Explain the Function of a n-p-n transistor as a switch with neces… - Vidyadip

Question

Explain the Function of a $n-p-n$ transistor as a switch with necessary circuit diagram.

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Answer

Any device which can turn ON and OFF current in an electrical circuits in known as a switch. Generally, mechanical switches are used to resume and break connection in electrical circuits. Digital devices like computers perform a very large number of switching operations every day. In computer circuits an electronic device like junction diode and junction transistor can be used to turn rapidly on or off the current.
Junction Transistor as a Switch : In order to understood the switching action of a junction transistor we should be familiar with three states of working of a transistor. In Fig. (a), circuit diagram of an $n-p-n$ transistor in common-emitter configuration is shown. In this circuit a battery of variable voltage V_BB IS connected to input circuit i.e., base-emitter circuit and another battery of variable voltage Vcc is connected to the output circuit i.e., emitter-collector circuit, R_B and R_e are the resistances in the input and output circuits, respectively.

Now applying Kirchhoff's voltage rule in the closed loop of input side and output side, we get respectively as :
$V_{BB}=I_{B} R_{B}+V_{BE} \text { and } V_{CE}=V_{CC}-I_{C} R_{C}$ ... (1)
But the voltage $V_{B B}$ can be regarded as d.c. input voltage V_i and V_CE as the d.c. output voltage V_O Hence the above equation can be written as :
$V _i= I _{ B } R _{ B }+ V _{ BE }$ and $V _o= V _{ CC }- I _{ C } R _{ C }$ ...(2)
With the help of series (2) of the equation, a graph between $V _o$ and $V _i$ is drawn, which is called transfer characteristic of the junction transistor Fig. (b). This typical characteristic curve has three well defined regions as follows :
(i) Cut-off region : In Si transistor, as long as input V_i is less than 0.6 V the transistor remains in cut off state and current I_cremains zero.
$\therefore \quad V _o= V _{ CC }$
This region is called cut-off region.
(ii) Active region : When V_ibecomes greater then 0.6 V tansistor is in active region with some current I_c in the output path and the output V_o decreases as the current $I _{ C }$ increases ( $V _o= V _{ CC }- I _{ C } R _{ C }$ ). With increase in $V _i, I _{ C }$ increases almost linearly and so $V _o$ decreases linearly till its value becomes less than about 1.0 V .

(iii) Saturation Region : When V_iis further increased to a high value, the large collector current flows which produces such a large potential drop across the load R_c that the emitter-collector junction gets also forward biased. The output voltage V_o decreases to almost zero. Since it cannot pass any more collector currect I_C, hence this situation is called saturation state and the transistor is said to be in saturation region.
A transistor can be used as a switch only if it is operated in its cut-off and saturation states. The electric circuit required to explain the switching action of a junction transistor is shown in Fig. (c) (i) in which common emitter $n-p-n$ transistor is used.

When the base input voltage V_in is negative or zero as shown in Fig. (c) (ii), the transistor is cut-off (i.e., base current I_B is zero) and no collector current flows in the load resistance R_L connected to the collector circuit. As such, there is no voltage drop across R_L. Hence the output voltage is equal to the collector supply voltage,
i.e. $V _{\text {out }}= V _{ CC }$
Strictly speaking, the collector current is not zero since a small collector-to-emitter leakage current always flows when the base input voltage is negative or zero.
When the base input voltage is positive [Fig. (c) (ii)], a base current I_B flows and an amplified collector-current I_c flows through the load R_L. Clearly, the output voltage is now given as follows :
$V _{\text {out }}= V _{ CC }- I _{ C } R _{ L }$
Thus, the transistor turns the current ON or OFF in the load R_L depending upon whether the input base voltage is positive or negative (or zero). In other words, it acts like a switch.
An compared to other types of switches, the transistor switch is compact, noiseless, trouble-free and has a very fast speed of operation.

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