The Hybrid equivalent model

Posted by AJ × 03:17

• 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

• 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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