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Important Points of Special Purpose Diodes

Zener Diode

  1. It is a specially designed diode which is optimized to operate in breakdown region.
  2. Zener diode is silicon diode.
  3. By varying the doping level silicon diode, a manufacturer can produce zener diodes with breakdown voltage from about 2 to 200V.

Applications of Zener Diodes

  • As voltage stabiliser
  • For meter protection
  • For wave shaping

Varactor Diodes

  1. Varactor diode operates under reverse biased conditions.
  2. Varactors are silicon diodes optimized for their variable capacitance.
  3. These diodes are voltage controlled diode.
  4. These diodes are used in television receiver, FM receivers, automobile radios and other communication equipment.

Light Emitting Diodes (LEDs)

  1. These are made from gallium arsenide phosphide (GaAsP) and Gallium phosphide (GaP).
  2. LEDs have a typical voltage from 1.5V to 2.5V for currents between 10mA to 50mA.
  3. LEDs have longer life, more than 20 years.
  4. They use in panel indicators, digital watches, calculators, multimeters, intercoms etc.

Photo Diodes

  1. It is also known as photodectector.
  2. Photodiode is optimized for its sensitivity to light.
  3. Photo diode operates under reverse biased conditions.

Schottky Diode

  1. Schottky diode is a special purpose diode.
  2. For its construction, a metal like gold, silver or platinum is used on one side of the junction and doped silicon is used on the other side.
  3. Schottky diode can easily rectify signals of frequency above 300MHz without distortion.
  4. These diodes are best suited for low voltage rectification.
  5. Schottky diodes find their best application in these computers because of their fast operation.
  6. These diodes are also used for rectification of high frequency (50MHz to 500MHz) signals such as in digital computers.

Tunnel Diodes

  1. By increasing the doping level, we can get breakdown at 0V, such diodes are known as tunnel diodes.
  2. This diode has negative-resistance region.
  3. The tunnel diode is useful in high-frequency oscillatory circuits.

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Important Points of Semiconductor Physics

  1. A semiconductor is a substance which has resistivity in between conductors and insulators.
  2. Germanium, silicon, selenium, carbon etc. are the example of semiconductor materials.

Properties of semiconductors

  • The resistivity of a semiconductor is less than insulator but more than a conductor.
  • Semiconductors have negative temperature coefficient of resistance.
  • Semiconductors formed by covalent bonds.
  • When a suitable impurity is added to a semiconductor its current conducting properties change appreciably.
  1. The two most frequently used materials are germanium (Ge) and silicon (Si).
  2. Germanium (Ge) and Silicon (Si) are tetravalent.
  3. Germanium (Ge) and Silicon (Si) have forbidden energy gap of 0.72 eV and 1.1 eV.
  4. A vacancy left in the valance band because of lifting of an electron from valance band to conduction band is known as hole.
  5. An extremely pure semiconductor is known as intrinsic semiconductor.
  6. A semiconductor to which an impurity at controlled rate is added to make it conductive is known as an extrinsic semiconductor.
  7. The process by which impurity is added to a semiconductor is known as doping.
  8. Extrinsic semiconductor may be classified as: (i) n-type semiconductor and (ii) p-type semiconductor.

N-type semiconductor

  • When a small amount of pentavalent impurity is added to a pure semiconductor providing a large number of free electrons in it, the extrinsic semiconductor thus formed is known as n-type semiconductor.
  • Arsenic (atomic number 33) and antimony (atomic number 51) are the example of pentavalent impurity.
  • Pentavalent impurities are also known as donor impurities.
  • In n-type semiconductors electrons are majority carriers and holes are minority carriers.

