"Walter Maxwell" wrote in
:
"Jim Lux" wrote in message
...
Owen Duffy wrote:
(Richard Harrison) wrote in news:23000-
:
Jim Lux wrote:
"in a linear system"
It produces no significant harmonics, so the system is linear.
That is a new / unconventional definition of 'linear'.
The term is usually used in this context to mean a linear transfer
characteristic, ie PowerOut vs PowerIn is linear.
Or, as I used it, that superposition holds.
One can build an amplifier or other device where the Pout(Pin)
=straight line, but is not linear in the formal sense. Say you built
a widget that measured the input frequency and amplitude, then drove
a synthesizer at that frequency and amplitude = 2*input amplitude.
Considering a typical valve Class C RF amplifier with a resonant
load:
Conduction angle will typically be around 120°, and to achieve
that, the grid bias would be around twice the cutoff voltage.
If you attempted to pass a signal such as SSB though a Class C
amplifier that was biased to twice the cutoff value, there would be
no output signal when the peak input was less than about 50% max
drive voltage, or about 25% power, and for greater drive voltage
there would be output. How could such a transfer characteristic be
argued to be linear?
It would not be.You're right
The active device isn't linear.
neither is the whole assembly.
I think, though, that sometimes we take a more casual view of linear
(e.g. people talk about the linearity of a log detector.. referring
to the deviation from a Voltage out=dBm in straight line.)
And, some confusion about nonlinear devices in a building block that
is, by and large, linear (e.g. a power op amp with an AB2 output
stage and a fair amount of negative feedback) with some constraints
on frequency and amplitude.
Owen
Owen, I didn't realize that this thread was specific to 'linear
transfer characteristic'. I thought the thread topic was sufficiently
Richard stated "It produces no significant harmonics, so the system is
linear." It is that with which I disagree.
....
And Owen, I'm somewhat surprised that you don't agree with the
flywheel analogy with respect to the smoothing effect of the energy
storage in the tank circuit. ...
I have not disagreed with that in anything that I wrote.
...
I am also curious as to why the subject of 'linear transfer
characteristic' with respect to Class C amps was even considered,
because the Class C amp has always been known to have a distorted
output relative to its input. I would agree that the subject is
appropriate when considering Class AB and B amplifiers, but not C.
Because Richards statement quoted above (which must be about transfer
linearity) is being used to support your assertion that the PA is linear
in its terminal V/I response with changing load.
Walt, the thread has become muddled with helpers muddying the water. Your
proposition needs to be argued with a single logically developed sound
argument. Your Chapter 19 tries to do that.
I have already stated that (as yet?) I am unconvinced, and I make the
observation that I am not alone. I will work through resolving the
apparent inconsistencies in my own time and without the confusion of
whether or not harmonics exist or more correctly the extent to which they
exist, and what that might mean.
Owen