Common Base Transistor Characteristics

Dr. Hari V S
Department of Electronics and Communication
College of Engineering Karunagappally

What You will Learn

• You will learn the operation of common base transistor stage.
• You will learn the experimental set up for plotting the current voltage characteristics at the input and output of a CB stage.
• You will learn to find out the common base short circuit current gain ($\alpha$), the input impedance ($r_{i}$) and output impedance ($r_{o}$)

Theory

The collector current of an actively biased CB (meaning that base is at ac ground) transistor, in terms of the emitter current and reverse saturation current is \begin{equation} I_{C}=-\alpha I_{E}+I_{CO} \end{equation} where $\alpha$ is the common base short circuit current gain. The short circuit current gain in the common emitter configuration. Input characteristics are plots of input current ($I_E$) vs. the input voltage ($V_{EB}$) for different output voltage ($V_{CB}$) values. They are very much like the characteristics of diode. The output characteristics are plots of the output current $I_{C}$ vs. the output voltage($V_{EB}$) for different input current ($I_{E}$) values.

Experiment - Input Characteristics

the Qucs schematic for plotting the input characteristics is given below.

Here, the variation of the input current ($I_E$) with input voltage ($V_{EB}$) is computed and plotted. The procedure is repeated for various $V_{CB}$ values.

These two variations are accomplished by two parameter sweeps.

The resulting input characteristics are shown below. It gives idea about the input impedance of the common base stage with active bias.

Experiment - Output Characteristics

The schematic for plotting the output characteristics is shown below.

The parameter sweep for $V_{CB}$ is set to vary from $0$ V to $10$ V.

The characteristics are plotted, for the input currents ranging from $1\,mA$ to $1$mA in $5$ steps, as shown below. The behaviour at saturation is observed from the plots.

Compute the input and output resistances from the above sets of plots.

Observations

The input resistance of CB stage = $\ldots\ldots\ldots\,$$\Omega$

The output resistance of CB stage = $\ldots\ldots\ldots\,$ $\Omega$

Common base short circuit current gain = $\ldots\ldots\ldots$

What You Learned

You understood the current voltage characteristics at the input and output of common base amplifier.

You found out the short circuit current gain, input impedance and output impedance of the common base stage.