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252 Linear Amplifier Design Tools
The negative input susceptance B M , was defined in (7.58). Given a network
characterized by short-circuit y or other parameters, it is important to realize
that the efficiency is a function of only the load and not the source.
Example 7.7.
Consider the transistor y parameters
y,,=13.008E-3 /29.46°, YI2=1.4000E-3 /-61.26°,
Yz,=34.4637E-3 /-90.26°, Yn=5.0772E-3 /86.31°.
(7.66)
What are the stability factor and maximum possible efficiency? From (7.57),
K=1.03253. From (7.64), T)m,,=19.088, or a 12.81-dB gain. Since K>l, the
transistor is stable for all possible right-half-plane loads (see also Example 7.10
in Section 7.4.3).
7.3.4. Conjugate Terminations. Roberts (1946) developed the concept of
conjugate-image impedances. This is the condition in which a linear two-port
network is conjugately matched at both ports. The development is worthwhile,
because it will be shown that the load impedance thus defined results in
maximum efficiency. Then, if the generator impedance is selected as the
conjugate of the corresponding input impedance, the maximum efficiency is
also the maximum transducer gain, i.e., maximum PL/P as '
Referring to Figure 7.15, if
then (7.49) yields
(7.67)
Y2lY 12 = (Y12 +YMc)(Y" - YM,)· (7.68)
Note that Y q
may be expressed in terms of the source admittance by using
(7.49) with subscripts I and 2 interchanged. Thus
Y2lY'2= (Yll + YM,)(Yn - YMc)· (7.69)
The last two expressions for Y2IY'2 may be equated. The real parts yield
G M , = GML =/J
gil g22 f'
(7.70)
Linear network
Z,Y,orS
parameters
2
"
+
v,
Y,
YMl- '" GML + jBML