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

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

or, in matrix form,

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

The 4 parameters are called:

• $h_{11}$ - short-circuit input impedance
• $h_{12}$ - open-circuit reverse voltage gain
• $h_{21}$ - short-circuit forward current gain
• $h_{22}$ - open-circuit output admittance

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

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

The above formulas can be used to build the circuit that we need to solve to measure the h-parameters. For instance, if we want to measure the $h_{21}$ 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 current source of 1 A at the input terminals (because the denominator in the definition of $h_{21}$ contains a current and the subscript is $1$, which denotes the input)
• Compute current $I_{2}$ at the output terminals (because the numerator in the definition of $h_{21}$ contains a current and the subscript is $2$, which denotes the output)