ANODE | ‘ CATHODE
Circuit symbol of a zener diode
Zener Breakdown!
It is a type of breakdown observed in a reverse biased p- n junction that has very high doping concentration on both-sides of the junction. The build-in-field is high and the depletion layer narrow as a result of high level of doping.
These are studied by plotting IE-VE and IC-VC curves,
(1) IE-VE curves : It show that emitter current,depends on the emitter yoltage and the collector. voltage
has a very little effect on the emitter current and hence transistor has a low input impedance,
2)IC-VC curves : It show that a transistor attains a high collector current at a low collector voltage and
hence the transistor has a high output impedance.
Current gain Alpha ()
It is ratio of the collector current variations to the emitter current variations on no-load and constant
voltages
where, IC = collector current variations
IC = emitter current variations
Note : Both current variations should be measured in
the same units.
(2) Beta (B)
It is ratio of the collector current variations to the base current variations on no load and constant voltages.
where, I, = base current variations. Relation between a and B
Alpha cut-off frequency
The frequency at which gain of a transistor is reduced to 10.1% of its gain at low frequency is called alpha cut-off frequency of the transistor.
Voltage gain of a transistor It is ratio of the output voltage to the input voltage
V, _ Output voltage
VG.= + = Tnput voltage
Power gain of a transistor é It is ratio of the output power to the input power
Output power _ Py
PG-= Toput power P,
Three Basic configurations of Transistors
There are three basic configurations of transistors.
1. Common base (CB) configuration :
It is the transistor circuit in which base is kept common to the input and output circuits.
Characteristics
() Low input impedance (50 to 500 ohms).
(i) High output impedance (1 to 10 mega ohms)
(iii) Current gain alpha = Jess than unity.
2.Common emitter (CE) configuration
It is the transistor circuit is which emitter is kept common to the input and output circuits.
Characteristics of CE
() High input impedance (500 to 5000 ohms)
(i) Low output impedance (50 to 500 kilo ohms)
(ii) Currem gain, fis = upto 98
(iv) Power gain uplo 5000 or 37 dB.
(vy) 180° out of phase output.
3, Common collector circuits (CC) configuration
Itis the transistor circuit in which collector is kept common to the input and output circuits.
It is also called emitter follower
Characteristics
() High input impedance (150 to 600 kilo ohms)
(i). Low output impedance (100 to 1000 ohms)
(ii) Current gain, pi = 99
(iv) Voltage and power gain is equal to or less than unity.
FET (Field Effect Transistor)
It is a solid state device in which conduction of: is controlled by an electrostatic field. Its
are called source, drain and gate.
Transistor Data
It is a reference book which contains information regarding characteristics of tr:
(I) Maximum collector-emitter voltage
(i) Maximum collector current
(iii), Leakage current
(iv) Input output impedances
(v) Current gain and power gain
(vi) Alpha cut off frequency
(vii) Power dissipation
(viii) Working temperature
(ix) Collector-base voltage and cap
(x) Base current ete,
Transistor Manufacture
Many transistors or other elements, their interconnecting links, are made ¢ to turn it into an IC, I usual
method of of monolithic ICs (mono=single, case silicon) has many similarities
A switch is used in an electric circuit as a device for making or breaking the electric circuit. switches may be classified as
1. Oil switches: These are usually use voltage heavy current circuits,
2. Air switches : These are further classified into
(a) Air-break switches
(d) Isolators
(o) Disconnected switches
BUS- BAR ARRANGEMENTS
Bus-bars are arranged to achieve
(a)dequate operating flexibility
(b) sufficient reliability
(c) minimum cost.
The cost can be minimised by reducing the number of circuit breakers to a minimum but complication
of the protective gear are increased. Some bus-bar arrangements.
(1) Single bus-bar arrangement. In this arrangement a set of bus-bars is used for complete power station and to this bus-bar are connected all generators, transformers and feeders through circuit breakers and isolating switches. Such a bus-bar arrangement is cheaper in initial as well as in maintenance cost and simple in operation and relaying.
