Introduction to Audio Power Amplifier
In this tutorial we shall study vacuum-tube amplifier
circuits which give a large output voltage of substantially the same waveform
as that of the input voltage, accompanied by a large curr ent of like waveform,
so that there is considerable power delivered to the load, rfhe operation
really is a power-conversion process, as in most cases all of the output power
derives from the plate d-c source, under the control of the signal voltage
applied to the grid. In general the treatment can often be made similar to that
used for voltage amplifiers, except that usually the operation is not linear
and therefore graphical or experimental procedures give most
accurate results.
Except for
details, a power tube resembles a voltage-amplifier tube. Power tubes vary in
size depending upon the amount of output power desired and the efficiency of
operation. For a small amount of output power a voltage tube may be used. The
tubes with the higher power ratings need larger anodes to dissipate by
radiation the heat derived from the energy of the impinging electrons, without
making too high a temp erature necessary. For tubes of very high power rating
the heat may be carried away by convection by an air or a water stream. A
second requirement for a power tube is a sufficiently large cathode to furnish
the necessary electron emission. This also means that a relatively large amount
of heating power will be needed. Small power tubes use oxide- coated cathodes,
while those of medium power employ thoriated tungsten, which will withstand
somewhat more ionic bombardment. High-power tubes with their very high voltage
drops use pure-tungsten filamentary cathodes. Tubes of small and moderate ratings
may be triodes, tetrodes, or pentodes, but for the very high power ratings
there is a problem of cooli ng the screen grid of the latter two kinds, which
makes the triode usually preferred.
In making a choice of tube, the class of operation, which is related to
the efficiency, must be taken into account. If the very best quality of output
is required, as in some measurements circuits, it is well to use class A1
operation as this renders the output most free of distortion. In this
connection it may also be advisable to choose a tube of higher power rating
than actually required, so that the grid drive may be made small compared with the maximum allowable and thus decrease
the distortion.
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