PRACTICAL # 13Transistor as a switch

By:Engr.Irshad Rahim Memon

OBJECTIVE Objective of this practical is to learn that

how a transistor can operate as a switch.

SOME CONCEPT REGARDING TRANSISTOR

Transistor is semiconductor device, it is constructed of three doped semiconductor materials. Either n-type material is sandwiched between two p-type materials or p-type material is sandwiched between two n-type materials and these three materials make three regions of transistor as shown in figure 01

FIGURE 01

CONCEPT CONTINUED…. Majority carriers from emitter are

emitted and those majority carriers flow through base and collector. As base is very thin and lightly doped, so most of the majority carriers travel towards collector. These majority carriers constitute current that is given in the following equation.

I E = I B + I C .........(i)

CONCEPT CONTINUED…. Transistor can be operated in one of

three following configurations, common terminal is grounded. 

Common Base Configuration   Common Emitter Configuration   Common Collector Configuration  

CONCEPT CONTINUED…. Transistor action is shown in figure 03

for common emitter configuration for which there can be following equation.

I C = BI B .............. (ii) Where b is known as transistor

current gain VBB = I B RB +VBE ............ (iii) Where VBE = 0.7volt VCC = I C RC +VCE ............ (iv) Where VCE =VCB +VBE

FIGURE 03

Figure 03: Transistor operation in common emitter configuration

CONCEPT CONTINUED…. Transistor works as an open switch in

any of the following conditions IB = 0 so IC = 0 so VCE ( cutoff ) =VCC Both junctions of transistor are reversed

biased Transistor works in its cut off region

CONCEPT CONTINUED…. Transistor works as a close switch in any

of the following conditions  IC =IC ( Sat ) VCE =VCE ( Sat ) Both junctions of transistor are forward

biased   Transistor works in its saturation region

CONCEPT CONTINUED…. Transistor as an open switch and close

switch is shown in figure 04

Figure 04: Switching action of Ideal Transistor

CHARACTERISTIC CURVE OF TRANSISTOR

COMING TOWARDS OBJECTIVE NOW

REQUIRED APPARATUS Few Transistors Few LEDs Breadboard Multimeter Connecting Wire etc.

PROCEDURE Prepare following circuit as shown in

figure 05. Depending upon the transistor model, set the values of RC, RB, VCC and Vin such that transistor should be operated in its saturation and then cutoff regions continuously with the gap of 1second.

TRANSISTOR CIRCUIT ON MULTISM

WHEN IB=0

WHEN IB>0

OBSERVATION It is observed that when transistor is in

its saturation region, LED is turning on for one second and when transistor is in its cutoff region, LED is turning off for one second. This process of LED blinking is continued with help of Switch.

ANY QUERY