- Information
- AI Chat
AEC Digital Notes
B.tech(IT), B.Tech(CSE)
Aryabhatta Knowledge University
Recommended for you
Preview text
ANALOG ELECTRONICS
II B I SEMESTER
FOR EEE
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING
MALLA REDDY COLLEGE OF ENGINEERING & TECHNOLOGY
####### Autonomous Institution – UGC, Govt. of India
(Affiliated to JNTU, Hyderabad, Approved by AICTE - Accredited by NBA & NAAC – 8A9 Grade - ISO 9001:2008 Certified) Maisammaguda, Dhulapally (Post Via Hakimpet), Secunderabad – 500100 PREPARED BY Dr.S RAO, Mr K LINGAM, Mr R RAO, Mr E REDDY Mr V SHIVA RAJKUMAR, MR KLN PRASAD, MR M ANANTHA GUPTHA,
(R18A0401) ANALOG ELECTRONICS OBJECTIVES This is a fundamental course, basic knowledge of which is required by all the circuit branch engineers .this course focuses:
- To familiarize the student with the principal of operation, analysis and design of junction diode .BJT and FET transistors and amplifier circuits.
- To understand diode as a rectifier.
- To study basic principal of filter of circuits and various types UNIT-I P-N Junction diode : Qualitative Theory of P-N Junction, P-N Junction as a diode , diode equation , volt- amper characteristics temperature dependence of V-I characteristic , ideal versus practical –resistance levels( static and dynamic), transition and diffusion capacitances, diode equivalent circuits, load line analysis ,breakdown mechanisms in semiconductor diodes , zener diode characteristics. Special purpose electronic devices: Principal of operation and Characteristics of Tunnel Diode with the help of energy band diagrams, Varactar Diode, SCR and photo diode UNIT-II RECTIFIERS, FILTERS: P-N Junction as a rectifier ,Half wave rectifier, , full wave rectifier, Bridge rectifier , Harmonic components in a rectifier circuit, Inductor filter, Capacitor filter, L- section filter, - section filter and comparison of various filter circuits, Voltage regulation using zener diode. UNIT-III BIPOLAR JUNCTION TRANSISTOR : The Junction transistor, Transistor current components, Transistor as an amplifier, Transistor construction, Input and Output characteristics of transistor in Common Base, Common Emitter, and Common collector configurations. α and β Parameters and the relation between them, BJT Specifications. BJT Hybrid Model, h-parameter representation of a transistor, Analysis of single stage transistor amplifier using h-parameters: voltage gain, current gain, Input impedance and Output impedance. Comparison of transistor configurations in terms of Ai, Ri ,Av,and Ro, UNIT-IV TRANSISTOR BIASING AND STABILISATION: Operating point , the D and A Load lines, Need for biasing , criteria for fixing, operating point, B.J biasing, Fixed bias, Collector to base bias ,Self bias techniques for stabilization, Stabilization factors, (s, sI, sII), Bias Compensation using diode and transistor , (Compensation against variation in VBE, ICO,) Thermal run away, Condition for Thermal stability. UNIT-V FIELD EFFECT TRANSISTOR AND FET AMPLIFIER JFET (Construction, principal of Operation and Volt –Ampere characteristics). Pinch- off voltage-Small signal model of JFET. FET as Voltage variable resistor, Comparison of BJT and FET. MOSFET (Construction, principal of Operation and symbol), MOSFET characteristics in Enhancement and Depletion modes. FET Amplifiers : FET Common source Amplifier, Common Drain Amplifier, Generalized FET Amplifier, FET biasing.
TEXT BOOKS:
- Integrated Electronics Analog Digital Circuits, Jacob Millman and D. Halkias, McGraw Hill.
- Electronic Devices and Circuits Theory, Boylsted, Prentice Hall Publications.
- Electronic Devices and Circuits, S,N kumar, McGraw Hill.
- Electronic Devices and Circuits,Balbir kumar ,shail b, PHI Privated Limted, Delhi.
o
VB
Forbidden band gap Eo j6eV
CB
VB
Eo =j6eV
CB CB
VB
UNIT-I
PN JUNCTION DIODE
####### INTRODUCTON
Based on the electrical conductivity all the materials in nature are classified as insulators, semiconductors, and conductors.
