Page 153 - Complete Wireless Design

Amplifier Design

152 Chapter Three

representation, or in a much easier to read tabular format on the device’s data
sheet. If desired, the series impedance values can be converted to a shunt
impedance (as presented earlier in this chapter), if this information is not so
provided. When provided, they may be in the form of a separate graph of input
and output parallel equivalent resistance versus frequency, and a graph of
input and output parallel equivalent capacitance versus frequency, at a set V
CC
and P .
OUT
Designing Class C power amplifiers with small-signal S parameters will
result in a circuit that is not optimized and will not function as intended. This
is because any transistor’s input resistance, capacitance, gain, and output
resistance will be significantly different when the device is run as a large-sig-
nal power amplifier as opposed to a small-signal, Class A amplifier. However,
sometimes S parameters may be placed on power MOSFET data sheets, but
are only to be utilized to approximate a beginning design, which must then be
tweaked in software, and then hardware.
Impedance matching, especially in RF power amplifiers, is required so that
the transfer of energy to the next stage is accomplished with as little wasted
power as possible. Matching that increases the return loss (decreasing the
VSWR) of a system or amplifier prevents ripples in the passband of filters
from forming and permits the active device to perform as designed with a flat
gain, proper NF, low distortion, and high stability.
Since the input and output impedances of a power transistor are a complex
impedance (Fig. 3.52), and this input impedance can be at very low values
(which can necessitate an impedance transformation ratio of up to 20 times for
a BJT), it can be seen that proper matching of a power amplifier to its source
and load is anything but trivial. And the higher the desired output power of
the large-signal transistor, the lower will be its output impedance, which can
make matching difficult, especially at these high power levels. In fact, the
design of power amplifiers just a few short years ago involved much trial and
error in the tweaking of matching and bias networks in order to obtain an effi-
cient and workable amplifier that did not self-destruct and had viable compo-
nent values. To a lesser extent, this is still the case, but usually only in order
to fine-tune the amplifier for low VSWR, high gain, maximum efficiency, and
maximum output power because of real-world component tolerances as well as
the effects of stray reactances on the completed circuit.
The transistor’s input and output impedances will also decrease with an
increase in frequency, which further complicates the design of matching net-
works for high power amplifiers—especially since these impedances can go
1
down to ohm or less.
2
However, by choosing a transistor with a high collector voltage requirement,
we can increase its output impedance over a transistor that operates at a low-
er value of collector voltage, or:
V 2
C
Z
OUT 2P
OUT

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