[sdiy] MOSFET ANGST

[Scott Gravenhorst]

I have started doing a few experiments, simple stuff just to see if what is
supposed to happen actually does.  So I built a little voltage controlled
attenuator.  It sort of works...  read on.

Please see http://home1.gte.net/res0658s/MOSFET_test.gif

In all cases, the AC input is a triangle waveform from a signal generator.

I started with "A" and noted what I would call unkindly behavior.  At zero
gate volts, the output is "full scale" and looks like a proper triangle
waveform.  However, as I increased the gate voltage, generally, the
amplitude of the output was attenuated as expected, but there is
assymetrical distortion.  I looks like as the gate voltage increases, the
negative portion starts to soft clip _first_, then after the voltage
increases more, the positive portion begins to drop in amplitude, but no
soft clipping occurs!

Ok, so I thought perhaps I needed to do experiment "B", which uses both the
P and the N transistors.  Guess what...  No difference, meaning that as the
gate voltage increases, there is attenuation, but the signal is still
asymmetrically distorted.  I also tried reversing the drain and source
connections of the P transistor - no difference.

WARNING: THIS MAY PROBABLY BE POORLY WORDED, NON-EE LANGUAGE.
I got suspicious that the P transistor wasn't doing anything because it
appears to me to be reverse biased, i.e., in normal inverter use, it's
source is more positive than it's drain and the more negative the gate is
with respect to it's source, the less it conducts.  It looks like both the
N and P MOSFETs are depletion mode, (I can't find this in the datasheet)
but the fact that they are opposite types (N vs P) means they work opposite
and make a nice inverter with 2 transistors.

Anyway, I did experiment "C" and found no change in output at all as gate
voltage increases.  As in nothing happens when I turn the knob.  To my way
of seeing it, this means that the idea of folding a 4069UB (i.e.,
connecting pin 14 to pin 7) has no advantage because the P transistors
won't do anything for us.

I'm looking for pointers, explanations, etc., like "HEY SCOTT, YOU
BONEHEAD, YOU NEED TO CONNECT THE FRAMUS TO THE PHLORBIS THROUGH A
HYPERSPACE MULTIZORCH!".

Is this kind of asymmetrical distortion expected?  If not, how the héll do
I fix it?  I thought that using such a low input would be wonderful...  Ok,
I tried reducing the input voltage to 40 millivolts, it looks better, but
still asymmetrical.

Help???




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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.