A total station (TS) or total station theodolite (TST) is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure both vertical and horizontal angles and the slope distance from the instrument to a particular point and on board computer to collect data and perform triangulation calculations.

Components of a Total Station

• EDM
• Prisms
• Electronic theodolite
• On-board microprocessor
• Data collector ( built in or separate unit )
• Data storage ( internal or memory card)

Operation of Total Station

• Distance Measurement

Electronic distance measurement (EDM) instrument is a major part of total station. Its range varies from 2.8 km to 4.2 km. The accuracy of measurement varies from 5mm to 10 mm per km. They are used with automatic target recognizer. The distance measured is always sloping distance from instrument to the object.

• Angle Measurement

The angle theodolite part of total station is used for measuring vertical and horizontal angle. For measurement of horizontal angles, any convenient direction may be taken as reference direction. For vertical angle measurement, vertical upward (zenith) direction is taken as reference direction. The accuracy of angle measurement varies from 2 to 6 seconds.

• Data processing

The instrument is provided with an inbuilt microprocessor. The microprocessor averages multiple observations. With the help of slope distance and vertical and horizontal angles measured, when height of axis of instrument and targets are supplied, the microprocessor computers the horizontal distance and X, Y, Z coordinates. The processor is capable of applying temperature and pressure corrections to the measurement, if atmospheric temperature and pressure are supplied.

• Display

Electronic display unit is capable of displaying various values when respective keys are pressed. The system is capable of displaying horizontal distance, vertical distance, horizontal and vertical angles, difference in elevations of two observed points and all the three coordinate of the observed points.

• Electronic Book

Each point data can be stored in an electronic note book (like compact disc). The capacity of electronic note book varies from 3000 points to 4000 points data. Surveyor can upload the data stored in note to computer and reuse the note book.

Application of Total Station

Total stations are mainly used by land surveyors and civil engineers, either to record features as in topographic surveying or to set features ( such as road, houses or  boundaries). They are also used by archaeologists to record excavations and by police, crime scene investigators, and private accident re-constructionist and insurance companies to take measurements of scenes.

• Mining

Total stations are the primary survey instrument used in mining surveying. A total station is used to record the absolute location of the tunnel walls, ceilings (backs) and floors as the drifts of an underground mine are driven. The recorded data are then downloaded into a CAD program and compared to the designed layout of the tunnel. The survey parts installs control station at regular intervals. These are small steel plugs installed in pairs in holes drilled into walls or the back. For wall stations, two plugs are installed in opposite walls, forming a line perpendicular to the drift. For back stations, two plugs are installed in the back forming a line parallel to the drift.

A set of plugs can be used to locate the total station set up in a drift or tunnel by processing measurements to the plugs by intersection and resection.

• Mechanical and Electrical Construction

Total stations have become the highest standard for most forms of construction layout. They are most often used in X and Y axis to layout the locations of penetration out of the underground utilities into the foundation, between floors of a structure as well as roofing penetrations.

• Meteorology

Meteorologists also use total stations to track weather balloons for determining upper-level winds. With the average ascent rate of the weather balloon known as assumed, the change in azimuth and elevation readings provided by the total stations as it tracks the weather balloon over time are used to compute the wind speed and direction at different altitudes. Additionally, the total station is used to track ceiling balloons to determine the height of cloud layers. Such upper level wind data is often used for aviation weather forecasting and rocket launches.

Application of Total Station in Civil Engineering

• To obtain the horizontal distance, inclined distance and vertical distance between these points.
• To find the length of a missing line.
• To locate the points at a predetermined distance along gridlines.
• To get the three dimensional coordinates of a point in space.
• To find the elevation of the remote object.
• To measure horizontal and vertical angles.

Use of EDM IN Surveying

Electronic distance measurement (EDM) is a method of determining the length between two points using phase changes that occur as electromagnetic energy waves travel from one end of the line to the other end. Electronic distance measurement in general is a term used as a method for distance measurement by electronic means. In this method, instruments are used to measure distance that rely on propagation, reflection and reception of electromagnetic waves like radio, visible light or infrared waves.

