Electronic Fundamentals Module-4 EASA Part-66

ELECTRONICS FUNDAMENTALS

for Aircraft Engineeers

EASA Part-66 Cat. B1,B2

ELECTRONIC FUNDAMENTALS

Shahzad Khalil EASA part-66-B1, B2

Electronic Fundamentals Page level

B1 B2

4.1 Semiconductors 4.1.1 Diodes

(a) Diode symbols; Diode characteristics and properties; Diodes in series and parallel; Main characteristics and use of silicon controlled rectifiers (thyristors), light emitting diode, photo conductive diode, varistor, rectifier diodes; Functional testing of diodes.

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(b) Materials, electron configuration, electrical properties; P and N type materials: effects of impurities on

conduction, majority and minority characters; PN junction in a semiconductor, development of a potential

across a PN junction in unbiased, forward biased and reverse biased conditions;

Diode parameters: peak inverse voltage, maximum forward current, temperature, frequency, leakage current, power dissipation; Operation and function of diodes in the following circuits: clippers, clampers, full and half wave rectifiers, bridge

rectifiers, voltage doublers and triplers;

Detailed operation and characteristics of the following devices: silicon controlled rectifier (thyristor), light emitting diode, Schottky diode, photo conductive diode, varactor diode, varistor, rectifier diodes, Zener diode.

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4.1.2 Transistors

(a) Transistor symbols; Component description and orientation;

Transistor characteristics and properties.

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(b) Construction and operation of PNP and NPN transistors; Base, collector and emitter configurations; Testing of transistors. Basic appreciation of other transistor types and their uses. Application of transistors: classes of amplifier (A, B, C); Simple circuits including: bias, decoupling, feedback and stabilisation; Multistage circuit principles: cascades, push-pull, oscillators, multivibrators, flip-flop circuits.

Electronic Fundamentals Page Level

B1 B2

4.1.3 Integrated Circuits

(a) Description and operation of logic circuits and linear circuits/operational amplifiers.

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(b) Description and operation of logic circuits and linear circuits; Introduction to operation and function of an operational amplifier used as: integrator, differentiator, voltage follower, comparator; Operation and amplifier stages connecting methods: resistive capacitive, inductive (transformer), inductive resistive (IR), direct; Advantages and disadvantages of positive and negative feedback.

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4.2 Printed Circuit Boards

Description and use of printed circuit boards.

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4.3 Servomechanisms

(a) Understanding of the following terms: Open and closed loop systems, feedback, follow up, analogue transducers; Principles of operation and use of the following synchro system components/features: resolvers, differential, control and torque, transformers, inductance and capacitance transmitters.

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(b) Understanding of the following terms: Open and closed loop, follow up, servomechanism, analogue, transducer, null, damping, feedback, deadband; Construction operation and use of the following synchro system components: resolvers, differential, control and torque, E and I transformers, inductance transmitters, capacitance transmitters, synchronous transmitters; Servomechanism defects, reversal of synchro leads, hunting.

Semiconductor Materials:

Semiconductor materials are insulators at absolute zero temperature that conduct electricity in a limited way at room temperature. They have negative temperature coefficient. There resistivity lies in between conductors and insulators. The defining property of a semiconductor material is that there electronic properties (conductivity) can be controlled either by increasing temperature or by throwing light or by doping or by increasing electrical potential across them.

Selected groups of Periodic Table of Elements (Semiconductors)

ii(+2) iii(+3) iv(+- 4) v (-3) vi (-2)

B Boron C Carbon N Nitrogen O oxygen Al Aluminum Si Silicon P Phosphorus S Sulphur Zn Zinc Ga Gallium Ge Germanium As Arsenic Se Selenium Cd Cadmium In indium Sn Tin Sb Antimony Te Tellurium Note: fig. within the bracket shows the valency.

Elemental semiconductors include Silicon and germanium; atoms of these materials are given below.

SILICON GERMANIUM

From figure it is clear that each atom has four electrons in its outer most shell; these electrons are known as valence electrons. Valence electrons are at a greater distance from the nucleus therefore these are less tightly bound and have an active role in electrical conduction.

There exists also Compound Semiconductors; composed of elements from two or more different groups of the periodic table. For e.g. group-III (B, Al, Ga, In) and group-V (N, P, As, Sb, Bi) combine to form binary (two elements, e.g. GaAs), ternary (three elements, e.g. InGaAs) and quaternary (four elements, e.g. AlInGaP). Same is the case for group-ii and vi elements.

