Question

Draw the circuit arrangement for studying the input and output characteristic of an n-p-n transistor in CE configuration. With the help of these characteristic define (i) input resistance, (ii) current amplification factor.

Answer 1

(a) Characteristic Curves: The circuit diagram for determining the static characteristic curves of an n-p-n transistor in common-emitter configuration is shown infigure.

Common Emitter Characteristics:

(i)Input characteristics: These characteristic curves are obtained by plotting base current $\left(I_B\right)$ versus base-emitter voltage $V_{BE}$ for fixed collector-emitter voltage $V_{CE}$. Fig. represents these characteristics.

(ii) Output characteristics: These characteristics are obtained by plotting collector current $I_C$ versus collector-emitter voltage $V_{CE}$ at a fixed value of base current $I_B$ . The base current is changed to some other fixed value and the observations of $I_C$ versus $V_{CE}$ are repeated. Fig. represents the output characteristics of a common-emitter circuit. Input Resistance. It is the ratio of change in base-emitter voltage $\left(\Delta V_{BE}\right)$ to the corresponding change in base current $\left(\Delta I_B\right)$ at constant collector-emitter voltage $V_{CE}$ i. e,.

$r_i=\frac{△V_{BE}}{△I_B}$ $V_{CE}=constant$

The input resistance of a common emitter circuit is of the order of a few hundred ohms.

Current amplification factors of a transistor (α and β):

The current gain a is defined as the ratio of change in collector current to the change in emitter current for constant value of collector voltage in common base configuration i.e.,

$\alpha =\left(\frac{△I_C}{△I_E}\right)$ $V_C=constant$ $\left(i\right)$

Practical value of a ranges from $0.9to0.99$ for junction transistor. The current gain α is defined as the ratio of change in collector current to the change in base current for constant value of collector voltage in common emitter configuration i. e.,

$\beta =\left(\frac{△I_C}{△I_B}\right)$ $V_C=constant$ $\left(ii\right)$

The value of $\beta$ ranges from 20 to 200. The current gains α and β are related as