[sdiy] current conveyor?

[David G Dixon]

Thanks to all for your suggestions.  Actually, in this particular case, I
figured out a very simple and elegant solution to my problem.  Through the
glories of feedback (around quite a large circuit, not just a single opamp),
I realized that I could just level-shift the output of my integrator, and
the integrator output itself would shift upward to compensate, while the
ultimate output of my VCO would stay perfectly centred.  In this way I could
reset my triangle integrator to ground (i.e., normally) and it looks like
resetting to -5V at the triangle output.  So now I've got hard sync in my
triangle core VCO, the 2164 expo is still terminating to virtual ground, and
all it took was one more opamp.

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of ASSI
> Sent: Monday, February 06, 2012 10:23 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] current conveyor?
> 
> On Monday 06 February 2012, 12:47:39, David G Dixon wrote:
> > The current needs to be able to flow in both directions.
> 
> Some sleep and a coffee later... you do need an active 
> current source rather than a passive level shift to have the 
> current flowing back, of course.
> 
> An OTA with linearizing diodes will do, although the biasing 
> is a bit tricky and I don't know how good the precision is at 
> low currents and you will probably need an offset trim.  In 
> essence, the output of the OTA goes to the -5V integrator 
> input; both Iabc and Idiode need to be the same current (you 
> need a mirror there since one will be sitting at +0.7V and 
> the other 1.4V above the negative supply), slightly larger 
> than the maximum current you're going to use; the negative 
> input goes to ground (here's where you add the buffered 
> offset trim, since it needs to be low-impedance); and the 
> positive connects to the output of the 2164.  It may be 
> advantageous to rig the supply of the OTA so that the 
> mid-supply becomes close to -5V (requires a 
> +-15V system w/ additional 5V rail due to head room requirements, 
> +probably
> not worth the trouble if you only have +-12V).
> 
> It may be easier to level shift all the inputs to the 2164 
> down by 5V with some zero-offset, zero drift opamp.  I assume 
> you do not want to change the integrator to have its output 
> symmetric about zero?
> 
> 
> Regards,
> Achim.
> -- 
> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
> 
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