The essential characteristic of Silicon crystal structure is that each atom has four electrons to share with adjacent atoms in forming bonds. The nature of a bond between two silicon atoms is such that each atom provides one electron to share with the other. The two electrons thus shared are in fact shared equally between the two atoms. This type of sharing is known as a covalent bond. Such a bond is very stable, and holds the two atoms together very tightly, so that it requires a lot of energy to break this bond. This is the reason that pure Si behaves as an insulator.

At room temperature the atoms are vibrating sufficiently in the lattice for a few bonds to break, setting free some valence electrons, leaving a hole where an electron was. Free electrons are attracted

towards the hole as the atom considered is now positively charged.

Covalent bonds break when temperature increases

If an electric potential is applied across pure semiconductor material, electrons are attracted towards positive terminal and holes towards negative terminal of the battery. This current flow is very small and is called as ‘intrinsic conduction’ and the pure semiconductor material itself is known as ‘intrinsic material’.

The concept of hole is understood by considering it as a vacancy or deficiency of electron. As the electron moves in one direction, this vacancy moves in opposite direction.

If the temperature is increased, electron pairs break and more electron-holes are generated which increases conductivity and hence decreasing resistance. More heat is generated and increasing more conduction and leads to thermal runaway. This eventually destroys crystal structure.

Doping:

The conductivity of semiconductors is altered by adding some impurities in a small quantity typically 1 in billionth. The material is then called as extrinsic semiconductor.

An N-type semiconductor (N for Negative) is obtained by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free charge carriers. When the doping material is added, it gives away (donates) weakly-bound outer electrons to the semiconductor atoms. This type of doping agent is also known as donor material since it gives away some of its electrons.

The purpose of N-type doping is to produce an abundance of mobile or "carrier" electrons in the material. To help understand how n-type doping is accomplished, consider the case of silicon (Si). Si atoms have four valence electrons, each of which is covalently bonded with each of the four adjacent Si atoms. If an atom with five valence electrons, such as those from group V(e.g. phosphorus (P), arsenic (As), or antimony (Sb)), is incorporated into the crystal lattice in place of a Si atom, then that atom will have four covalent bonds and one un-bonded electron.

This extra electron is only weakly bound to the atom and can easily be excited into the conduction band. At normal temperatures, virtually all such electrons are excited into the conduction band. Since excitation of these electrons does not result in the formation of a hole, the number of electrons in such a material far exceeds the number of holes. In this case the electrons are the majority carriers and the holes are the minority carriers. Because the five-electron atoms have an extra electron to "donate", they are called donor atoms. Note that each movable electron within the semiconductor is never far from an immobile positive dopant ion, and the N-doped material normally has a net electric charge of zero.

Free electrons can migrate through the inter-atomic space and can therefore act as current carriers when a very low voltage is applied.

A p-n junction is a junction formed by combining P-type and N-type semiconductors together in very close contact. Both pieces are neutral up to the instant of contact.

The term junction refers to the region where the two regions of the semiconductor meet. It can be thought of as the border region between the p-type and n-type blocks as shown in the following diagram:

‘+’ represents a hole and ‘–‘ an electron

As the holes are the vacancies for the electrons so as the two regions contact each other, electrons migrate towards the junction to fill in the holes.

As electron leaves the N type material it becomes positively charged and the P-type material which acquires an electron becomes negatively charged.

In an equilibrium PN junction, electrons near the PN interface tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (donors) on the n region. Similarly holes near the PN interface begin to diffuse in the n-type region leaving fixed ions (acceptors) with negative charge. The regions nearby the PN interfaces lose their neutrality and become charged, forming the space charge region or depletion layer.

T ‐  ‐    ‐   ‐  ‐     ‐    ‐  ‐   ‐  ‐   ‐  ‐  ‐    ‐  ‐  ‐  ‐      N  +  + +   +  +   +  + +  + +  +  +  + + +    P 

Forward Bias

Forward-bias occurs when the P-type semiconductor material is connected to the positive terminal of a battery and the N-type semiconductor material is connected to the negative terminal, as shown below.

With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed towards the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type material repels the holes, while the negative charge applied to the N-type material repels the electrons. As electrons and holes are pushed towards the junction, the distance between them decreases. This lowers the barrier in potential.