EDM instrument are classified based on the type of carrier wave as:

• Microwave instruments
• Light wave instruments
• Infrared wave instruments

Operations of Electronic Distance Measurement Instruments

The electromagnetic waves propagate through the atmosphere based on the equation;

V= f λ = (1/T) λ

Where, V is the velocity of electromagnetic energy in m/sec

             f is the modulated frequency in Hz

             λ is the wavelength measured in meters.

             T is the time in seconds = 1/f

Mainly the waves that are propagated can be represented like a sine wave as shown in figure below.

Another property of wave called as phase of wave Φ, is very convenient method of small fraction of wavelength during measurment of EDM. The points A,B, C, D etc. represents  various  phase points.

Say AB is the survey line to be measured, having a length of D. The EDM equipment is placed at ends A and B. A transmitter is placed at A and a receiver is placed at B. The transmitter lets propagation of electromagnetic waves towards B. A timer is also placed. At the instant of transmission of wave from A, the timer at B starts and stops at the instant of reception of incoming wave at B. This enable us to known the transmit time for the wave from the point A to B.

From the transit time and known velocity, the distance can be easily measured. Now to solve the problem arise due to difficulty in starting the timer at B, a reflector can be placed as shown below instead of a receiver at B.

Let the waves get transmitted from A and reflected from B. If the received signal is out of phase by a measure of △Φ, then equivalent distance is;

d= △Φ * λ/360°

Thus, the distance,

D= ½ { n λ + (△Φ * λ/360° ) }

Where, n is the integral number of wavelength, λ in the double path.

Schematic Diagram of EDM System

Application of EDM to Civil Engineering and Surveying

Generally, the use of EDM in engineering surveying operation results in a saving in time and in most case, an improvement  in the accuracy of distance measurment when compared with taping and with optical methods.

When using EDM, rapid and accurate surveying of detail is possible owing to the long ranges attainable and fewer control stations are required.Consequently, EDM has replaced tacheometric methods and chain surveying for the production of site plans. In addition, both the combined theodolite/EDM and electronic tacheometer systems are extremely well adapted to forming digital terrain models and if data storage units are used these can be interfaced directly with the computer forming the model.

As angles and distances can be measured simultaneously with the latest infrared short-range equipment, many setting out operations are now simplified. Some instruments have a continuous readout facility and this enables distances of many hundred of meters to be set out in one sighting often over ground that would be unsuitable for taping. The ability to take measurements across congested sites is also a great advantage.

In road works, EDM can be used to coordinate the major control points for the initila survey of the route and then can  use these stations to establish the road centerline by polar methods rather than tangential angle methods. Another instance of changing techniques is that buildings can be set out from two or more instrument stations by polar coordinates rather than by using a theodolite to establish right angles at each other.

These methods have been helped by the advent of advanced pocket calculators and computers and many local authorities now issue contractors with a computer printout for setting out engineering works in the form of bearing and distance tables.

A further use of EDM in civil engineering is in tunneling where it is used in the surveying necessary for the establishment of headings at ground level and for the measurement of the depths of shafts. EDM has also been used successfully for positioning piles and other inshore marine structures.

Strain Gauze Load Cell

A load cell is a transducer tha is used to create an electrical signal whose magnitude is directly proportional to the force being measured.

Basic Principle of Strain Gauge Load Cell

When steel cylinder is subjected to a force, it tends to change in dimension. On this cylinder, if the strain gauges are bounded, the strain gauge also gets streched or compressed causing a change in its length and diameter. This change in dimension of the strain gauge causes its resistance to change. This change in resistance or output voltage of the strain gauge becomes a measure of applied force.

Construction

They are a cylinder made up of steel on which four identical strain gauge are mounted and out of four strain gauge, two of them ( R₁ and R₄) are mounted along the direction of the applied load ( vertical gauges). The other two strain gauges ( R₂ and R₃) are mounted circumferentially at right angles to gauges R₁ and R₄. The four resistance strain gauges are connected electrically as a four arms of wheatstone bridge. The circuit diagram of wheatstone bridge is also shown below.

