Two Port Devices and the Hybrid Model

Two Port Devices and the Hybrid Model

• The terminal behaviour of a large class of two-port devices is specified by two voltages and two currents.

• The amplifier is a two port network having two input terminals and two output terminals, as shown in Fig. 6.2.1.

Here, I₁ is the input current to the amplifier.

V₁ is the input voltage to the amplifier.

I₂ is the output current of the amplifier and

V₂ is the output voltage of the amplifier.

• As we know transistor is a current operated device, input current is an independent variable. The input current, Ii and output voltage V2 derives the input voltage Vi  I2 input voltage Vi and output current I2 are the dependent variables, whereas input current I1 and output voltage V2 are independent variables. Thus we can write,

V₁ = f₁ (I₁ , V₂ ) … (6.2.1)

I₂ = f₂ (I₁ , V₂ ) … (6.2.2)

This can be written in the equation form as follows,

V₁ = h₁₁ I₁ + h₁₂  V₂ … (6.2.3)

I₂ = h₂₁ I₁ + h22  V₂ … (6.2.4)

Definitions of h- parameter

The parameters in the above equation are defined as follows:

• The four hybrid parameters have different units and hence referred to as hybrid parameters.

• As we use small letter for a.c. analysis, these are commonly known as h-parameters.

Notations :

• The standard notations can be given as,

i = 11 = Input o = 22 = Output

f = 21 = Forward transfer r = 12 = Reverse transfer.

• Thus we can write h-parameters as follows :

a) With output short circuited :

h11 = hi : Input resistance.

h21 = hf : Short circuit current gain.

b) With input open circuited :

h12 = hr : Reverse voltage transfer ratio.

h22 = ho : Output admittance.

Model

• The mathematical model for two port device shown in Fig. 6.2.1 is constructed with the use of four h-parameters and it is shown in Fig. 6.2.2.

• The model satisfies equations (6.2.3) and (6.2.4) by writing Kirchhoff's voltage and current laws for the input and output ports, respectively.

• The input circuit contains a dependent voltage generator and the output circuit has a dependent current source.