With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field can't counteract charge carrier motion across the p-n junction, consequently reducing electrical resistance. The electrons which cross the p-n junction into the P-type material (or holes which cross into the N-type material) will diffuse in the near-neutral region. Therefore, the amount of minority diffusion in the near-neutral zones determines the amount of current that may flow through the diode.

To maintain the flow of current through the PN junction requires a voltage greater than barrier potential.

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Diode parameters:

Peak Inverse voltage: PIV is the maximum voltage that a diode can withstand in the reverse

direction without breaking down or avalanching. If this voltage is exceeded the diode may be destroyed. Diodes must have a peak inverse voltage rating that is higher than the maximum voltage that will be applied to them in a given application.

Maximum Forward Voltage (VF): usually specified at the diode's rated forward current. Ideally, this

figure would be zero: the diode providing no opposition whatsoever to forward current. In reality, the forward voltage is described by the “diode equation.”

Maximum (average) forward current (IF(AV)): the maximum average amount of current the diode is

able to conduct in forward bias mode. This is fundamentally a thermal limitation: how much heat can the PN junction handle, given that dissipation power is equal to current (I) multiplied by voltage (V or E) and forward voltage is dependent upon both current and junction temperature. Ideally, this figure would be infinite.

Maximum (peak or surge) forward current (IFSM or if(surge)): The maximum peak amount of current

the diode is able to conduct in forward bias mode. Again, this rating is limited by the diode junction's thermal capacity, and is usually much higher than the average current rating due to thermal inertia (the fact that it takes a finite amount of time for the diode to reach maximum temperature for a given current). Ideally, this figure would be infinite.

Maximum total dissipation (PD): The amount of power (in watts) allowable for the diode to

dissipate, given the dissipation (P=IE) of diode current multiplied by diode voltage drop, and also the dissipation (P=I2_{R) of diode current squared multiplied by bulk resistance. Fundamentally limited by}

the diode's thermal capacity (ability to tolerate high temperatures).

Maximum DC reverse voltage (VR or VDC): The maximum amount of voltage the diode can

withstand in reverse-bias mode on a continual basis. Ideally, this figure would be infinite.

Operating junction temperature (TJ ): The maximum allowable temperature for the diode's PN

junction, usually given in degrees Celsius (o_C).

Maximum reverse current (IR): The amount of current through the diode in reverse-bias operation,

with the maximum rated inverse voltage applied (VDC). Sometimes referred to as leakage current.

Ideally, this figure would be zero, as a perfect diode would block all current when reverse-biased. In reality, it is very small compared to the maximum forward current.

Typical junction capacitance (CJ): The typical amount of capacitance intrinsic to the junction, due

to the depletion region acting as a dielectric separating the anode and cathode connections. This is usually a very small figure, measured in the range of picofarads (pF).

Reverse recovery time (trr): The amount of time it takes for a diode to “turn off” when the voltage

across it alternates from forward-bias to reverse-bias polarity. Ideally, this figure would be zero: the diode halting conduction immediately upon polarity reversal. For a typical rectifier diode, reverse

v

Half Wave rectifier

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification.

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one phase supply, or with three diodes in a three-phase supply.

When anode is positive with respect to cathode the diode conducts, this causes a current to flow across the circuit and a voltage will be developed across R. When the input polarity reverses the diode becomes reverse biased and will switch off.

The voltage developed across R is therefore half sine wave and is known as half wave Rectifier. The output is DC but its magnitude varies. The average value is half of that of supply i.e. 0.318 of peak voltage. The output ripple frequency is equal to supply frequency.

Half wave Rectifier Characteristics

1st Approx. 2nd Approx.

Peak input voltage Vp Vp

Peak output voltage Vp Vp-0.7

DC Output Vp (Output)/π Vp (Output)/π

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Parallel Diode Clipping Circuit

In this type of clippers, the diode is connected between output terminals. The on/off state of diode directly affects the output voltage. Following figures illustrate the clipping process.

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Light Emitting Diode-LED

A light-emitting-diode (LED) is a semiconductor diode that emits light when an electric current is applied in the forward direction of the device. The effect is a form of electroluminescence where incoherent and narrow-spectrum light is emitted from the p-n junction in a solid state material. LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. An LED is usually a small area (less than 1 mm2₎

light source, often with optics added directly on top of the chip to shape its radiation pattern and assist in reflection. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. Besides lighting, interesting applications include using UV-LEDs for sterilization of water and disinfection of devices, and as a grow light to enhance photosynthesis in plants.