Case I

When there is no load (force) on the steel cylinder, all the four gauge will have the same resistance. As the terminals N and P are at the same potential, the wheatstone bridge is balanced and hence the output voltage will be zero.

Case II

Now, the load ( force) to be measured ( say compression force) is applied on the steel cylinder. Due to this, the vertical gauges R₁ and R₄ will undergo compression and hence there will be decrease in resistance. At the same time, the horizontal gauges R₂ and R₃ will undergo tension and there will be an increase in resistance. Thus, when strained, the resistance of the various gauges change.

Now, the terminal N and P will be at different potential and the change in output voltage due to the applied force becomes a measure of the applied force when calibrated.

Uses of Strain Gauge Load Cells

• They are used for weighting purposes.
• They are used for material testing in process industry.
• They are used in tensile test machines as a major component.
• They are used for both expansion and compression.

Remote Control Sensing and Robotics

Remote sensing is the science of acquiring information about earth’s surface without actually being in contact with it. This is done by sensing and recording reflected or emitted energy and processing, analyzing and applying that information. Human apply remote sensing in their day to day business, throught vision, hearing and sense of smell. The data collected can be of many forms; variations in acoustic wave distributions (e.g. sonar), variations in force distributions (e.g. gravity meter), variations in electromagnetic energy distributions (e.g. eye) etc. These remotely collected data through various sensors may be analyzed to obtain information about the objects or features under investigation.

Thus, remote sensing is the process of inferring surface parameters from measurement of the electromagnetic radiation (EMR) form the earth’s surface. This EMR can either be reflected or emitted from the earth’s surface. This EMR can either be reflected or emitted form the earth’s surface. In other words, remote sensing is detecting and measuring electromagnetic (EM) energy emanating or reflected from distant objects made of various materials so that we can identify and categorize these objects by class or type, substance and spatial distribution.

Remote sensing provides a means of observing large areas at finer spatial and temporal frequencies. It finds extensive applications in civil engineering watershed studies, hydrological states and fluxes simulations, hydrological modeling, disaster mangaement services such as flood and drought warning and monitoring, damage assessment in case of natural calamities, environmental monitoring, urban planning, etc.

Principle of Remote Control Sensing

Different objects reflect or emit different amounts of energy in different bands of the electromagnetic spectrum. The amounts of energy reflected or emitted depends on the properties of both the materials and the incident energy (angle of incidence, intensity and wavelength). Detection and discrimination of objects or surface features is done through the uniqueness of the reflected or emitted electromagnetic radiation from the object.

A device to detect this reflected or emitted electro-magnetic radiation from an object is called sensor. A vehicle used to carry the sensor is called a platform ( e.g. aircrafts and satellites).

The main stages in remote sensing are as follows:

• Emission of electromagnetic radiation

The sun or an EMR source located on the platform

• Transmission of energy from the source to the object

Absorption and scattering of the EMR while transmission.

• Interaction of EMR with the object and subsequent reflection and emission
• Transmission of energy from the object to the sensor
• Recording of energy by the sensor
• Transmission of the recorded information to the ground station
• Processing of the data into digital or hard copy image
• Analysis of data

Depending on the source of electromagnetic energy, remote sensing can be classified as passive or active remote sensing.

• Passive Remote Sensing

In case of passive remote sensing source of energy is the naturally available such as the sun. Most of the remote sensing of EMR. Solar energy reflected by the targets at specific wavelength bands are recorded using sensors onboard air-borne or space borne platforms. In order to ensure ample signal strength received at the sensor, wavelength (energy bands capable of traversing through the atmospheric interaction are generally used in remote sensing. Passive sensors can also be used to measure the earth’s radiance but they are not very popular as the energy content is very low.

• Active Remote Sensing

In case of active remote sensing, energy  is generated and sent from the remote sensing platform towards the target. The energy reflected back from the targets are recorded using sensors onboard the remote sensing platform. Most of the microwave remote sensing is taking a picture with camera having built-in flash.

