Common Emitter Transistor Characteristics

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

What You will Learn

• You will learn the theory of common emitter transistor configuration and the corresponding collector current equation.
• You will understand the common emitter short circuit current gain ($\beta$).
• You will learn the input and output characteristics of common emitter BJT.
• You will understand the experimental set up for plotting the characteristics and draw the characteristics.

Objectives

• To plot the input and output characteristics of common emitter BJT.
• To compute the short circuit current gain ($\beta$), input impedance ($r_{i}$) and output impedance ($r_{o}$).

Theory

The collector current of an actively biased CE (meaning that emitter 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 ($\beta$) is related to $\alpha$ as \begin{equation}\label{eqbeta} \beta = \frac{\alpha}{1-\alpha} \end{equation} $I_{C}$ is related with $\beta$ as \begin{equation}\label{eqcollectorcurrentbeta} I_{C}=\beta I_{B}+(1+\beta)I_{CO} \end{equation} Input characteristics are plots of input current ($I_B$) vs. the input voltage ($V_{BE}$) for different output voltage ($V_{CE}$) 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($V_{CE}$) for different input current ($I_{B}$) values.

Experiment - Input Characteristics

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

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

These two variations are accomplished by two parameter sweeps. The first sweep varies $I_B$ from $10$ nA to $15$ mA. This loop is repeated for $V_{CE}$ values from $10$ V to $20$ V in steps of $5$ V, accomplished by the second parameter sweep.

The resulting input characteristics are shown below. It gives idea about the input impedance of the common emitter stage with active bias. Since the emitter base junction is forward biased, the characteristics resemble diode characteristics.

Experiment - Output Characteristics

The schematic for plotting the output characteristics is shown below.

The parameter sweep for $V_{CE}$ is set to vary from $-0.8$ V to $10$ V.

The initial negative value of $V_{CE}$ (to effect a small forward bias of $J_{2}$) is to understand the behaviour at saturation.

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

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

Observations

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

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

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

What You Learned

You learned the expression for collector current in common emitter configuration.

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

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