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**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 â€“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 V_{1 }and current I_{1 }at the input port, and voltage V_{2 }and current I_{2} 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,V_{1},V_{2}, I_{1},I_{2} . 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 h_{11}, h_{12}, h_{21}, h_{22 }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

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### Test: Signals

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