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Owen Duffy · June 13th 08, 10:39 PM posted to rec.radio.amateur.antenna

(Richard Harrison) wrote in
:

Owen Duffy wrote:
"The Class C amplifier is a switch...
If you say it enough times, will it become true?"

True is true no matter what anyone says. I`ve never seen Terman
misspeak.

On page 255 of his 1955 opus Terman wrote:
"---the Class C amplifier is adjusted so the plate current flows in
pulses that last less than half a cycle."

On page 450 he wrote:
"The high efficiency of the Class C amplifier is a result of the fact
that plate current is not allowed to flow except when the
instantaneous voltage drop across the tube is low; i.e. Eb supplies
energy to the amplifier only when he largest portion of the energy
will be absorbed by the tuned circuit."

Sounds like a switch to me. When switched on, voltage drop across the
tube is low.

Lets plug some real world numbers in...

Take a DC supply of 1000V, and a valve that saturates at 200V, the RF
approximately sinusoidal voltage swing on the anode is from 200V to
1800V. (For avoidance of doubt, whilst the RF voltage on the anode is
approximately sinusoidal, the anode current waveform is not.)

If the conduction angle is 120 degrees (typical for Class C amplifiers),
the valve starts conducting at about 1000-800*sin((180-120)/2) or 600V
instantaneous anode voltage... and continues conducting as the
instantaneous anode voltage passes through the minimum and rises again,
cutting off when then instantaneous anode voltage again reaches 600V. In
this case, the anode voltage during conduction varies between 400 and
600V, 40% to 60% of the supply voltage.

The switch analogy is not a good one.

When switched off, voltage drop across the tube is Eb, ...

That is wrong. The anode voltage is approximately a sinusoidal voltage
swing of almost (70% to 90%) Eb zero to peak superimposed on the DC
supply voltage (Eb).

You have taken a quote from Terman and weaved your own flawed extensions
(being the switch analogy and the statement about instantaneous anode
voltage).

Owen