State the principle of working of p-n diode as a rectifier.Explain, with the help of a circuit diagram, the use of p-n diode as a full wave rectifier. Draw a sketch of the input and output waveforms.

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Draw the symbolic representation of a (i) p- n- p, (ii) n- p- n transistor.Why is the base region of transistor thin and lightly doped?With proper circuit diagram, show the biasing of a p- n- p transistor in common base configuration. Explain the movement of charge carriers through different parts of the transistor in such a configuration and show that .

Principle: A junction diode offers a low resistance to current in one direction(when forward biased) and a high resistance in the other direction(when reverse biased). Thus, the diode acts as a rectifier.

Full wave rectifier:

When the diode rectifies the whole of the AC wave, it is called full wave rectifier.

The figure shows the arrangement for using diode as full wave rectifier. The alternating input signal is fed to the primary P₁P₂ of a transformer. The output signal appears across the load resistance R_L.

During the positive half of the input signal, suppose P₁ and P₂ are negative and positive respectively. This would mean that S₁ and S₂ are positive and negative respectively. Therefore, the diode D₁ is forward biased and D₂ is reverse biased. The flow of current in the load resistance R_L is from A to B.

During the negative half of the input signal, S₁ and S₂ are negative and positive respectively. Therefore, the diode D₁ is reverse biased and D₂ is forward biased. The flow of current in the load resistance R_L is from A to B.

 

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The symbols of p-n -p and n- p -n transistors are given below.

The base region is made thin and lightly doped so that it contains very few majority charge carriers.This reduces the recombination rate of electrons and holes across the emitter-base junction.This increases collector current and hence increases current gain of the transistor.

Action of p- n- p transistor: The emitter base junction is forward biased by battery and collector-base junction is reverse biased by battery.

The positive terminal of the battery repels the holes of the emitter towards the base. About 5% of the holes recombine with the electrons of the base. The remaining 95% of the holes enter the collector region.As the holes reach the end of the collector region, they attract the electrons from the negative terminal of the battery and combine with them. At the same time covalent bonds are broken in the emitter region, and an equal number of electrons get attracted towards the positive terminal of and holes flow towards the base.Hence