Transistor as a switch By:Engr.Irshad Rahim Memon

 Objective of this practical is to learn that how a transistor can operate as a switch.

 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

 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)

 Transistor can be operated in one of three following configurations, common terminal is grounded.  Common Base Configuration  Common Emitter Configuration  Common Collector Configuration

 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  V BB = I B R B +V BE  (iii) Where V BE = 0.7volt  V CC = I C R C +V CE  (iv) Where V CE =V CB +V BE

Figure 03: Transistor operation in common emitter configuration

 Transistor works as an open switch in any of the following conditions  I B = 0 so I C = 0 so V CE ( cutoff ) =V CC  Both junctions of transistor are reversed biased  Transistor works in its cut off region

 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

 Transistor as an open switch and close switch is shown in figure 04 Figure 04: Switching action of Ideal Transistor

 Few Transistors  Few LEDs  Breadboard  Multimeter  Connecting Wire etc.

 Prepare following circuit as shown in figure 05. Depending upon the transistor model, set the values of R C, R B, V CC and V in such that transistor should be operated in its saturation and then cutoff regions continuously with the gap of 1second.

 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.