There is no conjugate match to the output of any active source, whether
vacuum tube, or bipolar transistor, or FET. Maybe there is in traveling
wave devices or modulated wave devices such as TWT and klystrons, but
the control element controlled active device as in triode, tetrode,
pentode, bipolar transistor and FET does NOT conjugately match the load
because each of these devices is a time varying impedance. Bruene is
right and has been involved in the creation of much RF power. All those
that demand conjugate matching through to the active device are wrong.
The concept of conjugate match is handy for those working without
complete understanding of amplifiers so its a useful tool (Bruene won't
agree with that). Its given the more maddening appearance of there being
a conjugate match because of the way RF solid state devices are
measured. Commonly they are installed in a test jig, and the universal
matching network is adjusted for maximum power output (a side effect on
conjugate matching of none time varying components) and then the test
jig is split and the Z shown to the active device is measured and its
conjugate declared the output Z of the device. It is NOT. The impedance
measured is the load the allows the active device to produce maximum
power (within distortion and efficiency and gain limits). For circuit
designers, it is convenient to say the output impedance of the device is
r + jx and the load then must r - jx to be a conjugate match but that's
an improper statement of fact. Its plain wrong, but it makes circuit
designers produce output networks that supply the appropriate load for
the device.
If the load were the conjugate match for the active device, then the
load could be easily derived from the output of the active device but
that's not the case. During any one cycle of RF the output Z of the
active device varies from open to nearly shorted and that's nothing that
can be matched by a constant impedance. Any time the active device is a
constant impedance, its not producing any output, its output Z MUST VARY
to produce output.
In the simple case the active device is a switch, with two conditions,
open and shorted. Neither is a matchable condition, nor a desirable
condition because a zero impedance condition connected to the power
supply demands an unreasonably large power. But if the load Z is set for
the available voltage swing to produce the desired output power, the
power supply is saved and the power output is controlled.
73, Jerry, K0CQ
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