The LED consists of a chip of semiconducting material doped, with impurities to create a p-n

junction. As in other diodes, current flows easily from the p-side to n-side, but not in the reverse

direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.

It is often said that the Schottky diode is a "majority carrier" semiconductor device. This means that if the semiconductor body is doped n-type, only the n-type carriers (mobile electrons) play a significant role in normal operation of the device. The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons (keep in mind the mobility of electron is greater than holes). Therefore no slow, random

recombination of n- and p- type carriers is involved, so that this diode can cease conduction faster than an ordinary p-n rectifier diode. This property in turn allows a smaller device area, which also makes for a faster transition. This is another reason why Schottky diodes are useful in switch-mode power converters; the high speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 5 GHz.

The Schottky diode is used in logic gates. Schottky metal-semiconductor junctions are featured in the successors to the 7400 TTL family of logic devices, the 74S, 74LS and 74ALS series.

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        Typical SCR’s  

Photodiodes

A photodiode is a diode optimized to produce an electron current flow in response to irradiation by ultraviolet, visible, or infrared light. Silicon is the most often used to fabricate photodiodes; though, germanium and gallium arsenide can be used. The junction through which light enters the

semiconductor must be thin enough to pass most of the light on to the active region. As it operates in reverse bias mode there will be leakage current (minority carriers) which increase in proportion to the amount of light falling on the junction. The light energy breaks the bond in the crystal lattice of the semiconductor and produces electrons and holes to increase the leakage current.

4. Capacitance

Compared to zener diodes, varistors have a higher capacitance. Depending on the application, transient suppressor capacitance can be a desirable or undesirable feature. In DC circuits, the capacitance of varistors provides both decoupling and transient voltage clamping functions.

5. Less Expensive

Varistors are both cost and size effective compared with diode.

Type Surge capability (typical) Lifetime - number of surges Response time Shunt capacitance Leakage current (approximate) Metal-oxide

varistor (MOV) Up to 70,000 Amps

@ 100 Amps, 8x20 µs pulse shape: 1000 surges

~1

nanosecond Typically 100 - 1000 pF +++ 10 microamps

Avalanche diode 50 Amps @ 50 Amps, 8x20 µs pulse shape: infinite Sub-microsecond 50 pF 10 microamps

Gas tube > 20,000 Amps @ 500 Amps, 8x20 µs pulse width: 200 surges

< 5

Testing Silicon Diodes (Not LED Or Zener)

To test a silicon diode such as a 1N914 or a 1N4001 all you need is an ohm-meter. If you are using an analog VOM type meter, set the meter to one of the lower ohms scales, say 0-2K, and measure the resistance of the diode both ways. If you get zero both ways, the diode is shorted. If you get INFINITY both ways, the diode is open. If you get INFINITY one way but some reading the other way (the value is not important) then the diode is good. If you use a digital multi-meter (DMM), then there should be a special setting on the Ohms range for testing diodes. Often the setting is marked with a diode symbol:

Measure the diode resistance both ways. One way the meter should indicate an open circuit. The other way you should get a reading (often a reading around 600). That indicates the diode is good. If you measure an open circuit both ways, the diode is open. If you measure low resistance both ways, the diode is shorted.

Testing Diodes in Circuit

The procedures described above assume the diode under test is not part of any circuit. If you are trying to test a diode that is on a circuit board or otherwise connected to other components, then you should disconnect one end of the diode. On a circuit board you can unsolder one end of the diode and lift it off the board. Make sure that you first disconnect all power going to the circuit before you disconnect the diode. After disconnecting one end, proceed as described above.

Transistor

A transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals.

Construction and Theory

The bipolar or junction transistor consists of two p-n junctions back to back in the same crystal. If two PN junctions are fused together so that the two P regions form a very thin (0.1-1mm thick) lightly doped layer between the two more heavily doped N regions an NPN transistor is formed. Figure shows the layout of transistor and symbol.

Collector Base

Emitter

The three electrodes are called as Emitter (represented by arrow in symbol), Base and the Collector. The emitter is more heavily doped than Collector which is more heavily doped than Base. The physical size of collector is much higher than emitter and that of Base is very much small as compared to Emitter.

Similarly if two heavily doped P regions are separated by a very thin and lightly doped N regions then a PNP transistor is formed.

