"Walter Maxwell" wrote in
:
"Owen Duffy" wrote in message
...
(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.
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?
Owen
Owen, 'linear transfer characteristic' isn't the only context for the
use of the word 'linear'. Even though the input circuit of a Class C
amplifier is non-linear, the output is linear due to the energy
storage of the tank circuit that isolates the input from the output,
therefore, the output is linear. Proof of this is that the output
signal is a sine wave. In addition, the voltage and current at the
output terminals of the pi-network are in phase. Furthermore, the
ratio E/I = R appearing at the network output indicates that the
output source resistance R is non-dissipative, because a ratio cannot
dissipate power. This resistance R is not a resistor.
Hi Walt,
A few issues....
Yes, I understand the context in which you mean linear (though I have
issues with your proposition)... but my comment was referring to the
assertion that 'no harmonics' relates to linear operation which seems to
me to refer to the transfer characteristic linearity context.
I do have issue with your stated 'proof'. Firstly, I must qualify that we
are talking steady state... the mention of resonant loads means we are in
the frequency domain. Whilst it might seem that the tank circuit / pi
coupler / whatever is just a network of passive parts and they are all
linear, the energy that is supplied to that circuit in each cycle depends
on the resonant load impedance and traditional PA design methods suggest
that that Eout/Iout relationship is not linear for changes in load Z,
although it might be approximately linear over a small range.
I recognise a distinction between resistance (the ratio of E/I) and a
resistor (one type of component that exhibits resistance)... but I would
not claim that resistance is just a 'ratio' because it implies it is a
dimensionless ratio.
Owen