Single Stage Amplifiers - Electrical Engineering (EE) PDF Download

1 Introduction

V-I characteristics of an active device such as BJT are non-linear. The analysis of a non- linear device is complex. Thus to simplify the analysis of the BJT, its operation is restricted to the linear V-I characteristics around the Q-point i.e. in the active region. This approximation is possible only with small input signals. With small input signals transistor can be replaced with small signal linear model. This model is also called small signal equivalent circuit.

1.2 Two –Port  Devices and Network Parameters

Small signal low frequency transistor Models: 

All the transistor amplifiers are two port networks having two voltages and two currents. The positive directions of voltages and currents are shown in fig. 1.
 

A two-port network is represented by four external variables: voltage V₁ and current  I₁ at the input port, and voltage V₂ and current  I₂ at the output port, so that the two-port network can be treated as a black box modeled by the relationships between the four variables,V₁,V₂, I₁,I₂ . Out of four variables, two can be selected as independent variables and the other two as dependent variables. The dependent variables can be expressed in terms of independent variables. This leads to various two port parameters out of which the following three are important:

1.      Impedance parameters (z-parameters)

2.      Admittance parameters (y-parameters)

3.      Hybrid parameters (h-parameters)
 

1 z-parameters

A two-port network can be described by z-parameters as

In matrix form, the above equation can be rewritten as

Where

    Input impedance with output port open circuited

    Reverse transfer impedance with input port open circuited

  Forward transfer impedance with output port open circuited
  Output impedance with input port open circuited
 

Y-parameters

A two-port network can be described by Y-parameters as

In matrix form, the above equation can be rewritten as


    Input admittance with output port short circuited

  Reverse transfer admittance with input port short circuited

   Forward transfer admittance with output port short circuited

 Output admittance with input port short circuited
 

Hybrid parameters (h-parameters)

     If the input current I1 and output voltage V2 are taken as independent variables, the dependent variables V1 and I2 can be written as
 

Where h₁₁, h₁₂, h₂₁, h₂₂ are called as hybrid parameters.

     Input impedance with o/p port short circuited

    Reverse voltage transfer ratio with i/p port open circuited      

Forward voltage transfer ratio with o/p port short circuited

          output impedance with i/p port open circuited