• For the hybrid equivalent
model, the parameters are defined at an operating point.
• The quantities hie, hre,hfe,
and hoe are called hybrid parameters and are the
components of a small – signal
equivalent circuit.
• The description of the hybrid
equivalent model will begin with the general two
port system.
• The set of equations in which
the four variables can be related are:
• Vi = h11Ii + h12Vo
• Io = h21Ii + h22Vo
• The four variables h11, h12, h21
and h22 are called hybrid parameters ( the mixture
of variables in each equation
results in a “ hybrid” set of units of measurement for
the h – parameters.
• Set Vo = 0, solving for h11, h11
= Vi / Ii Ohms
• This is the ratio of input
voltage to the input current with the output terminals
shorted. It is called Short
circuit input impedance parameter.
• If Ii is set equal to zero by
opening the input leads, we get expression for h12:
h12 = Vi / Vo , This is called
open circuit reverse voltage ratio.
• Again by setting Vo to zero by
shorting the output terminals, we get
h21 = Io / Ii known as short
circuit forward transfer current ratio.
• Again by setting I1 = 0 by
opening the input leads, h22 = Io / Vo . This is known as
open – circuit output admittance.
This is represented as resistor ( 1/h22)
• h11 = hi = input resistance
• h12 = hr = reverse transfer
voltage ratio
• h21 = hf = forward transfer
current ratio
• h22 = ho = Output conductance
Hybrid Input equivalent circuit
Hybrid output equivalent circuit
Complete hybrid equivalent circuit
Common Emitter Configuration - hybrid equivalent circuit
• Essentially, the transistor model is a three terminal two – port system.
• The h – parameters, however, will change with each configuration.
• To distinguish which parameter has been used or which is available, a second
subscript has been added to the h – parameter notation.
• For the common – base configuration, the lowercase letter b is added, and for
common emitter and common collector configurations, the letters e and c are used
respectively.
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