Vth2

Vth2 is the Thevenin equivalent voltage seen at the second port (port 2) of a multi-port electrical network. It represents the open-circuit voltage across the terminals of that port when no load is connected and the network’s independent sources are in their reference states.

In practice, Vth2 is found by removing the load from port 2 and calculating the voltage across the port’s terminals with all independent sources active. If the network includes dependent sources, their behavior must be accounted for as well. In networks with multiple ports, Vth2 can depend on how the other ports are terminated or loaded, since the ports are often coupled through the network elements.

The Thevenin model seen from port 2 consists of a voltage source equal to Vth2 in series with an equivalent impedance Rth2 (or Zth2 for non-resistive cases). When a load RL (or ZL) is connected to port 2, the port voltage is given by Vload = Vth2 · ZL / (Rth2 + ZL) for a linear, time-invariant network. This relationship enables analyzing how the port responds to different loads without re-solving the entire network each time.

Common applications include simplifying the analysis of cascaded stages, impedance matching, and multi-output or multi-port circuits where individual ports are of interest. Vth2 is especially useful in RF design, power distribution, and network synthesis, where a compact Thevenin representation aids intuition and calculation.

Limitations include the requirement that the network be linear and bilateral with fixed ports. Thevenin equivalents are invalid for nonlinear or time-varying circuits unless the operating point is fixed and small-signal linearization is valid.