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808 ⏐⏐⏐ NETWORK THEOREMS (ac)
I1 = 0
+
Isc I R1 V = 0
–
�V �
– +
R 1 V E g
Z Th
FIG. 18.54
Determining Z Th using the approach Z Th �
E g / I g.
E Th = (1 + m)IR 1
+
–
(1 + m)R 1
FIG. 18.55
The Thévenin equivalent circuit for the
network of Fig. 18.50.
+
–
I sc
FIG. 18.53
Substituting V � 0 into the network of Fig.
18.52.
I g
I L
R L
with
Eoc (1 � m)IR1 ZTh ����� �(1 � m)R1 (correct)
Isc I
Method 3: See Fig. 18.54.
Eg � V � mV � (1 � m)V
or V �
V Eg and Ig ���� R1 (1 � m)R1
Eg and ZTh � �(1 � m)R1 (correct)
� Ig
E g
� 1 � m
The Thévenin equivalent circuit appears in Fig. 18.55, and
(1 � m)R1I IL ���
RL � (1 � m)R1 which compares with the result of Example 18.6.
The network of Fig. 18.56 is the basic configuration of the transistor
equivalent circuit applied most frequently today (although most texts in
electronics will use the circle rather than the diamond outline for the
source). Obviously, it is necessary to know its characteristics and to be
adept in its use. Note that there are both a controlled voltage and a controlled
current source, each controlled by variables in the configuration.
+
V i
–
I
R 1
+
k1V2 –
k 2 I
R 2
+
V 2
FIG. 18.56
Example 18.13: Transistor equivalent network.
–
Thévenin
EXAMPLE 18.13 Determine the Thévenin equivalent circuit for the
indicated terminals of the network of Fig. 18.56.
Solution: Apply the second method introduced in this section.
E Th
E oc � V 2
Vi � k1V2 Vi � k1Eoc I ��� ���
R1 R1
and Eoc ��k2IR2 ��k2R2��� R1 �
� � k �k2R2Vi 1k2R2Eoc ��
� R1
R1
V i � k 1E oc
Th