What is rectification? Explain the working of a bridge rectifier.

Process in which alternating voltage or alternating current is converted into direct voltage or direct current is known as rectification. The device used for this process is called rectifier. Working of a Bridge rectifier : Bridge rectifier is shown in figure. There are four diodes D1,D2,D3 and D4 used in the circuit, which are connected to form a network. The input ends A and C of the network are connected to the secondary ends S1 and S2 of the transformer. The output ends B and D are connected to the load resistance RL. During positive input half cycle of the arc voltage, the point A is positive with respect to C. The diodes D1 and D3 are forward biased and conduct, whereas the diodes D2 and D4 are reverse biased and do not conduct. Hence, current flows along S1ABCDS2 through RL. During negative half cycle, the point C is positive with respect to A. The diodes D2 and D4 are forward biased and conduct, whereas the diodes D1 and D3 are reverse biased and they do not conduct. Hence, current flows along S2CBADS1 through RL. The same process is repeated for subsequent half cycles. It can be seen that current flows through RL in the same direction, during both half cycles of the input a.c. signals. The output signal corresponding to the input signal is shown in figure. The efficiency of the bridge rectifier is approximately 81.2%. Advantages of bridge rectifier : (i) Centre tap on the secondary of the transformer is not necessary, (ii) Smaller transformer can be used and (iii) It is suited for high voltage applications.

Deduce the relation between the current amplification factors α and β of a transistor.

The relation between the current amplification factors α and β of a transistor. The current amplification factor or current gain of a transistor is the ratio of output current to the input current. If the transistor is connected on base mode the current gain α= I C I E and if the transistor is connected in common emitter mode, the current gain α= I C I B . Figure shows a NPN transistor connected in the common base and common emitter configurations. Since, 95% of the injected electrons reach the collector, the collector current is almost equal to the emitter current. Almost all transistors have a, in the range 0.95 to 0.99. We know that α= I C I E =IB+ I C I B + I C (∵ I E = I g + I C ) 1 α = I B + I C I C = I B I C + 1 1 α − 1 = 1 β ∴ β = α 1 − α Usually β lies between 50 and 300. Some transistors have β as high as 1000.

Draw the circuit diagram of OR-gate using diodes.

Let the load resistance to be bulb Case(i) A = 0 and B = 0 . If A and B both are open then there is no current in Y, hence bulb is not lighted. In mathematical form A = 0 , B = 0 then Y = A + B = 0 . Case(ii) A = 1 and B = 0 . If switch A is closed (A = 1) and B is open (B = 0) then battery will flow the current through switch A and light the bulb Y, hence Y = 1 . ∴ A = 1 , B = 0 then Y = A + B = 1 . Case(iii) A = 0 and B = 1 . If A is open and B is closed then also bulb will be lighted. ∴ A = 0 , B = 1 , Y = A + B = 1 . Case(iv) A = 1 and B = 1 . If A and B both are closed ( A = B = 1 ) then bulb will be lighted ( Y = 1 ) . ∴ A = B = 1 , Y = A + B = 1 . Thus, the output will be high only when the inputs A and B are high. The Table summaries the function of an OR gate.

What are the advantages of negative feedback amplifier?

The advantages of negative feedback amplifier are as follows: • The negative feedback amplifier reduces noise • It has less frequency distortion means improved frequency response • Better stabilised voltage gain