Kevin Aylward wrote:

gwhite wrote:
Kevin Aylward wrote:

Don Pearce wrote:
On Mon, 01 Sep 2003 14:16:51 +0100, Paul Burridge
wrote:


Is a tuned load (tank circuit) a viable load for an RF amplifier
operating in class A? Or is this type of load only really suitable
for class C?

Of course you can use a tuned load with class A. But the nice thing
about a tuned load is that you don't *have* to use class A to
achieve a clean output. Of course, if you are using an amplitude
modulated signal, then you will need class A.

That's what receivers would use since power consumption doesn't matter
much. It would be true for low level TX'er stages too, again because
of power consumption. But for things like "Ham linears" it is not
necessarily true.

That is, if the input signal is already AM, you need a class A or
linear amplifier.

Let's be specific regarding the your word "linear" since "or" implies
something else other than class-A may be possible.

Yes.

Single-ended
"linear" narrow band amps may be biased class-B if the tank has a high
enough Q. The missing half cycle is restored by the so-called
"flywheel effect." In practice deep AB is also used.


If the AM is to be done at that stage itself, then its the
class c non-linear bit that makes the multiplication modulation work.

No.

No. Its Yes. Non-linear action generates the multiplication products. I
was not drawing any real distinction between class c and b in this
context. The practical difference is minimal. They both do not amplifier
the waveform in a linear manner. I was not meaning to infer that it was
an "only" c. I was referring to the fact that you need at least some
method that generates non linarity.

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):

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.

Amplitude modulation can be "made" via linear methods. "Multipliers"
cannot be generally stated to be either linear or non-linear. 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.

+------+
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)

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


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

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. No non-linear circuit was used but yet AM was produced.
Not convenient, but it does dispel the "non-linearity is required"
myth. Also, a multiplier can be viewed as a MISO system.