Two-port networks

Ports hide the interior

• A two-port is defined only at its terminal pairs: $V_{1}, I_{1}$ in and $V_{2}, I_{2}$ out. ABCD uses current out of port 2 ($-I_{2}$); z-parameters use it into the port — pick one convention and stay.
• The series-Z block is $(1, Z; 0, 1)$ and the shunt-Y block $(1, 0; Y, 1)$; a cascade multiplies the matrices, in order.

{exercise:uni-ee4x-cascade}

Reciprocity reads in the numbers

• Converting to z-parameters ($z_{11} = A/C$, $z_{12} = Δ/C$, $z_{21} = 1/C$, $z_{22} = D/C$), reciprocity shows up as $z_{12} = z_{21}$ — visible, not assumed; controlled sources break it.
• With port 2 open ($I_{2} = 0$), $V_{1} = z_{11}I_{1}$: the open-circuit input is one z-parameter read directly.

{exercise:uni-ee4x-reciprocity}

{exercise:uni-ee4x-openport}