Radio Frequency Integrated Circuit Design - Webs

Power Amplifiers
10.8.1 Variation on Class F: Second-Harmonic Peaking
A second resonator allows the introduction of a second-harmonic voltage into
the collector voltage waveform, producing an approximation of a half sinusoid,
as seen in Figure 10.31. It can be shown that the amplitude of the secondharmonic
voltage should be a quarter of the fundamental.
It can be shown that the peak output voltage is given by
and the efficiency is given by
Vom = 4
3 V CC
379
(10.36)
� = 8
� ≈ 84.9% (10.37)
3
10.8.2 Variation on Class F: Quarter-Wave Transmission Line
A class F amplifier can also be built with a quarter-wave transmission line as
shown in Figure 10.32 with waveforms shown in Figure 10.33.
A quarter-wavelength transmission line transforms an open circuit into a
short circuit and a short circuit into an open circuit. At the center frequency,
the tuned circuit (L o and C o ) is an open circuit, but at all other frequencies,
the impedance is close to zero. Thus, at the fundamental frequency the impedance
into the transmission line is R L . At even harmonics, the quarter-wave transmission
line leaves the short circuit as a short circuit. At odd harmonics, the short
circuit is transformed into an open circuit. This is equivalent to having a
resonator at all odd harmonics, with the result that the collector voltage waveform
is a square wave (assuming that the odd harmonics are at the right levels).
The collector current consists of the fundamental component (due to the
load resistor) and all even harmonics. We note that there are no odd harmonics,
Figure 10.31 Second-harmonic peaking waveforms.