Electronic Circuit Analysis

High Frequency Transistor Circuits 113
Example : 3.4
Show that at low frequencies, the hybrid 1t - model with rb'e and ree taken as infinite reduces to the
approximate CE h-parameter model.
B
~~ +--Ic
C
h
Ie
Solution:
Eo---------~----------+_--------------_oE
Fig. 3.17 Circuit diagram for EX.3.4
The h-parameters equivalent circuit in Common Emitter configuration is as shown in
Fig. 1.49. We have to show that the hybrid 1t equivalent circuit will also be the same with the
approximation given in the problem.
E----------~---------+---------------oE
Fig. 3.18 Simplified circuit
If the expressions for the output current I and input impedance hie are same in the hybrid 1t
equivalent circuit and h-parameter equivalent circuit, we can say that both the circuits are identical.
This is the hybrid - 1t equivalent circuit neglecting capacitances
Ii, = r = 00
be ee (since at low f, they are (0).
1= gm' Vb'e;
Vb'e = lb' rb'e
I = gm' lb' rb'e
we know that
Example : 3.5
rb'e gm = hfe
1= hfe · Ib
hie = rbb' + rb'e
[in the derivation of gb'e]
[which is the same as in h-parameters circuit]
The following low frequency parameters are known for a given transistors at Ie = lOrnA, V ce = 10v
and at room temperature hie = soon, hoe = 10- 5 AN, h fe
= 100, h re
= 1 0-4U .
At the same operating point, fT = 50 MHz, and Cob = 3 pf, compute the values of all the
hybrid - 1t parameters.