Lecture - 26: h-Parameters

Small signal low frequency transistor Models:

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 i1 and output voltage v2 are taken independent then other two quantities i2 and v1 can be expressed in terms of i1 and V2.

The equations can be written as

where h11, h12, h21 and h22 are called h-parameters.

        = hi = input impedance with output short circuit to ac.

=hr = fraction of output voltage at input with input open circuited or reverse voltage gain with input open circuited to ac (dimensions).

= hf = negative of current gain with output short circuited to ac.

The current entering the load is negative of I2. This is also known as forward short circuit current gain.

= ho = 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. hfe ,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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