Page 157 - Complete Wireless Design

Amplifier Design

156 Chapter Three

impedances of both of the transistors as a complex R ± jX value, and we must
form a matching network that forces the driver transistor’s output impedance
to match the load resistance (R ) value as demanded for maximum power out-
L
put from the driver at its V . In other words, the PA’s input now appears to
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the driver stage to be the R that it must be in order to be able to output the
L
required power as needed by the PA stage. This must, as well, match the resis-
tive input to the PA and absorb or resonate out any ±jX part for a conjugate
match.
Matching networks for power amplifiers should normally consist of the T
type, rather than the pi type. Pi-type matching networks for high-powered
amplifiers sometimes result in unrealistic component values at the higher
operating frequencies encountered today into a 50-ohm load; T networks are
capable of much higher frequency operation before this becomes a major prob-
lem. Both T and pi networks can be used, however, if the output impedance of
the transistor is higher than its load, or the power output of the amplifier is
under 15 W.
To begin the design of a power amplifier, follow these steps for power tran-
sistor impedance matching:
1. Look through the transistor’s data sheet for the output power versus input
power graph (an output power versus frequency graph is similar) to find out
how much input power is needed to drive the amplifier for a specific output
power, and at the desired frequency of operation. As necessary, apply the
common formula to see the gain in dB:

P OUT
dB 10 log
P
IN
2. Search the data sheet for the series equivalent impedance on a Smith chart
or in a tabular chart to obtain the transistor’s series Z and Z (Z ) at
IN OUT OL
the frequency, power, and V of interest (Fig. 3.52).
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3. Now follow the same matching network design procedures as for small-sig-
nal amplifiers to obtain a conjugate match.
4. If a wideband power amplifier is required, then frequency-flatten as dis-
cussed at the end of this chapter.
5. Complete the bias network design for a Class C amplifier as presented in
Sec. 3.3, “Amplifier Biasing.”

Stability, tests, and cures. Class C power amplifiers must remain stable under
any load or V , since instability can “smoke” the transistor because of
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increased collector currents and high voltages. One way to test stability is to
decrease V to a quarter of its typical value while, with the output placed into
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its characteristic impedance, varying the input drive level. If the amplifier
remains stable, then there is a very good likelihood that it will not oscillate

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