AIML I-SEM: PN Junction Diode and Rectifiers

PN Junction Diode

• A PN junction diode is formed by combining p-type and n-type regions.
• Due to doping, electrons are present in the n-region, and holes are present in the p-region.
• When p and n regions are joined, electrons and holes diffuse, and ions are formed.
• The depletion layer, also known as the barrier potential or built-in potential, contains thermally generated ions.
• The built-in potential values for silicon and germanium are 0.6-0.7V and 0.3V, respectively.

Forward and Reverse Bias

• In forward bias, electrons from the n-type region move towards the p-type region, and holes from the p-type region move towards the n-type region.
• The barrier potential is reduced, allowing current to flow.
• In reverse bias, electrons from the n-type region move away from the junction, and holes from the p-type region move away from the junction.
• The depletion layer widens, and current is low.

PN Junction Diode Characteristics

• The forward voltage at which the diode starts allowing large electric current is called the cut-in voltage or knee voltage.
• The cut-in voltage for silicon and germanium diodes are approximately 0.7V and 0.3V, respectively.
• The dynamic resistance of a diode is the ratio of the small change in voltage to the resulting change in current.

Applications of PN Junction Diode

• Rectifiers in DC power supplies
• Switch in digital logic circuits
• Clipping circuits
• Demodulation circuits
• Detectors in optical communication circuits

Zener Diode

• A zener diode is a type of diode that permits current to flow in both forward and reverse directions.
• The zener diode is normally operated in reverse breakdown, and the current direction is from anode to cathode.
• Zener diodes are highly doped diodes.
• The breakdown voltage is known as the "Zener knee voltage" or "Zener voltage."

Zener Diode Characteristics

• The forward characteristics of a zener diode are similar to those of an ordinary forward-biased junction diode.
• In reverse direction, there is a very small leakage current between 0V and the Zener voltage.
• When the voltage reaches the breakdown voltage, the current flow is dramatic.

Operation of Zener Diode

• In forward bias, the operation is similar to that of an ordinary PN junction diode.
• In reverse bias, the breakdown depends on the amount of doping.
• There are two types of breakdowns: Zener breakdown and avalanche breakdown.

Breakdowns

• Zener breakdown occurs in heavily doped diodes, with a smaller depletion region and lower breakdown voltage.
• Avalanche breakdown occurs in lightly doped diodes, with a larger depletion region and higher breakdown voltage.

Applications of Zener Diode

• Zener regulators
• Zener comparators
• Zener limiters
• Zener in power supplies

Zener Diode Regulator

• A zener diode is always operated in its reverse-biased condition.
• The voltage regulator circuit can be designed using a zener diode to maintain a constant DC output voltage.
• The output voltage is selected to be the same as the breakdown voltage of the diode.

Basic Building Block of LMPS

• Transformer: inputs AC voltage at a required voltage level.
• Rectifier: converts AC to pulsating DC.
• Filter: used to get a pure DC voltage.
• Regulator: gives a steady DC output independent of load variations and input supply fluctuations.

Rectifiers

• Electronic devices used for converting AC voltage into unidirectional voltage.
• Types of rectifiers: half-wave rectifier, full-wave rectifier.

Half-Wave Rectifier

• Operation:
  • During the positive half cycle, the diode is forward-biased, and current flows through the load resistor.
  • During the negative half cycle, the diode is reverse-biased, and there is no current flow through the load resistor.

Full-Wave Rectifier

• A center-tapped full-wave rectifier uses a center-tapped transformer and two diodes to convert the complete AC signal into a DC signal.
• Operation:
  • During the positive half cycle, D1 is forward-biased, and D2 is reverse-biased.
  • During the negative half cycle, D2 is forward-biased, and D1 is reverse-biased.

Filters

• Used to minimize the undesirable AC component in the output of a rectifier.
• Types of filters: inductor filter, capacitor filter, LC filter, and CLC filter.

Inductor Filter

• When the output of the rectifier passes through the inductor filter, it blocks the AC component and allows the DC component to pass through the load.
• The output of the filter can be analyzed using Fourier series.

Capacitor Filter

• Used for light loads and connected directly across the load.
• The capacitor filter takes in the ripple and shorts it to the ground, while the DC component appears at the load.

CRO (Cathode Ray Oscilloscope)

• Gives a visual representation of time-varying signals.
• Used as an electronic test equipment in laboratories.
• The CRO plays an important role in electronic circuits.

Block Diagram of CRO

• Major subsystems: cathode ray tube (CRT), vertical amplifier, horizontal amplifier, sweep generator, and power supplies.

Applications of CRO

• Voltage measurement
• Measurement of phase
• Measurement of frequency
• Display of different types of waveforms
• Measurement of short time intervals
• Used in voltmeter to measure potential difference