[sdiy] tri-sin shaper and THD

[jhaible at debitel.net]

The basic idea about using feedback was to emulate the degenerate
emitter diff amp sine shaper method.
(As the diff pair isn't available separately in the OTA, you can't 
put in emitter resistors. So you have to partly linearise the circuit
with other means, like an overall feeback loop.)

Now, some of the better sine shapers use a 2 stage approach, using
one degenerate emitter diff amp, and one straight diff amp.
This should easily be emulated with a dual OTA, feedback loop
around the first stage, but no feedback loop around the second stage.

JH.


> Hello Ryan and all --
> 
> Several months ago I built a Sin shaper for the aux VCO in my chaos 
> box.  It incorporated the suggestion by JH to add negative feedback to 
> suppress the points at the peaks of the usual OTA shaper design.  I 
> adjusted both the amount of input drive and feedback empirically to obtain 
> the best looking and sounding waveform.
> 
> After Ryan's nice analysis of this method, I had another look at my 
> circuit, measuring the direct and feedback inputs to the OTA.  I found that 
> these inputs were almost exactly the same as Ryan found in his analysis: 
> 0.26 V from the direct input and 0.26 * 0.47 = 0.12 V from the 
> feedback.  Note that this is fairly heavy feedback (~ 32%).
> 
> The circuit and its spectrum are now up at my site:
> 
> http://home.earthlink.net/~ijfritz/xfer.htm
> 
> These probably won't be up for long, so please grab the schematic now if it 
> is of interest!
> 
>    Ian 
> 
> 
> 




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