Radio Frequency Integrated Circuit Design - Webs

Power Amplifiers
added to the fundamental in order to produce a collector voltage more like a
square wave. This means the collector voltage is lower while current is flowing,
but higher while current is not flowing, so the overall efficiency is higher.
The typical waveforms for a class F amplifier are shown in Figure 10.28,
with the collector voltage having a squared appearance while the output voltage
is sinusoidal. Current only flows for half the time or less in order to ensure
there are third-harmonic components and to maximize the efficiency (zero
current while there is finite collector voltage).
The transistor behaves as a current source producing a half sinusoid of
current iC (� ), similar to class B operation. L o and C o make sure the output
is a sinusoid. The third-harmonic resonator (L 3, C 3) causes a third-harmonic
component in the collector voltage. At the correct amplitude and phase, this
third-harmonic component produces a flattening of vC as shown in Figure
10.29. This results in higher efficiency and higher output power.
If the amplitude of the fundamental component of the collector voltage
is Vcm and the amplitude of the third harmonic is V cm3, then it can be shown
that maximum flatness is obtained when Vcm3 = Vcm/9.
Thus, with
Vcm3 = Vcm
9
it can be seen from Figure 10.29 that the peak collector voltage is
Figure 10.28 Class F waveforms.
377
(10.32)