tection for the body current and the collector current.
In most systems, the collector and the body operate at
nearly the same potential. Any potential difference is
usually only the difference in voltage drop across the
various metering circuits.
MULTICAVITY POWER KLYSTRON AM-
PLIFIERS. The simple, two-cavity power klystron
amplifier is not capable of high-gain or high-power
output, or suitable efficiency. However, with the
addition of intermediate cavities and other physical
modifications, the basic two-cavity klystron may be
converted into a multicavity power klystron, capable
of both high-gain and high-power output.
In addition to the intermediate cavities, there are
several physical differences between the basic two-
cavity klystron and the multicavity klystron. The
cathode of the multicavity power klystron must be
larger to be capable of emitting large numbers of elec-
trons. The shape of the cathode is usually concave,
which aids in focusing the electron beam. The col-
lector must also be larger to allow for greater heat
dissipation. In a high-power klystron, the electron
beam may strike the collector with sufficient energy
to cause the emission of X-rays from the collector.
Many klystrons have a lead shield around the col-
lector as protection against X-rays. Most high-power
klystrons are liquid-cooled and must be constructed to
facilitate cooling the collector.
Klystron amplifies have as many as seven cavities,
including five intermediate cavities. The intermediate
cavities improve the bunching process, resulting in
increased efficiency. Adding more intermediate cavi-
ties is roughly analogous to adding more stages to an
IF amplifier; that is, the overall amplifier gain is in-
creased and the overall bandwidth is reducedif all
the stages are tuned to the same frequency.
The same effect occurs with klystron amplifier
tuning. A given klystron amplifier tube will deliver
high gain and narrow bandwidth if all the cavities are
tuned to the same frequency-this is called synchron-
ous tuning. If the cavities are tuned to slightly dif-
ferent frequencies, the gain of the klystron amplifier
will be reduced and the bandwidth may be appre-
ciably increasedthis is called asynchronous tuning.
Most klystron amplifiers that feature relatively wide
bandwidths are stagger-tuned.
The klystron is not a perfectly linear amplifier;
that is, the RF power output is not linearly related to
the RF power input at all operating levels. In other
words, the klystron amplifier will saturate, just as a
triode amplifier will limit if the input signal becomes
too large. In fact, if the RF input is increased to levels
above saturation, the RF power output will actually
To better understand the reason for this decrease,
remember that electron bunches are formed by the
action of the RF voltage across the input cavity gap.
This RF voltage accelerates some electrons and slows
down other electrons, resulting in the formation of
bunches in the drift tube region. Obviously, this
speeding up and slowing down effect is increased as
the RF drive power is increased.
The saturation point is reached when the bunches
are perfectly formed at the instant they reach the out-
put cavity gap. This results in the maximum power
output condition. When the RF input is increased
beyond this point, the bunches are perfectly formed
before they reach the output gap; that is, they form too
soon. By the time the bunches have reached the output
gap, they tend to debunch because of the mutual
repulsion of electrons and because the faster electrons
have overtaken and passed the slower electrons. This
causes the output power to decrease.
FOCUSING KLYSTRON AMPLIFIERS.
One very important item that is required for high-
power klystron amplifier operation is an axial
magnetic field (a magnetic field parallel to the axis of
the klystron). In klystron amplifiers, which are physi-
cally long, it is difficult to keep the electron beam
properly formed during its travel through the RF
section. The mutual repulsion between electrons
causes the beam to spread in a direction perpendicular
to the axis of the tube. If this is allowed to occur, elec-
trons will strike the drift tube and be collected there,
rather than passing through the drift tube to the