ABCD parameters relate the input and output voltages and currents of a transmission line. They allow the input voltage and current to be expressed in terms of the output voltage and current. To analyze the ABCD parameters, the receiving port is assumed to be either open circuit, where the receiving current is 0, or short circuit, where the receiving voltage is 0. This leads to equations relating the input and output voltages and currents. The parameters A, B, C, and D then represent the voltage ratio, short circuit resistance, open circuit conductance, and current ratio respectively.

1. ABCD – parameter is a set of parameter relates to
variables at the input port to output port.
It also called transmission parameter because this
parameter is useful in the analysis & calculation of
transmission lines in
Sending of and Receiving of
(Vs& Is) and (Vr &Ir).

2. It is a linear & bilateral passive network,
the input voltage & current expressed in terms
of output voltage & current.
Therefore,
input voltage & current of transmission line
can be expressed in terms of output voltage &
current.

4. in Matrix form

6. The matrix is
called the
transmission
matrix of the
network.

7. To analyse, the values of the ABCD
parameters, two cases has to assume
Case 1. Wnen Receiving port Open
circuit
i.e. Ir =0
Case 2. When Recieving port, short
circuit
i.e. Vr =O

9. From the figure, when
output/recieving port
Open circuit,i.e. Ir =0
Then,
Equation (a) & (b) written as

12. From the above figure,. When
output/Sending port short circuit
i.e,Vr =0
Then
equation (a) & (b) written as

14. The conclusion is
A is called as Voltage Ratio,then
A = Vs / Vr
B is called as Short circuit resistance,then
B = Vs / Ir
C is called as Open Circuit Conductance,then
C= Is / Vr
D is called as Current Ratio,then
D = Is / Ir