VC envelopes

[Scott Gravenhorst]

On Fri, 5 Jun 1998 09:05:05 +0200 (MET DST), Martin Czech
<martin.czech at intermetall.de> wrote:

>> Consider a dual OTA such as the LM13600.  OTA #1 is configured as a normal
>> VCA.  OTA #2 is gain controlled by the same signal as OTA #1, but it's
>> input signal is a constant voltage provided by a reference source and a
>> trimmer.  Now the CV feedthru should appear on both OTAs, so couldn't the
>> output of OTA #2 simply be applied to OTA #1 at the inverting input?
>> Would that not cancel most of the CV feedthru?  These parts are rather
>
>If you look at the harris (ca3280 ?) dual ota data sheet (or app note) 
>you'll see this idea applied for a sine shaper. 
>
>But: the corrective current is added at the end of the circuit, ie. the
>current to voltage converter (in this case a simple R).
>I have the feeling that the error will cancel better this way.
DOH! Yes.  But at least the error voltage could be produced in this
manner.  The last stage of the VCA would then be a voltage follower
op-amp with OTA #1 feeding the non-invert side and the error voltage
feeding the invert side.

-- Scott Gravenhorst
-- FatMan Site: www.teklab.com/~chordman