Insulator : An insulator is a material that offers a very low level (or negligible) of conductivity when voltage is applied. Eg: Paper, Mica, glass, quartz. Typical resistivity level of an insulator is of the order of 10 10 to 10 12 '-cm. The energy band structure of an insulator is shown in the fig.1. Band structure of a material defines the band of energy levels that an electron can occupy. Valance band is the range of electron energy where the electron remain bended too the atom and do not contribute to the electric current. Conduction bend is the range of electron energies higher than valance band where electrons are free to accelerate under the influence of external voltage source resulting in the flow of charge. The energy band between the valance band and conduction band is called as forbidden band gap. It is the energy required by an electron to move from balance band to conduction band i. the energy required for a valance electron to become a free electron. 1 eV = 1 x 10-19 J For an insulator, as shown in the fig.1 there is a large forbidden band gap of greater than 5Ev. Because of this large gap there a very few electrons in the CB and hence the conductivity of insulator is poor. Even an increase in temperature or applied electric field is insufficient to transfer electrons from VB to CB.
Insulator Semiconductor Conductor FiG:1 Energy band diagrams insulator, semiconductor and conductor
Conductors: A conductor is a material which supports a generous flow of charge when a voltage is applied across its terminals. i. it has very high conductivity. Eg: Copper, Aluminum, Silver, Gold. The resistivity of a conductor is in the order of 10-4 and 10-6 '-cm. The Valance and conduction bands overlap (fig1) and there is no energy gap for the electrons to move from valance band to conduction band. This implies that there are free electrons in CB even at absolute zero temperature (0K). Therefore at room temperature when electric field is applied large current flows through the conductor.
Semiconductor : A semiconductor is a material that has its conductivity somewhere between the insulator and conductor. The resistivity level is in the range of 10 and 10 4 '-cm. Two of the most commonly used are Silicon (Si=14 atomic no.) and germanium (Ge=32 atomic no.). Both have 4 valance electrons. The forbidden band gap is in the order of 1eV. For eg., the band gap energy for Si, Ge and GaAs is 1, 0 and 1 eV, respectively at absolute zero temperature (0K). At 0K and at low temperatures, the valance band electrons do not have sufficient energy to move from V to CB. Thus semiconductors act a insulators at 0K. as the temperature increases, a large number of valance electrons acquire sufficient energy to leave the VB, cross the forbidden bandgap and reach CB. These are now free electrons as they can move freely under the influence of electric field. At room temperature there are sufficient electrons in the CB and hence the semiconductor is capable of conducting some current at room temperature. Inversely related to the conductivity of a material is its resistance to the flow of charge or current. Typical resistivity values for various materials9 are given as follows.
Insulator Semiconductor Conductor 10 -6 '-cm (Cu) 50'-cm (Ge) 1012 '-cm (mica)
50x10 3 '-cm (Si)
Typical resistivity values
Semiconductor Types
A pure form of semiconductors is called as intrinsic semiconductor. Conduction in intrinsic sc is either due to thermal excitation or crystal defects. Si and Ge are the two most important semiconductors used. Other examples include Gallium arsenide GaAs, Indium Antimonide (InSb) etc.
The absence of electrons in covalent bond is represented by a small circle usually referred to as hole which is of positive charge. Even a hole serves as carrier of electricity in a manner similar to that of free electron.
The mechanism by which a hole contributes to conductivity is explained as follows:
When a bond is in complete so that a hole exists, it is relatively easy for a valance electron in the neighboring atom to leave its covalent bond to fill this hole. An electron moving from a bond to fill a hole moves in a direction opposite to that of the electron. This hole, in its new position may now be filled by an electron from another covalent bond and the hole will correspondingly move one more step in the direction opposite to the motion of electron. Here we have a mechanism for conduction of electricity which does not involve free electrons. This phenomenon is illustrated in fig1.
Electron movement Hole movement
Fig. 1
Fig. 1 Fig. 1
Fig 1 show that there is a hole at ion 6 that an electron from ion 5 moves into the hole at ion 6 so that the configuration of 1 results. If we compare both fig1 &fig 1, it appears as if the hole has moved towards the left from ion6 to ion 5. Further if we compare fig 1 and fig 1, the hole moves from ion5 to ion 4. This discussion indicates the motion of hole is in a direction opposite to that of motion of electron. Hence we consider holes as physical entities whose movement constitutes flow of current.
In a pure semiconductor, the number of holes is equal to the number of free electrons.
EXTRINSIC SEMICONDUCTOR
Intrinsic semiconductor has very limited applications as they conduct very small amounts of current at room temperature. The current conduction capability of intrinsic semiconductor can be increased significantly by adding a small amounts impurity to the intrinsic semiconductor. By adding impurities it becomes impure or extrinsic semiconductor. This process of adding impurities is called as doping. The amount of impurity added is 1 part in 10 6 atoms.
