[sdiy] MOSFET ANGST

[Scott Gravenhorst]

Many thanks.  (*slap*, I needed that!)

Well, dang, I knew this and STOOPIDLY didn't apply it (he says as the palm of
the hand smacks the forehead).  Looks like I need those silly resistors to do
the drain to gate feedback deal.  I will test that next.

So my other question is why do some applications such as hex voltage
controlled mixer using 4046UB (or 4049UB) connect pin 14 to pin 7?  Other
than static defense, I see no purpose in this because it appears that the P
transistors don't do anything.  In fact, AN105 shows a voltage controlled
attenuator with a single FET, yet it's input is AC.  Am I, therefore, not
hallucinating that nothing is changes when I connect the P transistor?

"Theo" <t.hogers at home.nl> wrote:
>Think Siliconix application note AN105 may interest you "FETs as VCR"
>I dump you a copy off list.
>
>HTH
>Theo
>
>----- Original Message -----
>From: Scott Gravenhorst <music.maker at gte.net>
>To: <synth-diy at dropmix.xs4all.nl>
>Sent: Sunday, July 18, 2004 2:19 AM
>Subject: [sdiy] MOSFET ANGST
>
>
>> 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???
>>
>>
>>
>>
>> ---------------------------------------------------------
>> - Where merit is not rewarded, excellence fades.
>> - Hydrogen is pointless without solar.
>> - What good are laws that only lawyers understand?
>> - The media's credibility should always be questioned.
>> - The only good terrorist is a dead terrorist.
>> - Governments do nothing well, save collect taxes.
>>
>> -- 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.
>>
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- 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.