Any linear two-port network is described by the following equations

$$\begin{equation}I_1=y_{11}V_1 + y_{12}V_2\end{equation}$$ $$\begin{equation}I_2=y_{21}V_1 + y_{22}V_2\end{equation}$$

or, in matrix form,

$$\begin{bmatrix}I_1\\I_2\end{bmatrix} = \begin{bmatrix}y_{11} & y_{12}\\y_{21} & y_{22}\end{bmatrix} \begin{bmatrix}V_1\\V_2\end{bmatrix}$$

The 4 parameters are called:

• $y_{11}$ - short-circuit input admittance
• $y_{12}$ - short-circuit transfer admittance
• $y_{21}$ - short-circuit transfer admittance (similar to $y_{12}$)
• $y_{22}$ - short-circuit output admittance

Using the above definition, once can show that the admittance parameters can be computed by using the following equations

$$\begin{equation}y_{11}=\left.{\frac{I_1}{V_1}} \right|_{V_2=0}\end{equation}$$ $$\begin{equation}y_{12}=\left.{\frac{I_1}{V_2}} \right|_{V_1=0}\end{equation}$$ $$\begin{equation}y_{21}=\left.{\frac{I_2}{V_1}} \right|_{V_2=0}\end{equation}$$ $$\begin{equation}y_{22}=\left.{\frac{I_2}{V_2}} \right|_{V_1=0}\end{equation}$$

The above formulas can be used to build the circuit that we need to solve to measure the y-parameters. For instance, if we want to measure the $y_{11}$ parameter, we need to:

• Short-circuit the output terminals (because the constraint requires $V_{2}=0$ V, which is equivalent to a short-circuit)
• We connect a voltage source of 1 V at the input terminals (because the denominator in the definition of $y_{11}$ contains a voltage and the subscript is $1$, which denotes the input)
• Compute current $I_{1}$ at the input terminals (because the numerator in the definition of $y_{11}$ contains a current and the subscript is $1$, which denotes the input)