Collector Base

Emitter

Operation of a Transistor (PNP)

Again the base-emitter junction id forward biased and the collector-base junction is reverse biased. Under the influence of the electric field due to battery VCC, holes cross the junction into the base. Only 1-2% of

holes recombine with the free electrons in the base due to it being very thin and lightly doped. The majority of the holes (98-99%) are accelerated towards the very strong negative influence of battery VCC.

Holes are the majority carriers in the PNP transistor.

Due to recombination of electrons and holes in the base, the base region loses free electrons and will therefore exhibit a positive charge. The electrons will be attracted by the battery VCC into the base to

make up for those lost by recombine ing with holes.

R3 1.0kΩ R4 1.0kΩ Vcc 12 V Q2 BJT_PNP_4T_VIRTUAL 6 7 Vbb 6 V 8 10 9

Conventional Current flow in PNP Transistor

The arrow on the emitter of the transistor symbol points in the direction of conventional current. IE= IB + IC

Transistor Characteristics and Parameters:

Consider the fig. shown where transistor is biased by two batteries, although in actual circuits a single supply VCC is normally taken directly from the power supply output and VBB which is smaller can be

produced with a voltage divider bias circuit.

DC Beta (βdc) and DC Alpha (α): The ratio of the dc collector current (IC) to the base current (IB) is the dc

beta (βdc), which is the dc current gain of a transistor.

βdc = IC/IB

Typical values of βdc range from less than 20 to 200 or higher. βdc is designated as hFE on transistor data

sheets.

The ratio of the dc collector current (IC) to the emitter current (IE) is the dc alpha (α dc).

αdc = IC/IE

Typical values of αDC range from 0.95-0.99 or greater, but αdc is always less than 1.

Example: Determine βdc and IE for a transistor where IB =50µA and IC = 3.65mA.

Solution: βdc = IC/IB = 3.65mA/50µA = 73

IE= IB + IC = 3.65mA + 50µA = 3.70mA.

Current and Voltage Analysis:

Consider the basic transistor bias circuit configuration in figure. Three transistor dc currents and three dc voltages can be identified.

IB: dc base current

IE: dc emitter current

IC: dc collector current

VBE = dc voltage at base w.r.t. emitter

VCB = dc voltage at collector w.r.t. base

VCE = dc voltage at collector w.r.t. emitter

VBB forward biases the emitter base junction and VCC reverse biases the base collector junction. When the

BE junction is forward biased, it is like a forward biased diode and has a nominal voltage drop of 0.7 volt. Although in actual transistor VBE can be as high as 0.9 volt and is dependent upon current.

Since the emitter is at ground (0v), by Kirchhoff’s Voltage Law, the voltage across RB is

VRB =VBB - VBE

Also by ohm’s Law VRB = IBRB

Substituting for IBRB = VBB -VBE

Solving for IB, IB = (VBB -VBE) / RB

The voltage at the collector w.r.t. the grounded emitter is VCE = VCC - VRC

Since the drop across RC is, VRC = ICRC

The voltage at the collector can be written as, VCE = VCC - ICRC

Where IC = βdc IB

The voltage across the reverse biased collector base junction is VCB = VCE- VBE

Testing of Transistor

Meter readings will be exactly opposite, of course, for an NPN transistor, with both PN junctions facing the other way.

Class AB

Class AB Amplifier Operation Amplifiers designed for class AB operation are biased so that collector current is zero (cutoff) for a portion of one alternation of the input signal. This is accomplished by making the forward-bias voltage less than the peak value of the input signal. By doing this, the base-emitter junction will be reverse biased during one alternation for the amount of time that the input signal voltage opposes and exceeds the value of forward-bias voltage. Therefore, collector current will flow for more than 180 degrees but less than 360 degrees of the input signal, as shown in figure view B.

As compared to the class A amplifier, the dc operating point for the class AB amplifier is closer to cutoff. The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion.

Class C:

In class C operation, collector current flows for less than one half cycle of the input signal, as shown in figure. The class C operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow.

Common Emitter Amplifier (Class A, voltage divider bias)

Let us first consider the biasing of the circuit. Here voltage divider bias is used. R1 and R2 divide the

supply voltage into the same ratio as that of the resistors. So if the resistor values are 16kΩ and 4kΩ then with a supply voltage of 10volts, the voltage across R1 and R2 will be 8v and 2v respectively.

