Page 102 - Schaum's Outline of Theory and Problems of Electric Circuits

91
AMPLIFIERS AND OPERATIONAL AMPLIFIER CIRCUITS
CHAP. 5]
5.16 (a) Find the The ´ venin equivalent of the circuit to the left of nodes A-B in Fig. 5-39(a) and then
find v 2 for R l ¼ 1k
,10 k
,and 1. (b) Repeat for Fig. 5-39(c) and compare with part (a).
(a) The The ´ venin equivalent of the circuit in Fig. 5-39(a) is shown in Fig. 5-39(b).

Fig. 5-39

6 ð3Þð6Þ
v Th ¼ ð15Þ¼ 10 V and R Th ¼ ¼ 2k
6 þ 3 3 þ 6
By dividing v Th between R Th and R l we get
R l
v 2 ¼ ð10Þ
R l þ 2
For R l ¼ 1k
, v 2 ¼ 3:33 V
For R l ¼ 10 k
, v 2 ¼ 8:33 V
For R l ¼1 v 2 ¼ 10 V
The output v 2 depends on R l . The operation of the voltage divider is also affected by R l .
(b) The The ´ venin equivalent of the circuit in Fig. 5-39(c) is shown in Fig. 5-12(d). Here we have

v Th ¼ 10 V and R Th ¼ 0
and v 2 ¼ v Th ¼ 10 V for all values of R l , that is, the output v 2 depends on R 1 , R 2 , and v s only and is
independent of R l .