Radio Frequency Integrated Circuit Design - Webs

Power Amplifiers
Solution
The first thing to note is that the current extremes I max and I min will still be
approximately the same as they were for the class A amplifier even though
current will flow for a smaller percentage of the time. Examination of Figure
10.14 shows that the output fundamental current will be roughly constant for
any conduction angle between 180° and 360°. Thus, it would seem that it
should be possible to reduce the nominal current through this amplifier and
achieve roughly the same output power by driving the amplifier into compression.
In practice, with reduced bias, the achievable output power is reduced, as
shown by simulation results summarized in Table 10.2.
It can be seen that by reducing the bias current to 105 mA, approximately
the same results are obtained as for the class A amplifier. However, there are
a few important differences. Because the amplifier is driven into compression,
the efficiency is now 57.2% and the amplifier is now nonlinear. If instead the
same amplifier is used as in Example 10.1, a bias current of 147 mA, output
power is increased to 24.1 dBm and efficiency is increased to 58.9%. Timedomain
waveforms for this case can be seen in Figure 10.21. It can be seen
that waveforms do not match simple theory in that voltage is not sinusoidal
and current goes negative due to transistor capacitance. By considering the
positive portion of the current, the conduction angle for a 14-dBm input is
estimated to be about 260° (� = 130°), and from Figure 10.11, efficiency is
expected to be about 60%, which is close to the simulated value.
10.6 Class D Amplifiers
Two examples of class D amplifiers are shown in Figure 10.22. The two
transistors alternately switch the output to ground or to VCC . The output filter,
consisting of L o and C o , is tuned to the fundamental frequency. This serves
to remove the dc component and the harmonics, resulting in a sine wave at
the output. While class D amplifiers can have high efficiency and have been
Table 10.2
Simulation Results for Class AB Power Amplifier Example
P in
I bias Opt PAE (opt PAE) P out Compression
(mA) (%) (dBm) (opt PAE) (dB) � opt
63 55.8 12 20.7 2.5 11.5 + j 18.7
105 57.2 13 22.6 3.2 12.2 + j 14.3
147 58.9 14 24.1 3.7 13.1 + j 10.3
367