04 Tube Operation.pdf - Kambing UI

3.13.2 Correction of Parasitic Oscillations
The usual self-oscillations in r-f power amplifiers have been found
to fall in the following three classes:
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(a) Oscillation at VHF from about 40 MHz to 200 MHz,
regardless of the normal frequency of the amplifier.
(b) Self-oscillation on the fundamental frequency of the
amplifier.
(c) Oscillation at a low radio frequency below the normal
frequency of the amplifier.
The low frequency oscillation in an amplifier usually involves the r-f
chokes, especially when chokes are used in both the output and
input circuits.
Oscillation near the fundamental frequency involves the normal
resonant circuits, and brings up the question of neutralizing
the r-f power amplifier. This general subject is discussed under
“Neutralization,” Section (5).
When a parasitic self-oscillation is found on a very high frequency,
the interconnecting leads of the tube, the tuning capacitor and
the by-pass capacitors are involved. This type of oscillation
does not usually occur when the power amplifier is designed for
operation in the VHF region and where the r-f circuits external to
the tube have negligibly small tuning capacitors. Without tuning
capacitors, the highest frequency oscillating circuit possible is then
the fundamental, and there would be no higher frequency circuit
available for the parasitic. The only exception would be where
higher order modes of transmission line circuits might provide a
parasitic circuit.
The VHF oscillation occurs commonly in amplifierconstructions
where the radio frequency circuits are coils and capacitors, as in
the HF and LF region, in audio amplifiers or voltage regulators.
As in Figure 41, the parasitic oscillation uses the capacitors and
the associated grid and anode leads for the inductances of the
parasitic circuit. The tube capacitances help form the tuned-anode
tuned-grid oscillation circuits. The circuit is indicated by the heavy
lines in Figure 41.