[sdiy] An Improved Sine Shaper Circuit

[Pete Hartman]

Same here Jimmy, I contacted Dave off list about it.  I suspect that
perhaps they exceed the size limits that Ben described earlier this week.

Pete

On Tue, Apr 21, 2020 at 3:58 PM Jimmy Moore <jamoore84 at gmail.com> wrote:

> Not sure if the issue is on my end, but the picture attachments aren't
> rendering.  Anyone else?
>
> On Tue, Apr 21, 2020 at 2:29 PM David G Dixon <dixon at mail.ubc.ca> wrote:
>
>> I just simulated the Thomas Henry transistor-pair sine shaper, and
>> compared the output to a 90-degree phase shifted pure sine wave of equal
>> amplitude.  It is virtually impossible to tell the two apart – THD = 0.57%
>> -- light blue is the pure one, orange is the shaped one.
>>
>>
>>
>>
>>
>> Here’s the sine shaper circuit I’m simulating – this is exactly the
>> circuit I build into all my VCOs (transistors are 2N3904, opamp is TL07X –
>> the 11k resistors get me closer to 10Vpp than 10k):
>>
>>
>>
>>
>>
>> In what way does the output of the diff pair not look like a sine wave?
>>
>>
>> ------------------------------
>>
>> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of
>> *Donald Tillman
>> *Sent:* Tuesday, April 21, 2020 12:45 PM
>> *To:* René Schmitz
>> *Cc:* synth-diy at synth-diy.org
>> *Subject:* Re: [sdiy] An Improved Sine Shaper Circuit
>>
>>
>>
>>
>>
>> On Apr 17, 2020, at 8:53 AM, Donald Tillman <don at till.com> wrote:
>>
>>
>>
>> On Apr 17, 2020, at 1:56 AM, René Schmitz <synth at schmitzbits.de> wrote:
>>
>> Interesting circuit, and a great article.
>> I'm pretty sure I have seen a similar technique before, because I have
>> used it. (cusp canceling)
>>
>>
>> I am very familiar with cusp cancellation.  I've used it also.  And it's
>> mentioned in the article.
>> This is not cusp cancellation.
>>
>>
>>
>>
>>
>> I'd like to expand on this for a moment...
>>
>>
>>
>> I think the phrase "cusp cancellation" has, accidentally, been misused a
>> lot.  And that's caused confusion.
>>
>>
>>
>> "Cusp cancellation" should mean that we've already got a pretty good
>> approximation going, but the cusps of the triangle are still coming through
>> a little bit.  And we can cancel those by subtracting a small amount of the
>> original triangle wave.  Sweet!
>>
>>
>>
>> This would be because the transfer curve of the diff amp pair isn't
>> completely flat at the top and bottom.   The tanh() curve is asymptotic, so
>> there will always be a little slope on the peaks.
>>
>>
>>
>> The most common next step is to apply negative feedback around the diff
>> amp pair.  This could be in the form of a feedback resistor, or by adding
>> small emitter resistors.  The negative feedback plumps up the curve and
>> flattens the slope at the peaks for a better overall fit.  Nice!
>>
>>
>>
>> But here, with the Colin/Henry/Guest/Tillman (Have I got everybody?  In
>> order?) approach, the output of the diff amp pair isn't remotely close to a
>> sine wave.  Not even trying.  And none of us are using negative feedback to
>> plump out the curve.  We're not in the cusp cancelling business, we're
>> doing something else.
>>
>>
>>
>> I got here by applying actual cusp cancellation to an actual diff amp
>> pair with negative feedback and a pretty good sine approximation.  Then I
>> refined it with thousands of simulations, which lead me away from cusp
>> cancelling, and toward considering a compound curve of tanh(x) - βx,
>> expressly for the bumps and the sine shape in between.  And the rest as I
>> described.
>>
>>
>>
>> So I guess Dennis Colin (ARP, Aries) got to the circuit first.
>>
>>
>>
>> So, I'll claim that if a small amount of the original triangle wave is
>> subtracted from a wave that's roughly sinusoidal, then it's actual cusp
>> cancellation.
>>
>>
>>
>> But if the diff amp pair contribution doesn't look like a sine wave, and
>> there's no negative feedback, and the transfer function can be put into the
>> form tanh(x) - βx, then it's this other approach that Dennis Colin
>> pioneered.
>>
>>
>>
>>   -- Don
>>
>> --
>> Donald Tillman, Palo Alto, California
>> http://www.till.com
>>
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
>>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20200421/f611b109/attachment.htm>