The voltage across base-emitter must be 0.6volts to overcome the barrier potential. This could be achieved by removing RE

and making R2 of such a value that 0.6volts is dropped across

base-emitter junction but then R2 would be quite low and

amplification will be restricted. VBE = VR2 - VRE

So for this case VRE = 1.4volts leaving 0.6volts for VBE. So in

the static conditions quiescent current flows through the Q1, R1,

R2 and RE providing the bias necessary to make Q1 conduct.

When transistor is conducting there will be a voltage drop across RL. Let it be 5volts so that the remaining voltage is

5volts.

This is the condition that when dc is applied to the amplifier, all bias voltages available and a standing voltage is available at the collector of Q1.

Now a small ac signal is applied in the base of Q1which is superimposed on dc. Capacitor C1 blocks any dc component and also the amplified ac output must only be passed to the next stage if again dc

component is blocked using C3. These capacitors are known as coupling capacitors. It is also essential that VRE remains constant and

therefore VBE remains constant so that ac input

signal adds to and subtracts from the steady VBE

bias.

To ensure this capacitor C2 is connected across RE. This capacitor has a capacitive reactance lower than RE at the operating frequency. This means that if the ac bypasses RE then it will have a steady dc value. This capacitor is known as

Transistor Switching Circuit:

This is a Common Emitter arrangement. Here the transistor either turns fully "OFF" (Cut-off) or fully "ON" (Saturated). An ideal transistor switch would have an infinite resistance when turned "OFF" resulting in zero current flow and zero resistance when turned "ON", resulting in maximum current flow. In practice when turned "OFF", small leakage currents flow through the transistor and when fully "ON" the device has a low resistance value causing a small saturation voltage (Vce) across it. In both the Cut-off and Saturation regions the power dissipated by the transistor is at its minimum.

To make the Base current flow, the Base input terminal must be made more positive than the Emitter by increasing it above the 0.7 volts needed for a silicon device. By varying the Base-Emitter voltage Vbe, the Base current is altered and which in turn controls the amount of Collector current flowing through the transistor. When maximum Collector current flows the transistor is said to be saturated. The value of the Base resistor determines how much input voltage is required and corresponding Base current to switch the transistor fully "ON".

Example1: For example, using the transistor values from the previous tutorials of: β = 200, Ic = 4mA

and Ib = 20uA, find the value of the Base resistor (Rb) required to switch the load "ON" when the input terminal voltage exceeds 2.5v.

R

= (V

- V

) /I

= (2.5-0.7) / 20x10

= 90kΩ

Example 2:

IB = IC/ß = 200mA /200 = 1mA

Transistor switches are used for a wide variety of applications such as interfacing large current or high voltage devices like motors, relays or lamps to low voltage digital logic IC's or gates like AND Gates or OR Gates.

Types of Bias

1. Fixed bias

2. Voltage divider bias

3. Emitter bias

Fixed bias (Base bias)

This form of biasing is also called base bias. In the fig. on the right, the single power source is used for both collector and base of transistor, although separate batteries can also be used. In the given circuit, VCC = IBRB + Vbe

Therefore, IB = (VCC - Vbe)/RB

For a given transistor, Vbe does not vary significantly

during use. As VCC is of fixed value, on selection of RB, the

base current IB is fixed. Therefore this type is called fixed

bias type of circuit.

Also for given circuit, VCC = ICRC + Vce

Therefore, Vce = VCC - ICRC

From this equation we can obtain Vce. Since IC = βIB, we can obtain IC as well. In this manner, operating

point given as (VCE,IC) can be set for given transistor.

Merits:

•

It is simple to shift the operating point anywhere in the active region by merely changing

the base resistor (R

).

• Very few number of components are required.

Demerits:

• The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable.

• Changes in Vbe will change IB and thus cause RE to change. This in turn will alter the gain of the

stage.

• When the transistor is replaced with another one, considerable change in the value of β can be expected. Due to this change the operating point will shift.

Usage: Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits, ie. those circuits

which use the transistor as a current source. Instead it is often used in circuits where transistor is used as a switch. However, one application of 'fixed' bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a dc signal derived from the ac output of a later stage.