N type semiconductor: If the added impurity is a pentavalent atom then the resultant semiconductor is called N-type semiconductor. Examples of pentavalent impurities are Phosphorus, Arsenic, Bismuth, Antimony etc.
A pentavalent impurity has five valance electrons. Fig 1 shows the crystal structure of N-type semiconductor material where four out of five valance electrons of the impurity atom(antimony) forms covalent bond with the four intrinsic semiconductor atoms. The fifth electron is loosely bound to the impurity atom. This loosely bound electron can be easily
Fig. 1 crystal structure of N type SC
Fifth valance electron of SB
Donor energy level
Fig. 1 band diagram of N type
Ec Ed
Ev VB
CB
Thus in P type sc , holes are majority carriers and electrons are minority carriers. Since each trivalent impurity atoms are capable accepting an electron, these are called as acceptor atoms. The following fig 1 shows the pictorial representation of P type sc
hole (majority carrier)
Electron (minority carrier)
Acceptor atoms
Fig. 1 crystal structure of P type sc
The conductivity of N type sc is greater than that of P type sc as the mobility of electron is greater than that of hole.
For the same level of doping in N type sc and P type sc, the conductivity of an Ntype sc is around twice that of a P type sc
CONDUCTIVITY OF SEMICONDUCTOR
In a pure sc, the no. of holes is equal to the no. of electrons. Thermal agitation continue to produce new electron- hole pairs and the electron hole pairs disappear because of recombination. with each electron hole pair created , two charge carrying particles are formed. One is negative which is a free electron with mobility μn. The other is a positive i., hole with mobility μp. The electrons and hole move in opppsitte direction in a an electric field E, but since they are of opposite sign, the current due to each is in the same direction. Hence the total current density J within the intrinsic sc is given by
J = Jn + Jp
=q n μn E + q p μp E
= (n μn + p μp)qE
=ς E
Where n=no. of electrons / unit volume i., concentration of free electrons
P= no. of holes / unit volume i., concentration of holes
E=applied electric field strength, V/m
q= charge of electron or hole I n Coulombs
i GO
i
Hence, ς is the conductivity of sc which is equal to (n μn + p μp)q. he resistivity of sc is reciprocal of conductivity.
¤ = 1/ ς
It is evident from the above equation that current density with in a sc is directly proportional to applied electric field E.
For pure sc, n=p= ni where ni = intrinsic concentration. The value of ni is given by
n 2 =AT 3 exp (-E /KT)
therefore, J= ni ( μn + μp) q E
Hence conductivity in intrinsic sc is ςi= ni ( μn + μp) q
Intrinsic conductivity increases at the rate of 5% per o C for Ge and 7% per o C for Si.
Conductivity in extrinsic sc (N Type and P Type):
The conductivity of intrinsic sc is given by ςi= ni ( μn + μp) q = (n μn + p μp)q
For N type , n>>p
Therefore ς= q n μn
For P type ,p>>n
Therefore ς= q p μp
CHARGE DENSITIES IN P TYPE AND N TYPE SEMICONDUCTOR:
Mass Action Law:
Under thermal equilibrium for any semiconductor, the product of the no. of holes and the concentration of electrons is constant and is independent of amount of donor and acceptor impurity doping.
n= n 2
where n= eleetron concentration
p = hole concentration
ni 2 = intrinsic concentration
i
i A p A
Mass action law for P type, np pp= n 2
np= n 2 / N since (p j N )
####### QUANTITATIVE THEORY OF PN JUNCTION DIODE
PN JUNCTION WITH NO APPLIED VOLTAGE OR OPEN CIRCUIT CONDITION:
In a piece of sc, if one half is doped by p type impurity and the other half is doped by n type impurity, a PN junction is formed. The plane dividing the two halves or zones is called PN junction. As shown in the fig the n type material has high concentration of free electrons, while p type material has high concentration of holes. Therefore at the junction there is a tendency of free electrons to diffuse over to the P side and the holes to the N side. This process is called diffusion. As the free electrons move across the junction from N type to P type, the donor atoms become positively charged. Hence a positive charge is built on the N-side of the junction. The free electrons that cross the junction uncover the negative acceptor ions by filing the holes. Therefore a negative charge is developed on the p –side of the junction. net negative charge on the p side prevents further diffusion of electrons into the p side. Similarly the net positive charge on the N side repels the hole crossing from p side to N side. Thus a barrier sis set up near the junction which prevents the further movement of charge carriers i. electrons and holes. As a consequence of induced electric field across the depletion layer, an electrostatic potential difference is established between P and N regions, which are called the potential barrier, junction barrier, diffusion potential or contact potential, Vo. The magnitude of the contact potential Vo varies with doping levels and temperature. Vo is 0 for Ge and 0 V for Si.
