Vierpol

Vierpol (four-pole) is a term in electrical engineering for a network with four terminals that connects two pairs of terminals, commonly referred to as a two-port network. The two ports are typically labeled 1 and 2, each with a positive and a negative terminal. A Vierpol is characterized by the relation between the voltages across the ports (V1, V2) and the currents into the ports (I1, I2). Depending on the chosen model, this relationship can be expressed with a matrix such as the Z-parameter form: V1 = Z11 I1 + Z12 I2, V2 = Z21 I1 + Z22 I2; or with the Y-parameter form: I1 = Y11 V1 + Y12 V2, I2 = Y21 V1 + Y22 V2. Other parameterizations include T-, H-, and S-parameters. A Vierpol can be passive or active and may be linear or nonlinear, though linear two-port theory is most common.

Typical realizations include a transformer, which presents two windings as a coupled four-terminal device, or networks of resistors, capacitors, and inductors that couple input and output. The concept is used in circuit analysis for impedance matching, filtering, signal transfer, and measurement. In high-frequency engineering, S-parameters and related models are often preferred.

See also: two-port network, transformer, Z-parameters, Y-parameters, S-parameters.