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254 Linear Amplifier Design Tools
"" plane. However, a generalized reflection coefficient normalized to (7.79)
will produce a concentric family of constant-efficiency circles when projected
on the different map of the load admittance plane.
7.3.5. Maximum Added Power. Kotzebue (1976, 1979) has described a
method for maximizing the two-port added power for a fixed value of the
input port independent variable, e.g.• VI' It has been observed that highfrequency,
bipolar junction transistors tend to saturate at their input, while no
such clipping is observable at their output. The assumption of constant VI in
the preceding development enables the extension of the linear design approach
to some nonlinear cases, where the so-called large-signal parameters are more
appropriate. Kotzebue also argues in favor of the maximum added power
approach for the common situation where K < I, and the transistor is potentially
unstable.
The added power when V I is constant is shown as h in Figure 7.14. The
location of the maximum added power was given by (7.60). Figure 7.13, (7.46),
and (7.54) enable the expression of the "22 reflection coefficient at the point of
the maximum added power:
(7.80)
The corresponding power delivered to the load is available using (3.47) and
(7.46):
P _ IY211 2 -lyd 2
L-
4g 22
(7.81 )
Solving (7.48) for YL and using (7.80), the load admittance that produces the
maximum added power is
Y = _ + 2g"Y21
L Y22 Y +y' .
21 12
(7.82)
It is interesting that as the reverse parameter Y12~O, Y L---;loyi2'
Kotzebue calls the efficiency when the added power is maximized the
"maximally efficient gain." Its expression requires the input power, which is
simply Gin for V, = I volt. The input admittance for YL in (7.82) is obtained
by using (7.49):
Then its real part yields the input power
Y2lY 12 + Iyd 2
2g 22
(7.83)
K\Y2IYu!-\yd 2
2g 22
(7.84)