newbie diode/rectification ?s

[Don Tillman]

   Date: Mon, 18 May 1998 10:59:49 -0700
   From: dan mcmullen <dog at well.com>


   i had a notion over the weekend of using a diode in-line w/ my synth output
   to remove one half (< 0V) of the signal.  basically, i'm wanting to
   experiment w/ asymmetrical waveforms.  

   - the signal magnitude after the diode is initially connected is half that
   of the input as expected.  

Diodes aren't perfect rectifiers.  They have a 0.6 volt threshold to
overcome before they start to conduct in the forward direction.  So
what you end up with is a much larger fraction of the waveform being
removed than you expected, especially for lower signal levels.

There's not much you can do about this other than use a large enough
signal level so that it's not much of an issue.

The right thing is to avoid this approach and use a more exotic
rectifier circuit.  There's a reason folks go to the trouble of
building those! 

   gradually however, over a period of ~15 seconds,
   it *attenuates* down to less than 1/8 of the input. 

If the mixer or amp that you are connected to has a capacitor input
(which is very likely) you are going to end up charging that cap on
each conducting half cycle, and not discharging (or not discharging
nearly enough) on each nonconducting half cycle.  This builds up a
bias offset voltage where you didn't intend it, further snuffing out
your signal.

You can minimize this effect with a resistor to ground at the input of
your amp or mixer.  Maybe about 20k ohm.

   - diode orientation is significant.  

Right, the input impedance of the first stage of your mixer or amp is
going to react differently to a positive or negative bias buildup.

  -- Don