Small Signal Low Frequency h-parameter Model

Small Signal Low Frequency h-parameter Model:

Let us consider the transistor amplifier as a block box.

Where, I_i – input current to the amplifier

V_i - input voltage to the amplifier

I_o – output current of the amplifier

V_o – 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