Diodes OCT 19 2022 1 Introduction to Diodes A diode is a two-terminal electronic component that allows current to flow in one direction while blocking it in the reverse direction. This unidirectional behavior makes diodes essential in many electronic circuits. 1.1 Applications of Diodes Diodes have a wide range of applications, such as: • Rectification: Converting AC to DC. • Voltage regulation: Maintaining a stable voltage. • Signal clipping and clamping: Shaping waveforms. • Light emission (LEDs): Producing light in displays and lighting. • Photodetection: Sensing light in devices like solar cells and photodiodes. 2 2.1 PN-Junction Diode Construction and Operation A PN-junction diode is formed by joining p-type and n-type semiconductors. The region where the two materials meet is called the depletion region. 1 2.1.1 Forward Bias When the p-side is connected to the positive terminal of a voltage source and the n-side to the negative, the diode conducts current by reducing the width of the depletion region. 2.1.2 Reverse Bias When the p-side is connected to the negative terminal and the n-side to the positive, the diode blocks current as the depletion region widens. 2.2 I-V Characteristics The relationship between the diode current (I) and the applied voltage (V ) is given by the diode equation: qV I = IS e kT − 1 , where: • IS : Saturation current (reverse leakage current) • q: Charge of an electron (1.6 × 10−19 C) • k: Boltzmann constant (1.38 × 10−23 J/K) • T : Temperature in Kelvin • V : Applied voltage 3 3.1 Applications of Diodes Rectification Diodes are extensively used in rectifiers to convert AC to DC. 3.1.1 Half-Wave Rectifier A half-wave rectifier uses a single diode to allow only one half of the AC waveform to pass through. The output voltage is given by: Vavg = 2 Vpeak . π 3.1.2 Full-Wave Rectifier A full-wave rectifier uses both halves of the AC waveform. It can be implemented using: • A center-tapped transformer and two diodes. • A bridge rectifier with four diodes. The average output voltage for a full-wave rectifier is: 2Vpeak . Vavg = π 3.2 Voltage Regulation Using Zener Diodes Zener diodes operate in reverse breakdown mode to maintain a constant output voltage. They are widely used in voltage regulator circuits. The Zener current IZ is calculated as: Vin − VZ . IZ = R 3.3 3.3.1 Clipping and Clamping Clipping Circuits Clipping circuits limit the output voltage to a specific range using diodes. For example: • Positive clipper: Limits the positive half of the waveform. • Negative clipper: Limits the negative half of the waveform. 3.3.2 Clamping Circuits Clamping circuits shift the DC level of a waveform without altering its shape. Diodes and capacitors are the main components in these circuits. 3.4 Signal Detection and Mixing In communication systems, diodes are used as: • Demodulators to extract signals. • Mixers to combine signals and generate sum/difference frequencies. 3 4 4.1 Practice Problems Problem 1: Half-Wave Rectifier Given an AC input voltage with a peak value of 10 V, calculate the average DC output voltage of a half-wave rectifier. 4.2 Problem 2: Zener Voltage Regulator Design a voltage regulator circuit using a Zener diode with a breakdown voltage of 5 V and a load resistance of 1 kΩ. Assume the input voltage varies between 7 V and 12 V. 4.3 Problem 3: Clipping Circuit Design a clipping circuit that limits the output voltage between −2 V and +2 V using diodes. 4