Electronic Circuit Analysis

72 Electronic Circuit Analysis
2.7.7 AC Equivalent Circuit
The input resistance
v.
R = _I =h fe
h fe
Re
I Ibl 1 2
Ifwe take h fe as 50, Re = 4Kn, we get R j as 10Mn. Ifa transistor with h fe = 100 is taken, R j
will be much larger. The value ofXco is chosen such that at the lower frequencies, under consideration
XCo is a virtual short circuit. If the collector C 1 changes by certain potential, E2 also changes by the
1
same amount. So C 1 and E2 are boot strapped. There is h between BI and C 1
.
ob
Reff = h (1 - A )
ob v
h re V cel
V,
N
Fig. 2.33 AC equivalent circuit
Direct short circuit is not done between C 1 and E 2
. Since, DC condition will change, Xc is a
o
short only for AC signals and not for DC.
2.S The CASCO DE Transistor Configuration
V =V
o 2
1
Fig. 2.34 CASCO DE Amplifier (C.E, C.B configuration)