Radio Frequency Integrated Circuit Design - Webs

Example 10.4 Class F Power Amplifier
Power Amplifiers
Design a class F amplifier with third-harmonic peaking to deliver 200 mW
from a 3-V supply.
Solution
The maximum output voltage can be determined.
VCC = 8
9 Vom ⇒ Vom = 9
8 VCC = 3.375V
The required output resistance is found as
V 2 om
2R
3.3752
= 0.2 ⇒ R = = 28.5�
2 � 0.2
The maximum collector voltage swing, Vmax = 2V CC = 6V
Peak output current is io,peak = Vom /R = 3.375/22 = 148 mA
Imax = 2 � io,peak = 237 mA � I dc = Imax/� = 0.237/� = 75.5 mA
Check powers, efficiencies ⇒ P dc = I dcVCC = 0.0755 � 3 = 0.2264W
� = Po 0.20
= ⇒ 88.4%
P dc 0.2264
Of course, in a real implementation, efficiency would be lower because
of losses due to saturation voltage, on resistance of the transistor, finite inductor
Q, imperfect RFC, and parasitics.
One useful application of a class F amplifier is as a driver for the class E
amplifier, as shown in Figure 10.34. A class E amplifier is ideally driven by a
square wave. Such a waveform is conveniently available on the collector of the
class F amplifier, so this node is used to drive the class E amplifier.
10.9 Class G and H Amplifiers
The class G amplifier shown in Figure 10.35 amplifier has been used mainly
for audio applications, although recently variations of this structure have been
used up to 1 MHz for signals with high peak-to-average ratios (high crest factor),
for example, in digital telephony applications.
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