Fig 1: Symbol of PN Junction Diode
The electrostatic field across the junction caused by the positively charged N-Type region tends to drive the holes away from the junction and negatively charged p type regions tend to drive the electrons away from the junction. The majority holes diffusing out of the P region leave behind negatively charged acceptor atoms bound to the lattice, thus exposing a negatives pace charge in a previously neutral region. Similarly electrons diffusing from the N region expose positively ionized donor atoms and a double space charge builds up at the junction as shown in the fig. 1
Fig 1
It is noticed that the space charge layers are of opposite sign to the majority carriers diffusing into them, which tends to reduce the diffusion rate. Thus the double space of the layer causes an electric field to be set up across the junction directed from N to P regions, which is in such a direction to inhibit the diffusion of majority electrons and holes as illustrated in fig 1. The shape of the charge density, Ä, depends upon how diode id doped. Thus the junction region is depleted of mobile charge carriers. Hence it is called depletion layer, space region, and transition region. The depletion region is of the order of 0μm thick. There are no mobile carriers in this narrow depletion region. Hence no current flows across the junction and the system is in equilibrium. To the left of this depletion layer, the carrier concentration is p= NA and to its right it is n= ND.
called the "knee" on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.
Forward Characteristics Curve for a Junction Diode
Fig 1: Diode Forward Characteristics
The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the "knee" point.
Forward Biased Junction Diode showing a Reduction in the Depletion Layer
Fig 1: Diode Forward Bias
Reverse Biased Junction Diode showing an Increase in the Depletion
This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0 for germanium and approximately 0 for silicon junction diodes. Since the diode can conduct "infinite" current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.
1.1 PN JUNCTION UNDER REVERSE BIAS CONDITION:
Reverse Biased Junction Diode
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N- type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode. The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.
Fig 1: Diode Reverse Bias
This condition represents a high resistance value to the PN junction and practically zero current flows through the junction diode with an increase in bias voltage. However, a very small leakage current does flow through the junction which can be measured in microamperes, (μA). One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will
ø = empirical constant, 1 for Ge and 2 for Si
Fig 1: Diode Characteristics
Temperature Effects on Diode
Temperature can have a marked effect on the characteristics of a silicon semiconductor diode
as shown in Fig. 11 It has been found experimentally that the reverse saturation current Io will just
about double in magnitude for every 10∞C increase in temperature.
Fig 1 Variation in Diode Characteristics with temperature change
It is not uncommon for a germanium diode with an Io in the order of 1 or 2 A at 25∞C to have a leakage current of 100 A - 0 mA at a temperature of 100∞C. Typical values of Io for silicon are much lower than that of germanium for similar power and current levels. The result is that even at high temperatures the levels of Io for silicon diodes do not reach the same high levels obtained. For germanium—a very important reason that silicon devices enjoy a significantly higher level of development and utilization in design. Fundamentally, the open-circuit equivalent in the reverse bias region is better realized at any temperature with silicon than with germanium. The increasing levels of Io with temperature account for the lower levels of threshold voltage, as shown in Fig. 1. Simply increase the level of Io in and not rise in diode current. Of course, the level of TK also will be increase, but the increasing level of Io will overpower the smaller percent change in TK. As the temperature increases the forward characteristics are actually becoming more <ideal,=
####### IDEAL VERSUS PRACTICAL RESISTANCE LEVELS
DC or Static Resistance
The application of a dc voltage to a circuit containing a semiconductor diode will result in an
operating point on the characteristic curve that will not change with time. The resistance of the diode
at the operating point can be found simply by finding the corresponding levels of VD and ID as shown
in Fig. 1 and applying the following Equation:
The dc resistance levels at the knee and below will be greater than the resistance levels obtained for
the vertical rise section of the characteristics. The resistance levels in the reverse-bias region will
naturally be quite high. Since ohmmeters typically employ a relatively constant-current source, the
resistance determined will be at a preset current level (typically, a few mill amperes).
AEC Digital Notes
Course: B.tech(IT), B.Tech(CSE)
University: Aryabhatta Knowledge University
- Discover more from:
