Sketch the output Y from NAND gate having input A and B given below-

For NAND gate the Boolean expression is A . B =y For the given wave from, we have the following values for A, B and y. For time t<t1 A=1,B=1 hence y=0 For time t1 to t2 A=0,B=1 hence y =1 For time t2 to t3 A=0,B=1 hence y =1 For time t3 to t4 A=1,B=0 hence y =1 For time t4 to t5 A=1,B=1 hence y =0 For time t5 to t6 A=0,B=0 hence y =1 For time t<t6 A=0,B=1 hence y =1 The wave from for output y is as given in Fig.

Drawing the circuit diagram, explain the working of p - n junction diode as a full wave rectifier.

Working: The a.c input signal is fed to the primary (P) coil of the transformer. The p-region of both the diodes D1 and D2 are connected to two ends of secondary coil (S). The load resistance (RL) across which output voltage is obtained is connected between common point of n-regions of diodes and central topping of the secondary coil. When positive half cycle of input a.c signal flows through the primary coil, induced emf is set up in the secondary coil due to mutual induction. The direction of induced emf is such that the upper end of secondary coil becomes positive while lower end becomes negative. Thus, D1 diode is forward biased and D2 is reverse biased. So, current through D1 flows through circuit in a direction shown. During negative half cycle of input a.c signal, diode D1 is reverse biased and D2 is forward biased. The current due to D2 flows through circuit in a direction shown. Since, both the halves of input a.c (wave) are rectified, so the junction diode is called Full-wave rectifier. Input and Output waveforms are as shown:

Drawing circuit diagram, explain the role of Zener diode as a voltage regulator.

A Zener diode is a diode, which works in a breakdown region, and it is used in the reversed biased condition. Let us draw a reverse characteristic of the Zener diode for a better understanding. As we can see that for the range I Z m i n t o I z m a x Zener voltage is approximately constant and this region for which zener voltage is constant is the breakdown region. I Z m i n is also called knee voltage, or we can say it is the minimum value of zener current whereas I Z m a x is the maximum value of zener current beyond this value the zener diode will burst. Therefore, zener diode is used within the range of I Z m i n t o I z m a x . Now, as Zener voltage remains the same for the range I Z m i n t o I z m a x It can be used as a voltage regulator when it is operated in the breakdown region. Because the work of a voltage regulator is to keep the output voltage the same for varying input. To understand how Zener diode works as a voltage regulator let us draw a circuit diagram for the voltage regulator using Zener diode. Here we can see from the diagram, there are two resistors R and RL Where R is connected in series to absorb all the fluctuation of the output voltage and RL is the load resistance across which output voltage is obtained. VZ is the Zener voltage and I 1 is the current flowing through the circuit which divides into I Z and I 2 . I z is the Zener current flowing through the Zener diode and I2 is the current flowing through load RL. Now if we apply Kirchoff's current law at node A we can write I 1 = I z + I 2 Now there can be two cases when the source is varying or input is varying and when the load resistance is varying. Let’s take the first case where input is varying, in this case, the extra voltage or the fluctuation is absorbed by series resistance R due to which total current or I 1 will increase, but this increase is balanced by Zener current I z and so I 2 doesn’t change resulting constant output voltage across load resistance. In the second case that is when load resistance changes and input is fixed then there should be an increase in load current I 2 . As input is constant I 1 remains the same and the change is compensated by Zener current I z . For example, if load resistance decreases then load current should increase so that output remains the same, therefore Zener current decreases to provide the amount of current required at load resistance to keep the voltage constant (because total current I 1 remains constant). Hence, this way Zener diodes can be used as a voltage regulator.