gwhite wrote:
Kevin Aylward wrote:

gwhite wrote:
Kevin Aylward wrote:


It can be done with class-A or class-B since the assumption is
that the amp will be driven (or nearly so) to the rails by the
carrier alone.

I agree, this is another method of generating x-product
multiplication terms. However, arguable, a class A amplifier is not
really a class A amplifier if it is driven to saturation. Its a
really a switching amp or, a pulse amplitude modulator if its rails
are varying.


Here's a "class-A" amplifier that can be amplitude modulated but yet
not saturated (assumes a constant load R):

Ho hummm...


V+
|
LC Tank
| AM RF_out
+--||--O
Carrier |
RF_in c
O--||---b Class A biased (no base bias details)
e
|
|
RF_Choke
Audio |
in c
O--||-- b Class A modulator (no base bias details)
e
|
GND


This is a single ended amplitude modulator. The top transistor could
be driven to the switch mode by the carrier, but this is not
necessary to produce AM. Practically, it will be driven to the
switch mode for efficiency reasons.

And you think that this is me to me? I suppose you aint read many of my
10,000+ posts.

Err..you've missed the cap from the top transistor emitter to ground. If
the bottom transistor circuit was a true current source at rf, the top
transistor could not effect the output current at all.

Oh, and its not very linear with required to audio input signal without
an emitter resister anyway. The distortion of an basic tranister amp for
2nd Harmonic = Vi(mv)%, that is 10 mv will get you 10% distortion.


Amplitude modulation can be "made" via linear methods.

Nope. Not a chance.

"Multipliers"
cannot be generally stated to be either linear or non-linear.

If one input of a multiplier is held constant, the other input has a
linear response. If the other input is a function of time, the response
to the first input is non-linear. That is, it dose *not* satisfy a(f(t))
= f(at).

The above circuit relies on the fact that the "re" of the top
transistor, is a function of its emitter current set by the bottom
transistor, via the equation re=1/40.Ic. e.g. see
http://www.anasoft.co.uk/EE/index.html

A system which includes a multiplier must be put through the linearity
test to see if the configuration is linear or non-linear. IOW, it
can be either.

Ho humm again. You confuse where the term linearity is to be applied.


+------+
x(t) O---| h(t) |---O y(t)
+------+


linearity:

a·x1(t) = a·y1(t)
b·x2(t) = b·y2(t)

if x(t) = a·x1(t) + b·x2(t)

then

y(t) = a·y1(t) + b·y2(t)

Linearity can more easily be expressed as:

a(f(t)) = f(at)


If that is true, then the system is linear. This can be true for
systems with multipliers.

Nope. A signal being acted on by a multiplier is a non-linear system if
the second input is non constant with time. Your way of base on this
one.



This system is linear and has a multiplier (it is not time invariant):


Nope, its not.

The System
+---------------+
| |
in | /¯¯¯\ | out
x(t) O--------( X )---------O y(t)
| \___/ |
| | |
| | |
| O |
| cos(w_c·t) |
+---------------+


It produces a DSB signal (y(t)). w_c·t could be "added in later"
(linearly) to y(t) in the proper amplitude and phase and the resultant
signal would for all practical purposes be indistiguishable from
standard AM.

This is not a linear circuit. You need to understand what linear means.
A linear system, cannot produce frequencies that are not in the input,
essentially, by definition. With all due respect, I would guess you
don't have an EE B.S. degree. This is all pretty basic stuff really.

No non-linear circuit was used but yet AM was produced.

Nonsense. Your pretty misguided on this. You can not achieve
multiplication without a non-linear circuit. For example, Gilbert
multipliers use the fact that Id=Is.exp(vd/Vt). That is it logs, adds
and antilog. Balanced switching mixers use switches. Fet mixers use
their square law response.

Not convenient, but it does dispel the "non-linearity is required"
myth.

Its not a myth. I know of no way whatsoever to generate an analogue
multiplication x product terms without having a device satisfying the
property of a.f(t) != f(at), i.e. a non-linear device. Please feel free
to suggest one, but file your patent first.

Oh, by the way...your trying to teach your granny to such eggs son.

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.