Sun Jan 9 09:51:38 EST 2011

Envelope: One pole variable filter using OTA.

Implement envelope generator using a full DC one-pole variable filter.

( Note that an envelope generator only needs a 1-quadrant multiplier
since both control voltage and output voltage do not cross zero, so
there might be a simpler way.  However, the 1-quadrant multiplier,
i.e. current-source biased commion emitter configuration has biasing
issues which are solved by the differential pair.  This then gives
2-quadrant operation for free.  Looks like the OTA (without
current-mirrir output) really is the simplest solution. )

A 1-pole lowpass OTA filter is a variation on the OTA integrator which
has current output into a capacitor.  The lowpass has additional
voltage feedback from the output to inverting input which makes
current into the capacitor depend on the voltage difference between
input and output, creating the typical low-pass response.

In the schematic this looks as if the (-) input transistor is replaced
by a diode.


  * If this is implemented with a non-linearized OTA, how important is
    the nonlinearity?  Quite.  For the envelope follower it limits the
    slew rate significantly.

    Seems the question is about how much of the wave shape will be
    effected by this.  A little bit of squashing on an exponential
    decaying curve isn't going to hurt much I think.  Maybe this can
    even be used as an effect?  To give more "punch" by driving a
    larger gate signal.

  * Offset: since we need true DC performance, offset needs to be
    compensated.  Otherwise the envelope won't go to zero, or will
    cross it.

The offset voltage needs to be taken care of.  This is one property
that makes the direct ramp->exp generater look interesting again.