[sdiy] An Improved Sine Shaper Circuit

Jimmy Moore jamoore84 at gmail.com
Tue Apr 21 22:54:16 CEST 2020


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
>
>
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