Radio Frequency Integrated Circuit Design - Webs

356 Radio Frequency Integrated Circuit Design
Table 10.1
PA Class A, AB, B, C Conduction Angle and Efficiency
Conduction Efficiency Ouput Power
Class Angle (�) (max theoretical) (%) (normalized)
A 360 50 1
AB 360–180 50–78.5 Nearly constant about 1
(theory: max of 1.15 at 240°)
B 180 78.5 1
C 180–0 78.5–100 1 at 180°, 0at0°
To obtain high efficiency, power loss in the transistor must be minimized,
and this means that current should be minimum while voltage is high, and
voltage should be minimum while current is high. It can be seen in Figures
10.7 and 10.8 that for all waveforms, maximum voltage is aligned with minimum
current, and maximum current is aligned with minimum voltage. It can further
be seen that for class B and class C, the current is set to zero for part of the
cycle where the voltage is high. This leads to increased efficiency; however,
there will still be some loss, since there is an overlap of nonzero voltage and
current. Other classes of amplifiers, to be described in later sections, namely,
classes D, E, F, and S, are designed such that the voltage across the transistor
is also nonlinear, leading to higher efficiencies, in some cases up to 100%. A
different way to improve efficiency, while potentially maintaining linearity, is
to power a linear amplifier from a variable or switched power supply. This is
the basis for class G and H designs. All of the above amplifiers will be discussed
in more detail in Sections 10.6 to 10.9.
Figure 10.9 shows a simplified power amplifier and a plot of transistor
current versus time for the various classes. The different classes can be obtained
with the same circuit by adjusting the input bias circuit. For example, in class
A, if the maximum current is Imax, the amplifier is set to have a nominal bias
of half of Imax so that current swings from nearly 0 to Imax. For class B, the
bias is set so that the transistor is nominally at the edge of conduction so that
positive input swing will cause the transistor to conduct, while negative input
will guarantee the transistor is off. Thus, the transistor will conduct half the
time.
10.5.1 Class A, B, and C Analysis
Except for class A, the current through the transistor is not sinusoidal, but may
be modeled as a biased sinusoid as shown in Figure 10.10.
The collector current can be expressed as