There is functional dependence between currents and voltages in this system

.

Then:

.

The increment of independent variable components dV₁ and dV₂ can be considered as low variable components of high frequency harmonic voltage with complex amplitudes V₁ and V₂. The increments dI₁ and dI₂ are also harmonic oscillations with the comple amplitudes I₁ and I₂. The partial derivatives, being situated in front of the independent variable components are the complex conductivities Y₁₁, Y₁₂, Y₂₁, Y₂₂.

Thus

.

Hence

- the transistor input conduction,

- the conduction of the transistor backward gain,

- the conduction of the transistor direct gain,

- the output conduction of the trasistor.

The above equations correspond to transistor circuit shown in Fig. 4.7 b. All Y -parameters are defined in the short circuit mode for a variable current component which is on the opposite side of the four-terminal network: at the input (V₁=0) for Y₂₂ and Y₁₂; at the output (V₂=0) for Y₁₁ and Y₂₁. Y - parameter system represents high frequency transistor characteristics.