All the transistor amplifiers are two port networks having two voltages and two currents. The positive directions of voltages and currents are shown in **fig. 1**.

Fig. 1

Out of four quantities two are independent and two are dependent. If the input current i_{1} and output voltage v_{2} are taken independent then other two quantities i_{2} and v_{1} can be expressed in terms of i_{1} and V_{2}.

The equations can be written as

where h_{11}, h_{12}, h_{21} and h_{22} are called h-parameters.

= h_{i} = input impedance with output short circuit to ac.

=h_{r} = fraction of output voltage at input with input open circuited or reverse voltage gain with input open circuited to ac (dimensions).

= h_{f} = negative of current gain with output short circuited to ac.

The current entering the load is negative of I_{2}. This is also known as forward short circuit current gain.

= h_{o} = output admittance with input open circuited to ac.

If these parameters are specified for a particular configuration, then suffixes e,b or c are also included, e.g. h_{fe} ,h _{ib} are h parameters of common emitter and common collector amplifiers

Using two equations the generalized model of the amplifier can be drawn as shown in **fig. 2**.

**Fig. 2**

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