Advantage of remote sensing

• Provide data of large areas.
• Rapid production of maps for interpretation.
• Easy and rapid collection of data.
• Relatively inexpensive when compared to employing a team of surveyours.
• Able to obtain imagery of any area over a continous period of time through which any anthropogenic or natural changes in the landscape can be analyzed.
• Provides data of very remote and inaccessible regions.

Disadvantages of remote sensing

• Objects can be misclassified or confused
• Data from multiple source may create confusion.
• Needs cross verification with grounds (field) survey data.
• The interpretation of imagery required a certain skill level.
• Distortion may occur in an image due to the relative motion of sensor and source.

Reference 1 : A reference book basic electronics engineering , Er. Sanjaya Chauwal, G.L Book house Pvt.Ltd.

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What is an Op-Amp ?

• Low cost integrating circuit consisting of:
  • Transistors
  • Resistors
  • Capacitors
• Able to amplify a signal due to an external power supply
• Name derives from its use to perform operations on a signal.

Applications of Op-Amps

• Adder
• Subtraction
• Comparators
• Multiplication/ Division
• Integrators
• Differentiators
• Analog to Digital Converters

Pin configuration of op amp

Pin 1 and 5 Offset null — With the 741, you provide the same voltage (signal) to the input pins and adjust the offset null to make sure the output is zero.(common mode)

An ideal op-amp has no offset because its gain is infinite, there are no leakage currents, and there’s no undesired impedance anywhere. Any real op-amp will necessarily have these things, the 741 being no exception

Differential amplifier (pin 2 and 3 input)

Pin 2– inverting input

Pin 3 – non inverting i/p

Pin 6 – output

Symbol for an Op-Amp

Open loop gain and output voltage of an op-amp

Since input 2 and 3 are out of phase (inverting and non-inverting) they are called differential mode signals. The operational amplifier amplifies the differential mode signals.

V_out = A_VOL * V_D

Where,

A_VOL = open loop voltage gain or simply gain of an op amp

V_D      = V₁-V₂ (V₁ and V₂ are input signals to pin 2 and 3)

Common Mode signals

Input 2 and 3 same magnitude and same phase then output = 0 (common mode signals)

V_out = A_vol * V_d

(since v₁=v₂ thus  V_d=0) for common mode

V_out = A_vol * 0 = 0

Characteristics of an Ideal Op-Amps

1. Infinite input impedance
2. Infinite open loop gain (A_vol)—typically more than 200000
3. Zero output impedance
4. Infinite bandwidth
5. Common mode rejection ratio = infinite

CMRR = Ad/Acm= Ad/0 = infinite (since Acm for ideal op amp = 0 → CMRR = infinite)

              Ad = differential voltage gain               Acm = common mode gain

Importance of Common Mode Rejection Ratio (CMRR)

The CMRR is the ability of an op amp (differential amplifier) to reject the common mode signals. The larger the CMRR , the better the op-amp ability at eliminating common mode signals. The  common mode signals are usually undesired signals caused by external interferences. (for e.g. RF signals picked up by op-amp inputs).

Configuration of an Op-amp

Inverting Configuration

Non-Inverting Configuration

Oscillators

• A clock pendulum is a simple type of mechanical oscillator. Electronic oscillators are used to generate signals in computers, wireless receivers and transmitters, and audio-frequency equipment, particularly music synthesizers.
• An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices
• The oscillator does not create energy, but it acts as an energy converter. It receives d.c. energy and changes it into a.c. energy of desired frequency. The frequency of the oscillations depends upon the constants of the device.
• Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.

Damped and undamped oscillation

Damped Oscillations:

The electrical oscillations whose amplitude goes on decreasing with time are called damped oscillations.

Undamped Oscillations:

The electrical oscillations whose amplitude remains constant with time are called undamped oscillations.

