CBSE Class 12-science Answered

Dear student,

 

Trasnsistor as switch:

Because a transistor's collector current is proportionally limited by its base current, it can be used as a sort of current-controlled switch. A relatively small flow of electrons sent through the base of the transistor has the ability to exert control over a much larger flow of electrons through the collector.

Suppose we had a lamp that we wanted to turn on and off with a switch. Such a circuit would be extremely simple.let's insert a transistor in place of the switch to show how it can control the flow of electrons through the lamp. Remember that the controlled current through a transistor must go between collector and emitter. Since it is the current through the lamp that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch were. We must also make sure that the lamp's current will move against the direction of the emitter arrow symbol to ensure that thetransistor's junction bias will be correct.



Transistor as an amplifier

 

A useful mode of operation is the common-emitter configuration.The resistors connected to the base ensure that the BE junctionis forward biased. They effectively form a potential divider toreduce the voltage supplied to the base.The emitter resistor work with the base resistors to stabilise theoperating point wrt variations in β due to component variationand temperature by providing negative feedback.Finally, the collector resistor provides the load.

How it works
A signal, such as music from a CD player, is applied to the inputLet’s examine what happens when such a signal increases thebase voltage by ∆Vin.The emitter voltage is always 0.7V below Vb, so if Vb changes by∆Vin , so does Ve.Thus the emitter current increases by ∆Vin /Re.But Ic=-αIe≅-Ie, so it also increases by ∆Vin /Re.Thus the voltage at the collector will increase by-∆Vin RL/Re (that is, it will decrease)In this case RL/Re is 10, so the circuit amplifiesthe input voltage signal by a factor of -10.In general, the gain is -RL/Re. The negative signindicates that a increase in input voltage leads toa decrease in output voltage.This is an example of an inverting amplifier.

 

 

 

Transistor as an oscillator

Transistor oscillators generate regular, repeating signals for electronic equipment, including music synthesizers, radio transmitters, clocks and computers. They come in two basic kinds: resonant circuits with feedback, and relaxation oscillators.

Resonance

A resonant network sets the transistor oscillator's frequency. It may consist of capacitors and resistors, capacitors and inductors, or an inductor and a quartz crystal. As with tuning forks and pendulums, a resonant network has a natural frequency.

Feedback

A transistor amplifies current. If you connect a transistor amplifier to a resonant network and feed the output back into the amplifier's input, the transistor produces a pure frequency. You must tune such oscillators carefully, however, as too little signal will cause the oscillation to die out, and too much will distort the signal.