Applications of Transistors
The transistor as an amplifier
1. A transistor can be used to amplify current. This is because a small change in base current causes a large change in collector current.
2. Example is a microphone.
3. Sound waves that are fed into the microphone cause the diaphragm in the microphone to vibrate.
4. The electrical output of the microphone changes according to the sound waves.
5. As a result, the base current is varying because of the small alternating voltage produced by the microphone.
6. A small change in the base current causes a large change in the collector current.
7. The varying collector current flows into the loudspeaker. There, it is changed into the sound waves corresponding to the original sound waves.
8. The frequencies of both waves are equivalent but the amplitude of the sound wave from the loudspeaker is higher than the sound waves fed into the microphone.
Microphone: To change sound signal to electrical signal
Capacitor: To block a steady current from flowing into the transistor and microphone.
Potential divider: To apply a proportion of the total voltage across the emitter-base junction so that the junction is forward-biased.
Transistor: To amplify the input wave form.
Loudspeaker: To change the electrical signal to sound wave.
The transistor as switch
1. In a transistor, no current can flow in the collector circuit unless a current flows in the base circuit. This property allows a transistor to be used as switch.
2. The transistor can be turned on or off by changing the base.
3. There are a few types of switching circuits operated by transistors.
(a) Light-Operated Switch
1. The circuit is designed to light the bulb in a bright environment and to turn it off in the dark.
2. One of the components in the potential divider is a light-dependent resistor (LDR). When it is placed in DARKNESS, its resistance is large. The transistor is switched OFF.
3. When LDR is lighted by bright light, its resistance falls to small value resulting in more supply voltage and raising the base current. The transistor is switched on, collector current flows and bulb lights up.
(b) Heat-operated switch
1. One important component in the circuit of a heat-operated switch is the thermistor.
2. Thermistor is type of resistor that responds to the surrounding temperature. Its resistance increases when the temperature is low and vice versa.
3. When heat is applied to the thermistor, its resistance drops and a greater share of supply voltage is dropped across R. The base current increases followed by a greater increase in the collector current. The bulb will glow and the siren will sound.
4. This particular circuit is suitable as a fire alarm system.
Integrated Circuits (C)
1. An integrated circuit (IC) consists of transistors, resistors, diodes and capacitors combined together in one wafer-thin chip of silicon.
2. This is one wafer-thin chip is called a microchip.
3. The microchip is only a few millimeters square with a thickness of 0.5 mm.
Advantages of an IC:
a. Consumes a small amount of electrical energy.
b. Very little heat is generated.
c. Occupies a small space which reduces the size of circuits.
d. Can be built at low cost.