Barkhausen criteria for undamped oscillation

1. The net phase shift around the loop should be 0 or integral multiple of 360 degree. In other words the feedback should be positive.
2. The feedback factor or loop gain should be infinite.

Barkhausen’s criterion states that,

It states that if A is the gain of the amplifying element in the circuit and β(jω) is the transfer function of the feedback path

output v_o is fed back into its input v_in through a feedback network β(jω).

For positive feedback,

Note : Output offset voltage – Some voltage appears at output even though no input is applied to the opamp , this is known as output offset voltage.

Reference 1 : A reference book basic electronics engineering , Er. Sanjaya Chauwal, G.L Book house Pvt.Ltd.

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Introduction TO Bipolar Junction Transistor

The transistor is a solid- state device whose operation depends upon the flow of electric charge carries within the solid. A transistor consists of two P-N junctions formed by sandwiching either P-type or N-type semiconductor between a pair of opposite types. Accordingly, there are two types of transistors, namely:

1. N-P-N transistor

2. P-N-P transistor

A  N-P-N transistor is compared of two N-type semiconductors separated by a thin section of P-type as shown in the figure below.

However, a P-N-P transistor is formed by two P-sections separated by a thin section of N-type.

Each type of transistor has two P-N junctions-one junction between the emitter and base called emitter-base junction or simply the emitter junction and the other junction between the base and collector called the collector-base junction or simply the collector junction. Thus, a transistor is like two P-N junction diodes connected back to back. The two junctions give rise to three regions provided with three terminals called the emitter, base and collector.

Transistor Terminals

1. Emitter

Its main function is to supply majority charge carriers (electrons in case of NPN transistors and holes in case of PNP transistors) to the base. The emitter is always forward biased with respect to base so that it is able to supply majority charge carries to the base. The emitter is heavily doped so that it may be able to inject a large number of charge carriers. It is of moderate size in order to maintain heavy doping without diluting it or mesh formation in it.

• Collector

It is the right hand section of the transistor and its main function is to collect majority charge carriers. Collector is always reverse biased so as to remove the charge carriers away from its junction with the base. It is large in size to withstand the temperature generated at the collector and is moderately doped.

• Base

It is the middle section of the transistor and is very lightly doped to reduce the recombination within the base so as to increase collector current and is very thin in comparison to either emitter or collector so that it may pass most of the injected charge carriers to the collector.

Current Flow Mechanism In NPN and PNP Transistor

• Current Flow Mechanism In NPN Transistor:

Figure below shows the NPN transistor with forward bias to emitter base junction and reverse bias to collector base junction. The forward bias causes the electrons in the N-type emitter to flow towards the bias.

This constitutes the emitter current I_E. As these electrons flow through the P-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore only a few electrons combine with holes to constitute base current I_B. The remainder electrons cross over into the collector region to constitute collector current I_c. In this way, almost the entire emitter current flows in the collector circuit. It is clear that emitter current is the sum of collector and base currents I.e.  I_E=I_B+I_C.

In actual practice, a very little current (a few μA) would flow in the collector current. This is called collector cut off current and is due to minority carriers.

• Current Flow Mechanism In PNP Transistor

Figure below shows the basic component of a PNP transistor.

This constitutes the emitter current I_E. As these holes cross into N-type base, they tend to combine with the electrons. As the base is lightly doped and very thin, therefore, only a few holes combine with electrons. The remainder cross into the collector region to constitute collector current I_C. In this way, almost the entire emitter current flows in the collector circuit. It may be noted that current conducting within PNP transistor is by holes. However, in the external connecting wires, the current is still by electrons.

Input And Output Characteristics Of CB And CE Transistor

A. Common Base (CB) Configuration

In common base configuration, input is connected between emitter and base and output is taken across collector and base. Thus, base is common between input and output circuits.

In this arrangement, the emitter- base junction is forward biased and collector- base junction is reversed biased. This emitter current I_E flows in the input circuit and collector current I_C in the output circuit.

The ratio of collector current, I_C to the emitter current, I_E is called current amplification factor, α.