P-type semiconductor

  • When a small amount of trivalent impurity is added to a pure semiconductor providing a large number of holes in it, the extrinsic semiconductor thus formed is known as p-type semiconductor.
  • Gallium (atomic number 31) and indium (atomic number 49) are the example of trivalent impurity.
  • Trivalent impurities are also known as acceptor impurities.
  • In n-type semiconductors holes are majority carriers and electrons are minority carriers.
  1. When a pn junction conducted across an electric supple, the junction is said to be under biasing.
  2. When the positive terminal of a dc source or battery is connected to p-type and negative terminal is connected to n-type of a pn junction the junction is said to be in forward biasing.
  3. When the negative terminal of a dc source or battery is connected to p-type and positive terminal is connected to n-type of a pn junction the junction is said to be in reverse biasing.


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Important points of Alternators

  1. Alternator is a device that converts mechanical power into electrical power.
  2. The basic principle of operation of synchronous generator is electromagnetic induction.
  3. First law of electromagnetic induction  states that whenever a conductor cuts by the magnetic flux or field an emf is induced in the conductor.

Advantage of Stationary Armature

  1. It is easy to insulate stationary winding for high voltage.
  2. Only two slip rings are required for dc supply to the field winding on the rotor.
  3. Solid connections can be obtained on stationary armature.
  4. Due to simple and robust construction of the rotor, higher speed of rotating dc flux is possible.


  1. Stator of the alternator is made of cast iron (for small machine) and fabricated steel (for large machine).
  2. Stator Core is made of laminated silicon steel.
  3. Rotor is divided into two types:- (a) Salient Pole Type and (b) Non-salient Pole type
  4. Salient pole type rotor is used in slow and medium-speed alternators.
  5. Salient pole type rotors have large diameters and short axial length.
  6. Non-salient pole type rotor is used high speed alternators.
  7. Non-salient pole type rotors have small diameters and very long axial length.

Frequency of induced emf

  1. Frequency of the alternator is given by the relation, f = NP/120 Hz.
  2. The frequency of induced emf or current induced in the armature conductors of the alternator depends upon the number of poles and speed of the rotor.

Armature reaction in Alternator

  1. When load power factor is unity, the effect of armature reaction is to distort the main flux.
  2. When load power factor is zero lagging, the effect of armature reaction is to weaken (demagnetize) the main flux.
  3. When load power factor is zero leading, the effect of armature reaction is to strengthen the main flux.

Voltage Regulation

  1. The change in terminal voltage from no-load to full load divided by full load voltage.

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Important Points of Multistage Transistor Amplifiers

A transistor circuit in which a number of amplifier stages are used in succession is called multi-stage or cascaded amplifier.


  1. The ratio of output to the input of an amplifier is called its gain.


  1. The common logarithm of power gain is known bel power gain.

Band Width

  1. The range of frequency over which the gain of an amplifier is equal to or greater than 70.7% of its maximum gain is known as band width.
  2. The frequency of f­1 or f2 ­is also called frequency or half power frequency.

Function of a coupling device

  1. To transfer only ac output of one stage to the input of the next stage.
  2. Another function of coupling is to block dc components and isolate the dc conditions of one stage from the other stage.

RC (Resistance-Capacitance) Coupled Transistor Amplifier

  1. At low frequencies (below 50Hz), the gain is small.
  2. At mid-frequencies (50Hz to 20 KHz) also called audio frequency range, the gain is almost constant.
  3. At high frequencies (above 20 KHz), the voltage gain is reduced.
  4. Light in weight
  5. RC coupled amplifiers provide excellent audio-fidelity over a wide range of frequencies. Therefore, they are widely used as voltage amplifier for example in the initial stage of public address system.

Transformer Coupled Transistor Amplifier

  1. These are also used at the output stage fir amplification because of their excellent impedance matching properties.
  2. Higher gain can be obtained using step-up coupling transformer.
  3. These amplifiers give very poor frequency response.
  4. They produce hum due to nearby power rating.

Direct Coupled Amplifiers

  1. When extremely low frequency (below 10Hz) signals are required to be amplified, direct-coupled transistor amplifiers are employed.
  2. Cheap in cost.
  3. It cannot be used to amplify high frequency signals.
  4. Temperature may shift the operating point.

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