# HYBRID EQUIVALENT FOR CC TRANSISTOR

The figure shows the transistor connected in common emitter configuration and the figure also shows the hybrid equivalent circuit of such a transistor.

_{bc}) and the output current (i

_{c}) are given by the following equations:

V_{bc} = h_{ic}.i_{b} + h_{re}.V_{ec}

i_{e} = h_{fe}.i_{b} + h_{oe}.V_{ec}

Contents

## Hybrid expression

Expression can be obtained from the general hybrid formulas derived in this article HYBRID EQUIVALENT OF TRANSISTOR by adding a second subscript letter ‘b’ (which stands for commonbase) with the h-parameters and are as discussed below.

## Current Gain

It is given by the relation,

A_{i} = -(h_{fc}/(1 + h_{oc}.r_{L}))

Where r_{L} is the A.C load resistance. Its value is equal to the parallel combination of resistance R_{E} and R_{C}. Since h_{fe} of a transistor is a positive number, therefore A_{i} of a common emitter amplifier is negative.

## Input Resistance

The resistance looking into the amplifier input terminals (i.e. base of a transistor) is given by the relation,

R_{i} = h_{ie} + h_{re}.A_{i}.r_{L} = h_{ie} – ((h_{rc}.h_{fc})/(h_{oc} + (1/r_{L})))

The input resistance of the amplifier stage (called stage input resistance R_{is}) depends upon the biasing arrangement. For a fixed bias circuit, the stage input resistance is,

R_{is} = R_{s}

## Voltage Gain

It is given by the relation,

A_{v} = A_{i}.r_{1}/R_{i}

Since the current gain (A_{i}) of a common base amplifier is positive, therefore the voltage gain (A_{v}) is also positive. It means that there is no phase difference between the input and output signals of the common base amplifier. The voltage gain, in terms of h-parameters, is given by the relation.

A_{v} = h_{fc}.r_{L}/(h_{ic} + ∆h.r_{L})

Where

∆h = h_{ic}.h_{oc} – h_{rc}.h_{fc}

## Output Resistance

The resistance looking into the amplifier output terminals is given by the relation,

R_{o} = (R_{s} + h_{ic})/(R_{s}.h_{oc} + ∆h)

Where

R_{s} = Resistance of the source, and

∆h = h_{ic}.h_{oc} – h_{rc}.h_{fc}

## Overall Voltage Gain

It is given by the relation,

A_{vc} = (A_{v}.R_{is})/(R_{s} + R_{is})

## Overall Current Gain

It is given by relation,

A_{is} = (A_{i} + R_{s})/(R_{i} + R_{is})