The collector current consists the current produced by normal transistor action i.e. component depending upon emitter current (αI_E) which is produced by majority carriers and the leakage current due to movement of minority carriers across base-collector junction on account of it being reverse biased. This leakage current flows in the same direction as that of I_CBO and is denoted by I_CBO.

∴ Total collector current (I_C) = αI_E+ I_CBO

The current I_CBO sometimes referred to as I_CO, is usually very small and may be neglected intransistor 

calculation.

So,

α = ( I_C – I_CO)/ I_E = I_C / I_E

Neglecting I_CO in comparison with I_C; we have,

I_B=I_E-I_C = I_E– (αI_E+ I_CBO) = ( 1 – α) * I_E – I_CO  [∴ I_C = αI_E+ I_CBO]

1. Input Characteristics

The curve drawn between emitter current I_E and emitter base voltage V_EB for a given value of collector base voltage V_CB is known as input characteristics. For determination of the input characteristics, the output voltage V_CB is maintained constant and the input voltage V_EB is set at several convenient levels. Figure shows the input characteristics of a typical transistor in CB arrangement.

The following points can be noted from these characteristics:

• Emitter current , I_E increases rapidly with small increase in emitter base voltage V_EB. It means that input resistance is very small.
• The emitter current is almost independent of collector base voltage V_CB. This leads to the conclusion that emitter current is almost independent of collector voltage.

Input resistance =  (Change in emitter – Base voltage)/( Change in emitter current)

∴ r_i = ▲V_EB / ▲I_E

As a very small V_EB is sufficient to produce a large flow os emitter current I_E, therfore input resistance is quite small of the order of a few ohms.

2. Output Characteristics

The curve drawn between collector current I_C and collector-base voltage V_CB for a given value of emitter current I_E is known as output characteristics.

For determination of output characteristics, the emitter current is held constant at each of several fixed levels. Figure aside shows the output characteristics of a typical transistor in CB arrangement.

The noteworthy points regarding output characteristics of common base configuration are given below:

• The collector current I_C varies with V_CB only for very low voltage but transistor is never operated in this region.
• In cutoff region (emitter and collector junctions forward biased) small collector current I_C flow even when emitter current I_E = 0. This is the collector leakage current I_CBO or I_CO.
• In active region ( emitter forward biased and collector reverse biased) collector current I_C is almost equal to I_E and appears to remain constant when V_CB is increased. In fact , there is very small increase in I_C with increase in V_CB. This is because the increase in V_CB expands the collector- base depletion region and thus shortens the distance between the two depletion regions. Transistor is normally operated in this active region.
• A very large change in collector voltage causes a very small change in collector current I.e. output resistance of CB configuration is very high-the dynamic output resistance r_out being given as the ratio of ▲V_CB and ▲I_C for a given value of I_E.

I.e. r₀ = ▲V_CB / ▲I_C  at constant I_E

• In saturation region ( both emitter and collector junction forward biased) collector current I_C flows even when V_CB = 0. Collector current I_C is reduced to zero when V_CB is increased negatively.

B. Common Emitter (CE) Configuration

In common emitter configuration, input is connected between base and emitter while the output is taken between collector and emitter. Thus, emitter is common to input and output circuits. In this configuration, the bias voltage are applied between collector and emitter and base and emitter. Emitter-base junction is forward biased and the base is made more positive than the emitter by V_BB, collector emitter junction is reverse biased and the collector is made more positive than emitter by V_CC. The value of V_CC must be greater than that of V_BB.

In this arrangement, the base current I_B flows in the input circuit and collector current I_C flows in the output circuit.

The ratio of change in collector current (output current) and change in base current (input current) is called the base current amplification factor, β.

