[sdiy] An Improved Sine Shaper Circuit

[David G Dixon]

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 ¨C THD = 0.57% -- light
blue is the pure one, orange is the shaped one.

 



 

Here¡¯s the sine shaper circuit I¡¯m simulating ¨C this is exactly the
circuit I build into all my VCOs (transistors are 2N3904, opamp is TL07X ¨C
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: Ren¨¦ 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, Ren¨¦ 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) - ¦Âx, 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) - ¦Âx, then it's this other approach that Dennis Colin
pioneered.

 

  -- Don

--
Donald Tillman, Palo Alto, California
http://www.till.com

 

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