a) Write the symbol and truth table of NAND gate. [2] b) Explain the full wave rectification using two P N junction diodes. [3]

a) Symbol and Truth Table of NAND Gate

The NAND gate is a universal logic gate whose output is the negation of the AND operation. Its symbol is shown above.

Truth Table:

---

b) Full Wave Rectification Using Two P-N Junction Diodes

Full wave rectification converts both halves of an AC input signal into a unidirectional (DC) output. It uses two P-N junction diodes arranged in a center-tapped transformer or bridge rectifier configuration. Here, we explain the center-tapped transformer method.

Circuit Operation:

1. Transformer Setup:

   • A step-down transformer provides two equal secondary voltages, $V_{in}/2$, with respect to the center tap.
   • The center tap acts as the reference point (ground).

2. Diode Arrangement:

   • Two diodes, $D_1$ and $D_2$, are connected to the two ends of the secondary winding.
   • The load resistor $R_L$ is connected between the common point of the diodes and the center tap.

3. Working Principle:

   • Positive Half-Cycle:
     • The upper end of the transformer becomes positive, and the lower end becomes negative.
     • Diode $D_1$ (anode positive, cathode negative) conducts, while $D_2$ is reverse-biased.
     • Current flows through $D_1$, the load $R_L$, and back to the center tap.
   • Negative Half-Cycle:
     • The upper end becomes negative, and the lower end becomes positive.
     • Diode $D_2$ conducts, while $D_1$ is reverse-biased.
     • Current flows through $D_2$, the load $R_L$, and back to the center tap.

Output Waveform:

• The output across $R_L$ is a full-wave rectified signal, meaning both halves of the input AC waveform appear as positive pulses.
• The peak output voltage is approximately $V_{in,peak}/2-V_d$ (where $V_d$ is the diode drop, ~0.7 V for silicon diodes).

Advantages:

• Higher DC output voltage compared to half-wave rectification.
• Better utilization of the transformer secondary winding.

Disadvantages:

• Requires a center-tapped transformer.
• Higher ripple frequency (easier filtering).