Also,

β = I_C / I_B = I_C / (I_E – I_C) = (I_C / I_E) / (1- I_C/ I_E) = α / ( 1- α)

In CE configuration, a small collector current flows even when base current is zero. This is the collector cutoff current and denoted by I_CEO. I_CEO must be larger than I_CBO. The current equations are:

I_E = I_B + I_C

And,

I_C = αI_E + I_CBO  = α(I_B + I_C) + I_CBO

Or, I_C(1- α ) = αI_B + I_CBO

Or, I_C = {α/(1- α) } * I_B + I_CBO/ (1- α) ———–(1)

And,

Total collector current,(I_C) =  βI_B + I_CEO —————(2)

Comparing equation (1) and (2); we have,(1

β = α / ( 1- α)

And,

I_CEO = {α /(1- α) } * I_CBO

Putting the value of I_CEO = {α /(1- α) } * I_CBO  in the equation (2); we have

I_C = βI_B +  {α /(1- α) } * I_CBO  

Or, I_C = βI_B +  (1+β )  * I_CBO  

1. Input Characteristics

The curve drawn between base current I_B and base-emitter voltage V_BE for a given value of collector- emitter voltage V_CE is known as the input characteristics. For determiantion of input characteristics, collector emitter voltage V_CE is held constant and base current i_B  is recorded for different values of base-emitter voltage V_BE.

The noteworthy points regarding input characteristics are given below:

• The input characteristics of CE transistors are quite similar to those of a forward biased diode because the base-emitter region of the transistor is a diode and it is forward biased.
• In comparison to common base arrangement base current increases less rapidly with the increases less rapidly with the increase  base- emitter voltage, V_BE. This indicates that input resistance is larger in common emitter configuration that in common base configuration. Due to initial non-linearity of the curve, input resistance varies from point to point in the initial part of the characteristics. Its value over the linear part of the curve is of the order of few hundred ohms.
• An increment in value of V_CE  causes the input current I_B to be lower for a given level of V_BE. This is because the higher levels of V_CE provide greater collector-base junction reverse bias causing depletion region penetration into the base and thus reducing the distance between the collector- base and emitter-base regions. As a resuilt, more of the charge carriers from the emitter flows across the collector-base junction, and few flow out across the base lead.

The ratio of change in base-emitter voltage to the resulting change in base current at constant collector-emitter voltage is known as dynamic input resistance.

I.e. r_in = ▲V_BE / ▲I_B  at constant V_CE

2. Output Characteristics

Output characteristics for a common emitter transistor in the curve  drawn between collector current I_C and collector- emitter voltage V_CE for a given value of base current I_B.

The noteworthy points regarding output characteristics are given below:

• The collector current I_C varies with V_CE for V_CE between 0 to 1 V  and then become almost constant and independent of V_CE . The transistor are always operate above 1V.
• Output characteristics in CE configuration has same slope while CB configuration has almost horizontal characteristics. This indicates the output reistance in case of CE configuration is less than in CB configuration.
• Moderate output to input impedence ratio makes this configuration an ideal one for coupling between various transistor stages.
• With low values ( ideally zero) or V_CE, the transistor is said to be operated in saturation region and in this region base current I_B does not cause a corresponding change in collector current I_C.
• With much higher V_CE, the collector-base junction completely breaks down and because of this avalanche breakdown collector current I_C increases rapidly and the transistor gets damaged.
• In cutoff region, small amount of collector current I_C flows even when base current I_B =  0. This is called I_CEO. Since, main current I_C is zero, the transistor is said to be cutoff.

The ratio of change in collector-emitter voltage to the change in collector current at constant base current is known as dynamic output resistance. It is calculated as the reciprocal of the slope of output characteristic at a given V_CE.

i.e. r_out = ▲V_BE / ▲I_C  at constant I_B

DC current gain, (β) = I_C / I_B

AC current gain, (β₀) = ▲I_C / ▲I_B  at constant V_CE

Reference 1 : A reference book basic electronics engineering , Er. Sanjaya Chauwal, G.L Book house Pvt.Ltd.

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Intrinsic conductor

An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form. A semiconductor is not truly intrinsic unless its impurity content is less than one part in 100 million parts of semiconductor.

