Paul Burridge wrote:
On Wed, 10 Sep 2003 10:25:49 +0100, "Kevin Aylward"
wrote:


That is, to achieve linear multiplication by this transistor gm
method, it necessarily requires a non-linear relation between I and
V for the transistors gm characteristics. This forms a proof of my
statement that the class A modulator achieves such modulation by a
non-linear process.

gwhite:

Please do not respond if you are going to past masses of waffle text
that you don't understand. Provide a *mathematical* *explicit*
disproof of my mathematics or present your retraction.

Well this is compelling stuff, I must say. I'd hoped to be able to
disappear on holiday for a week and return refreshed, but it looks
like I'm going to have to seek out cybercafes to keep up to date on
these exchanges instead. :-( Still no outright winner so far...

Not to me aint. The case is closed.

The problem with gwhite, is that his arguments are all based on "an
appeal to authority", and we all know that is not the way science is
done. He pastes reams of stuff without the slightest idea of what the
documents are talking about. The assumption being that such documents
back him up. They don't. He hasn't presented one, not even one
derivation of his claim, only end results. This is typical of all
vacuous claims. It says so in the bible, so it must be true sort of
thing.

On the otherhand, he does have a very valid point that one, could in
principle, make a modulator that does not depend on an inherent
non-linearity, he just happened to pick the wrong examples, and the
wrong person to debate with. He obviously learnt the basic concept from
a coarse he took, but never understood
enough to know when and how to apply it. For the active bipolar or fet
case, the gain setting is gm based, and this gm is electrically
controlled by the value of its
own current, hence, as a I proved, must have a non-linear V/I curve.
However, for example, in a passive case, things are different. If one
used a fet as a passive voltage controlled resister, the resistance is a
function of the gate source voltage, but the control voltage is not
connected accross the controlled resistance, and the resistance
variation is not implied to be a function of its own current, therefore
a non-linear resistance is not implied. Indeed, this technique is used
in guitar phaser pedals to produce a swept notch filter, and I designed
and built my
first one of those around 25 years ago, based on this concept. To make
the fet even more linear, as qwhite correctly suggested, I used, as is
well known standard practice, a series gate resister and feedback
resistor from drain to gate. Unfortunately, I made a mistake...in my
"there is only a world market for 5 computers" statement, I forgot what
I already knew:-)

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.