Z-parameter system

Currents are independent components in this system

.

Then

.

If the increments dI₁ and dI₂ are considered as low AC currents of a high frequency with complex amplitudes I ₁ and I ₂, the increments dV ₁ and dV ₂ will be low AC voltages with complex amplitudes V ₁ and V ₂, and partial derivatives will be complex resistances (Z ₁₁, Z ₁₂, Z ₂₁, Z ₂₂). Then the equations will turn in such a way:

V ₁ =Z ₁₁ I ₁ +Z ₁₂ I ₂, V ₂ = Z ₂₁ I ₁ +Z ₂₂ I _2,

– the transistor input resistance, – the resistance of feedback, – the resistance of the transistor direct gain, – the transistor output resistance.

These proportions correspond to the transistor circuit shown in Fig. 4.7c. All Z –parameters are defined in the open circuit mode for the AC component of current which is on the opposite side of the four-terminal network. At the input (I₁=0) for Z₂₂ and Z₁₂, at the output (I₂=0) for Z₁₁ and Z₂₁.