The small signal input impedance of a transistor when the output is shorted for the measuring signal, is (where the symbols have their usual meaning)
1
\({h_{11}} = {\left. {\frac{{{V_1}}}{{{I_1}}}} \right|_{{V_2} = 0}}\)
2
\({h_{12}} = {\left| {\frac{{{V_1}}}{{{I_1}}}} \right|_{{V_2} = 0}}\)
3
\({h_{21}} = {\left. {\frac{{{I_2}}}{{{I_1}}}} \right|_{{V_2} = 0}}\)
4
\({h_{22}} = {\left| {\frac{{{I_2}}}{{{V_2}}}} \right|_{{I_1} = 0}}\)