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1160 ⏐⏐⏐ SYSTEM ANALYSIS: AN INTRODUCTION
Recall an earlier conclusion that the larger the value of R L, the larger
the loaded voltage gain. For current levels, Equation (26.12) reveals that
the larger the level of R L, the less the current gain of a loaded
amplifier.
In the design of an amplifier, therefore, one must balance the desired
voltage gain with the current gain and the resulting ac output power
level.
For the system of Fig. 26.17, the power delivered to the load is
determined by E 2 o/R L, whereas the power delivered at the input terminals
is E 2 i/R i. The power gain is therefore defined by
A G � � � � � � 2
and AG � A (26.13)
2 Ri
v�� R
Expanding the conclusion,
A G � (A v) � A v � � (A v)(�A i)
so AG ��AvAi (26.14)
Don’t be concerned about the minus sign. A v or A i will be negative
to ensure that the power gain is positive, as obtained from Eq. (26.13).
If we substitute A v ��A iR L/R i [from Eq. (26.10)] into Eq. (26.14),
we will find
AG ��AvAi ��� � �Ai Ri
or AG � A (26.15)
2 i � RL
�
R
which has a format similar to that of Eq. (26.13), but now A G is given
in terms of the current gain of the system.
The last power gain to be defined is the following:
2
AGT ��� ��� 2 � � �
Pg
Eg/(R �Eg�� RL �
g � Ri) or AGT � A (26.16)
2 vT��R g �
R
Expanding:
P L
Po �
Pi
E 2 o/RL �
EgIg E 2 o/RL �2 Ei/Ri Ri �
RL
E Ri �
RL
2 o
E 2 o/R L
A GT � A vT� A v T � R g
�A iR L
Ri �� L
� Ri �
R �
and AGT ��AvTAiT (26.17)
� E 2 i
L
i
L
Eo �
Ei
E o
Ri �
RL
R g � R i