Speaking of Sine Shapers

[Jim Patchell]

    Another variation on the OTA sine shaper is the one that is shown in the
RCA data sheet for the CA3280 dual OTA.  This part has a pin that is
connected to the emmiters of the diff pair.  Using both OTA's in the package
in a sort of push-pull mode, a resistor is connected between the two emitter
pins which is used to trim distortion.  Sort of an expensive sine converter,
however, since it uses one whole CA3280, and these things are not exactly
cheap.

    -Jim


Haible Juergen wrote:

> Speaking of sine shapers.
>
> I bet this is not new either, but I discovered this just some weeks ago.
> I was looking for a simple but fairly good triangle to sine shaper
> (for the PWM of my PolyModular VCO).
>
> You know that a single overdriven OTA does the trick - somehow.
> Problem: When the input amplitude is trimmed for minimum THD,
> the waveform has a rather pointed peak. So you have to choose
> your compromise, different overdrive for VCO or LFO applications.
>
> There is the 2-stage solution used by Moog. Nice, and quite
> involved. (Two stages ...)
>
> And there is the degenerate emitter method, where a discrete
> differential pair is used instead of the OTA, and small emitter
> resistors are added. I've mostly used this method so far.
> (See FS-1)
>
> But then it struck me that these emitter resistors are just a method
> of (local) feedback, i.e. slightly linearizing the differential pair's
> input function.
> The same thing must work with a standard OTA and an external
> feedback loop ! And it does. 3080 driving inverting opamp at
> virtual GND summing node. Output of opamp *slightly* fed back
> to the OTA's noninverting input. Works like a charm.
> I'll post a circuit of this with the next PolyModular update.
>
> JH.