In an intrinsic semiconductor, even at room temperature, hole electron pairs are created. When electric field is applied across an intrinsic semiconductor, the current conduction takes place by two processes, namely by free electrons and holes as shown in the figure. The free electrons are produced due to the breaking up of some covalent bonds by thermal energy. At the same time, holes are created in the covalent bonds. Under the influence of electric field, conduction through the semiconductor is by both free electrons and holes. Hence, the total currents inside the semiconductor are the sum of current due to free holes and electrons.

It may be noted that current in the external wires is fully electronic i.e. by electrons. Holes being positively charged move towards the negative terminal B, electrons enter the semiconductor crystal under the terminal and combine with holes, thus cancelling them. At the same time, the loosely held electrons near the positive terminals A are attracted away from their atoms into the positive terminal, which again drift towards the negative terminals B. The electric current flows the negative terminals B. The electric current flows through the intrinsic semiconductor in the same direction as in which the holes are moving. Since, the electrons are negatively charged the direction of conventional current is opposite to that of movement of electrons. The total current inside the conductor is thus the sum of currents owing to free electrons and holes but the current in the external wires is only because of electrons.

Extrinsic conductor

A semiconductor to which an impurity at controlled rate is added to make it conductive is known as extrinsic semiconductor. The process by which an impurity is added to a semiconductor is known as doping. So, the impurity or doping material may be either pentavalent or trivalent. According to the impurity introduced, the extrinsic semiconductor can be divided into two classes, namely (i) N-type semiconductor and (ii) P-type semiconductor.

(i) N-type extrinsic semiconductors:

When a small amount of pentavalent impurity such as arsenic, antimony, bismuth or phosphorous is added to pure semiconductor crystal during the crystal growth, the resulting crystal is called the N-type extrinsic semiconductor; where N stands for negative.

When a pentavalent or a donor impurity is added to silicon or germanium the impurity atoms form covalent bonds with the silicon or germanium atoms; but since intrinsic semiconductor atoms have only four electrons and four holes in their valence shells, one spare valence shell electrons is produced for each impurity atom added. Each spare electrons so produced enters the conduction band of pure semiconductor as a free electron. The pentavalent impurity is called donor type impurity as it donates one electron to the conduction band of a pure semiconductor. Though each impurity provides only one free electron yet an extremely small amount of impurity provides enough atoms to supply millions of free electrons.

When an electric field is applied the excess electrons donated by impurity atoms move towards the positive terminal of the battery. This constitutes the electric current. This type of conductivity is called the negative or N-type conductivity because the current flows through the crystal due to free electrons (negatively charged particles).

Although N-type semiconductor has excess of electrons but it is electrically neutral. This is due to the fact the electrons are created by the addition of neutral pentavalent impurity atoms to the pure semiconductor i.e. there is no addition of either negative charges or positive charges. It is said that an N-type semiconductor has electron as majority carries and the holes as minority carriers.

(ii) P-type extrinsic semiconductor

When a small amount of trivalent impurity such as boron, gallium, indium or aluminum is added to a pure semiconductor crystal during the crystal growth, the resulting crystal is called the P-type extrinsic semiconductor; where P stands for positive.

When a trivalent impurity is added to silicon or germanium, these impurity atoms form covalent bonds with four surrounding intrinsic semiconductor atoms but one bond is left incomplete and gives rise to a hole. Such impurities make available and give rise to a hole. Such impurities make available positive carries because they create holes which can accept electrons. These impurities are consequently known as acceptor or P-type impurities. In P-type semiconductor, the majority carries are holes and minority carries are electrons.

P-type semiconductor remains electrically neutral as the number of mobile holes under all conditions remains equal to the number of acceptors. When an electric field is applied across a P-type semiconductor, the current conduction is primarily due to holes. Here, the holes are shifted from one covalent bond to another covalent bond. As the holes are positively charged, they are directed towards the negative terminal and constitute the hole current. The hole current flow more slowly than electron current in N-type semiconductor.

 Different between P and N type semiconductor

Reference 1 : A reference book basic electronics engineering , Er. Sanjaya Chauwal, G.L Book house Pvt.Ltd.

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