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Chapter: Electronic Circuits - BJT Amplifiers

Small Signal Low Frequency h-parameter Model

Let us consider the transistor amplifier as a block box.

Small Signal Low Frequency h-parameter Model:

 

Let us consider the transistor amplifier as a block box.


Where, Ii – input current to the amplifier

Vi - input voltage to the amplifier

Io – output current of the amplifier

Vo – output voltage of the amplifier

 

Input current is an independent variable. Input voltage and output current are dependent variables. Input current and output voltage are independent variables.


This can be written in the equation form as,


The above equation can also be written using alphabetic notations,



Definitions of h-parameter:

 

The parameters in the above equations are defined as follows:

 


 

h11 – input resistance with output short-circuited in ohms

h12 – fraction of output voltage at input with input open circuited, it is unitless

h21 – forward current transfer ratio or current gain with output short circuited, it is unitless

h22 – output admittance with input open circuited in mhos

 

Benefits of h-parameters:

 

1.     Real numbers at audio frequencies

2.     Easy to measure

3.     Can be obtained from the transistor static characteristic curve

4.     Convenient to use in circuit analysis and design

5.     Most of the transistor manufacturers specify the h-parameters

 

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