Electronic Circuit Analysis

28 Electronic Circuit Analysis
Example : 1.7
For the JFET amplifier circuit shown in Fig. 1.31, if gm = 2500 ~(j, and Vm = 5 m V, what is the value
ofVo?
1 MO
r----,--- + 30 V
V, 0---1 1---+--1-+1
Ca
1 MO
""":""'.f---i r--,---O Vo
3 KO
Solution:
Fig. 1.31 Circuit diagram of Ex : 1.7
1
--=4000
gm 2500 fl
Open circuit output voltage, that is without considering R L
, = (0.949) (5mV) = 4.75 mY.
Output impedance is, Zo = Rs II
1
ḡm
= 7500 0 II 400 0
=3800
This is the AC equivalent circuit. An AC source of 4.75 m V is in series drawn circuit with an
output impedance of 380 O.
5mV::1
500n I
3800
Fig. 1.32 AC Equivalent circuit
Therefore, AC voltage across the load resistor is,
3000
Vo = 3380 x 4.75 mV
= 4.22 mV