LTSpice 
>>>> and
>>>> see roughly what was going on.
>>>>
>>>> I've now dug a bit deeper. Here is a five transistor version of John May's four transistor version.
>>>
>>> Out of curiousity,
>>
>> Is that allowed Bill? I thought that making component changes to see if the circuit works better was design by evolution?
>
> I didn't make the change to see whether it worked better - I did it to see if I'd correctly understood what it was doing. The fact 
> that it made it work better was incidental.
>
>>> I upped the currents through Q1A and Q1B by about an order of magnitude (R27 down to 27k, R17 down to 22kk and R28 down to 68k)
>>> and the worst case harmonic became the second at 2kHz, 155dB below the the fundamental. The fourth was close behind at at about
>>> 157dB down.
>>>
>>> Essentially, their incremental resistance has dropped by an order of magnitude, and the ripple on the gain-control signal 
>>> produces
>>> less voltage excursion. >> > Did you mean R20? I don't see R27.
>
> I did indeed.
>
> > I think the only way forward with this circuit would be to build and
> test it.
>
> Agreed.
>
> > I'd do a first prototype with everything through hole except LT1679
> and NSS40301MDR2G.
>
> Why?

Changing almost anything in this circuit in LTSPice changes the residual harmonic levels.
Assuming the same is true in reality I'd want to be able to change components easily.

>
> > I'd also put four more resistors in series with each 68k (maybe
> reduce them to 56k) for the four diodes so I can make the current pulses in the four diodes exactly equal.
>
> Why? I can see an argument for removing all the 68k resistors so the current being fed through R11 is as high as possible, with 
> the smallest possible ripple. There is a risk that the diode current will turn off fast enough to drive them into snap-recovery, 
> but it is remote.
>
> Increasing the 68k resisitors reduces the effect of the tolerance on the forward voltage drop through each diode, but choosing 
> diodes with a closer tolerance on the forward voltage drop would be a better way to go. The 1N914 doesn't seem to have one at all.
>
> The Infineon-BAS3007ASERIES diodes at least specify 350mV typical and 400mV max at 100mA. I think NExperia had something better 
> back when it was Philips, but that's a long time ago.

Changing to schottky diodes changes the distortion but not always down.
So I'd want to be able to make changes easily on a real prototype.

>
> > And add a capacitor (100n min) to ground where the resistors join.
>
> Adding more phase delay along the feedback path and make the settling time even longer.

One of us doesn't care if he has to wait 5 minutes for the purest sinewave.
The other seems to put higher priority on the circuit settling in a few seconds.
I think we'll just have to differ there.

>
> > And use the remaining LT4167 (two quad packs) as an output buffer so
> that whatever is connected to the output doesn't disturb the
> > operation of D10.
>
> What D10?

The one that's D14 in your circuit.

>
> -- 
> Bill Sloman, Sydney
>
>
>