(2) Single bus- bar system with sectionalization . With increased number of generators and outgoing
feeders connected to the bus-bars, it becomes essential to provide arrangement for sectionalizing the bus-bars so that a fault on any one section of the bus-bars may not cause a complete shutdown. ‘This is achieved by providing a circuit breaker and isolating switches between the sections.
(3) Ring bus-bar system. In this arrangement each feeder is supplied from two paths, so that in case of failure of a section, supply is not interrupted.
(4) Duplicate bus-bars system. Duplicate bus-bar system with sectionalization is usually adopted in order to maintain continuity of supply. Such a system consists of two-bus-bar couplers and sectionalizing breaker converts the duplicate bus- bars into a ring system having greater flexibility.
1. Zener diodes
In zener region, the characteristic d™ potential almost vertical manner at a reverse : conduction denoted V , For Zener diode ,direction of co de. Si is of current is reversed to that of simple it a. Ss preferred in manufacturing of Zener diode higher temperature and current capability.
2. Light emitting diode (L.E.D) : When a junction diode is forward biased, energy is released at junction in the form of light due to recombination of electrons and holes. In case of Si or Ge diodes, the energy ereleased in the infra-red region. In the junction diode made of GaAs, InP etc energy is released in visible region such a junction diode is a light emitting diode (LED). Its symbol
Solar cell : Solar cell is a device for converting solar energy into electrical energy. Ajunction diode in which one of the P or N sections is made very thin (so that the light energy falling on diode is not greatly absorbed before reaching the junction can be used to convert light energy into electric energy such diode is called as sola cell. Its symbol is
3.Tunnel diode.
SOME SPECIAL DIODES
Photodiode : A kimctopm diode made from “light or photo sensitive semiconductor’ is called photo diode.
Its sysmbol is SS When light of energy “hv” falls on the photodiode (here hv > energy gap) more electrons move from valence band to conduction band, and due to this current in circuit of photodiode in reverse bias, increases. As light intensity is increased, the current goes on increasing so photodiode : photo diode is used, to detect light intensity.
Diode symbole.
(1) Permanent magnet
(2.)Self-excited
(3) Separately excited,
MOTOR
It is a machine which converts mechanical energy, into mechanical energy.
1) n-type semiconductors
semiconductors are formed elements from Vth group ¢ As and Sb. Addition of a percent of such elements may increase the conductivity by several powers of ten. Ag the impurity increases the ionization energy decreases.
(2) p-type semiconductor p-type
semiconductors are formed by doping with element from the third group, ie. B, Al, Ba, In. At absolute zero, the holes remains bound to the impurities, but as the temperature is increased, valence electrons may be excited into the bound holes with the result that holes are created in the valence band. At low temperature conductivity results predominantly from holes produced by excitation of valence electrons into acceptor levels.
For silicon and germanium, this energy is given by, Eg = 1.21-3.6 x 10+ x T eV (for Silicon)
Eg = 0.785 — 2.23 x 104 x T eV (for Germanium)
where, T = absolute temperature in °K Assuming room temperature to be 27° C,
i.e. 300 °K, the forbidden gap energy for Siand Ge can be calculated from the above equations. The
forbidden gap for the germanium is 0.72 eV while for the silicon it is 1.12 eV at room temperature. The
silicon and germanium are the two widely used semiconductor materials in electronic devices. At room
temperature, the materials classified as semiconductors have a resistivity between that of a typical metals
and that of a typical insulators, However resistivity of semiconductors in general depends strongly on
temperature. This classification is not satisfactory because at very low temperature, semiconductors may
behave as insulators,
The forbidden gap is very wide, approximately of about 7 eV is present in insulators. For a diamond, which is an insulator, the forbidden gap is about 6 eV. Such materials may conduct only at very high temperatures or if they are subjected to high voltage. Such a conduction is rare and is called breakdown of an insulator.
