[AMPS] Tuned Input, what me worry

[Peter Chadwick]

According to Pappenfus et al., the tank circuit plus the feed cable plus the
output network of the driver should be of such length as to make the driver for
a g-g stage appear as a constant voltage source. By making the total electrical
length a suitable multiple of 90 degrees, this can be achieved without the phase
distortion that would be introduced by a varying load on a non integral quarter
wave transmission line.

The exact length (even or odd multiple of 90 degrees) will depend on whether or
not the driver is a voltage or current source. Again, according to Pappenfus, a
tetrode with r-f voltage feedback  approaches a constant voltage source.

>From this viewpoint, then, the actual network doesn't appear to matter. 

In another part of the same reference, the cathode tank supplies power 'during
the negative half of the cathode r-f voltage swing'. It also says that if the
transmission line length between the output of the driver pi network and the
cathode is short (<1/20 wavelength) the output capacitor of the pi 'then
provides the low capacitive reactance from the amplifier cathode to ground for
the lower harmonic frequencies'. This agrees with Tom's point about using a
cathode tank that's a pi.

A staright forward pad will be good news for the driver, in that the input
impedance swing is limited, but I don't see how it helps maintain the sinusoidal
drive at the cathode. A parallel tuned circuit will do this, and provide the low
impedance to ground for the harmonics, although the values may be a bit large on
the lower bands - and the current ratings required in the capacitors. 

Of course, in all this, there's a big difference between that which is
theoretically the most perfect, that which is in practice totally acceptable,
and that which is a real no-no.
As the techniques that produce the results vary from design to design, some of
the 'tablets of stone' may prove at the end of the day to be rather
crumbly.........

73

Peter G3RZP





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