Resistive-Capacitive (RC) Coupling

Inductive-capacitive (LC) coupling: Transformer coupling: amplifier: Direct coupled amplifier:

Integrated Circuits:

Integrated Circuits are arrangements of several electronic components in a common housing. The major advantage is the very high density of the components; the total arrangement therefore will be very compact. As well they are quite resistant to mechanical stress. The small housing and therefore the small surface is a disadvantage because some additional cooling might be required. A heat sink or fan must be attached then. Another disadvantage is that IC’s cannot be repaired; a defective IC must always be replaced.

Usually the following components are integrated in IC’s: 1. Semiconductors (Transistors, Diodes)

2. Resistors 3. Capacitors

Inductances usually cannot be integrated due to their large space requirements. IC’s can be found in each and every modern appliance, in analogues as well as in digital ones. Functional blocks can be found in a single IC, requiring only a very small amount of space, i.e. Processors (Computer), Amplifier.

Differential Amplifier: It consists of two transistors with two inputs and a single output. The circuit is

symmetrical i.e. the two transistors have identical characteristics. The emitter resistor RE is common to

both transistors. Collector load resistors R2=R3. The two input (signals) circuits are also identical. And also R1=R4

Differential-mode input (Non-common mode operation): The two transistors are connected in

differential mode, receive input sine wave from opposite ends of a centre-tapped transformer. The input signals to the bases of Q1 and Q2 are equal in magnitude but opposite in phase, the condition for differential mode operation.

Assume an instant of time when input to the base of Q1 is positive going and that on Q2 is negative going. Now consider the action of Q1 as there is no Q2 connected. With a positive going signal on the base of Q1, an amplified negative-going waveform appears at the collector of Q1. Moreover a positive-going sine wave appears across RE, the un-bypassed emitter resistor,because of the emitter follower

action of Q1.

Now consider the action of Q2 as there is no Q1 connected. With a negative going signal on the base of Q2, an amplified positive-going waveform appears at the collector of Q2. Moreover a negative-going sine wave appears across RE, the un-bypassed emitter resistor,because of the emitter follower action of Q2.

The signal voltages appear across RE, because of the opposite actions of Q1 and Q2 are equal in

amplitude but 180o _{out of phase. Therefore when we consider the action of both the Q1and Q2 acting}

together, the signal voltages across the emitter resistor cancel each other and no signal is developed across RE. In this case RE does not introduce degeneration.

Now if VOUT is taken across the collector of Q1 and Q2, a positive going wave with amplitude twice the

amplitude of the signal voltage from either the collector to ground is received. However it is possible to take two outputs from the differential amplifier equal in amplitude but opposite in phase.

Operational Amplifier

The term operational amplifier or "op-amp" refers to a class of high gain DC coupled amplifiers with two inputs and a single output. Some of the general characteristics of the ideal op-amp are:

• Infinite voltage gain (on the order of a million )

• Infinite bandwidth

• Used with split supply, usually +/- 15V

• infinite input impedance

• Zero output impedance

Typically the output of the op-amp is controlled either by negative feedback, which largely determines the magnitude of its output voltage gain, or by positive feedback, which facilitates regenerative gain and oscillation.

Modern designs are electronically more rugged than earlier implementations and some can sustain direct short-circuits on their outputs without damage.

Various op-amp ICs in 8-pin dual in-line packages

"DIPs")

Symbol and terminals:

The circuit symbol for an op-amp is shown to the

right, where:

• V + : non-inverting input

• V − : inverting input

• Vout: output

• VS + : positive power supply

• VS − : negative power supply

The power supply pins (V

and V

) can be

labeled in different ways. Despite different

labeling, the function remains the same.

Comparator

makes it an extremely sensitive device for comparing its input with zero. For practical purposes, if

the output is driven to the positive supply voltage and if

it is driven to the negative supply voltage. The switching time for - to + is limited by the slew rate

of the op-amp.

The basic comparator will swing its output to at the slightest difference between its inputs. But there are many variations where the output is designed to switch between two other voltage values. Also, the input may be tailored to make a comparison to an input voltage other than zero

Voltage follower

Used as a buffer amplifier, to eliminate loading effects or to interface impedances (connecting a device with a high source impedance to a device with a low input impedance). Due to the strong feedback, this circuit tends to get unstable when driving a high capacity load. This can be avoided by connecting the load through a resistor.

• (realistically, the differential input impedance of the op-amp itself, 1 MΩ to 1 TΩ)

Summing amplifier

Sums several (weighted) voltages

• When , and Rf independent

• When

• Output is inverted

Hundreds of EASA Part-66 questions