"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 broad so as to
include the subject of linearity of the output of the tank circuit that permits
the use of theorems that require the output to be linear. Richard H's and my
posts were simply reminders that the energy storage in the tank circuit is the
reason for the linear relation between voltage and current at the output of both
Class B and C amplifiers that results in a sine wave. From that perspective I
believed our posts were legitimate to the thread topic. Apparently we were
wrong.
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.
This analogy has been around for decades--it's not my invention. IMHO, the
periodic energy spurts from the pistons entering the flywheel is precisely an
analog of the energy spurts of the periodic current pulses entering the tank
citcuit. Why do you not agree? Even the pendulum swing is appropriate, because
if you trace the position of the pendulum with respect to time you'll discover
the trace is a perfect sine wave, while the short spurt of energy supplied by
the spring at the beginning of each cycle is just sufficient to overcome the
energy dissipated due to friction at the axis plus the aerodynamic resistance.
How could this not be an appropriate analogy? Sorry to have forced you away from
the thread topic with questions not pertaining to the thread.
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.
Walt, W2DU