Consider the following driving point impedance functions:
\({Z_1}\left( s \right) = \frac{{\left( {s + 2} \right)}}{{\left( {{s^2} + 3s + 5} \right)}}\)
\({Z_2}\left( s \right) = \frac{{\left( {s + 2} \right)}}{{\left( {{s^2} + 5} \right)}}\)
\({Z_3}\left( s \right) = \frac{{\left( {s + 2} \right)}}{{\left( {{s^2} + 2s + 1} \right)}}\)
\({Z_4}\left( s \right) = \frac{{\left( {s + 2} \right)\left( {s + 4} \right)}}{{\left( {s + 1} \right)\left( {s + 3} \right)}}\)
Which of the above is not a positive real?1
Z1
2
Z2
3
Z3
4
Z4