EDC + Analog Electronics

Transcription

1 Engineering Services Personality test Practice Sheet Electronics & Telecommunication. EDC + Analog Electronics Panel Members (last year ESE-2012 interview ) : Prof. David Q. What is difference between Analog and digital signals? Ans. Analog signals can assume any value in a given range but digital can assume some fixed quantized signals in a given range. Analog signals cannot be easily processed by computers and algorithms unlike digital ones. Besides, analog signals are more memory occupying than digital. Digital signals are regarded as more environmentally friendly. Q. What is a semiconductor? Ans. The material whose conductivity lies in the middle of those metals and insulators is called semiconductors. Since they have bipolar currents and their conductivity can be modulated, they offer many advantages? Q. What is difference between Electrical and electronics engineering? Ans. In electrical engineering, we are mostly concerned with large magnitude unipolar currents and their power and transmission. In electronics, we are mostly have semiconductor which have bipolar currents of range of milliamperes. Q. What is difference between Si and Ge? What will you prefer? Ans. Both are semiconductors. But Si has lesser reverse saturation current so better thermal stability. Besides, Si is more readily available and can handle large currents with efficient power delivery. But Ge is more suited for high frequency applications. Also, Ge is better conductor, has lower energy gap. I think Si has more advantages, so I will use Si mostly. Q. What is Hall Effect? Ans. When a current carrying specimen is placed in a transverse magnetic field, a perpendicular field is developed inside it (or voltage). Developed by a scientist named Hall, this method is used to measure mobility of majority carriers. The magnitude of Hall voltage is stronger in semi conductors than in metals. Q. How to measure mobility of minority carriers?

2 Ans. Haynes- Shockley experiment. Q. How to you relate mobility with applied electric field? Ans. Mobility is drift velocity per applied electric field. In general, the mobility initially remains constant with the field applied, then decreases sub-linearly and finally linear decrement is observed. Q. How does conductivity of an extrinsic semi conductor varies with temperature? Ans. Initially, it increases linearly till room temperature because of either donor level ionization or bonding. By 300K this process outputs a maximum conductivity. Then it shows non linear decrement till Curie temperature at which the concentration of majority and minority become equal and we say that the purpose of doping has been lost. AT this point extrinsic becomes intrinsic and thereafter, the conductivity increases with temperature as it should in an intrinsic semi conductor. Q. What is Fermi level? Ans. The energy level whose probability of occupancy is ½ at a given temperature. Also it can defined as the maximum energy possessed by an electron at 0K. Q. Draw Fermi levels for different semi conductors. Ans. In intrinsic, it lies exactly at the middle of the forbidden energy gap. In n-type, the Fermi level is above this middle value and as doping is increased it moves upward towards conduction band but with it shifts below towards the middle value. In p-type, it lies below this middle value and with doping it moves downwards towards valence band but with temperature it moves upwards towards the middle. Q. What is diode? Can we use Ohm s law on it? Ans. Diode is a two terminal unidirectional noon-linear semiconductor device. We cannot use Ohm s law directly on a diode because its voltage and current are not directly proportional rather in a transcendental way. But we can develop linear models of a diode then use Ohm s law on them to obtain approximate results. Q. Can we measure cut-in voltage of a diode? What does it depend upon? Ans. We cannot measure but calculate cut-in voltage. Because to measure we will connect voltmeter across the terminals of the diode and this will give zero reading because the diode cut-in voltage will be exactly compensated by the metal-semiconductor contacts potential at the connection of voltmeter.

3 To calculate we use the formula V = kt/q ln(n a.n D/n i 2). So it depends upon doping levels and temperature. Q. Name some applications of diodes. Ans. Rectifiers, clippers, clampers, Voltage regulators, LEDs, Photo diodes and opto coupling. Q. What is difference between LED and LCD? What kind of TV do you use? Ans. Light emitting diodes are semiconductor devices and have more lifetime. Liquid crystal devices are based upon pneumatic crystals which require back lighting. Though LCD s have lesser power consumption but lesser life time. Nowadays, the two technologies have been fused. LCD displays use LED backlighting. Q. What breakdown of a diode? What are its types and differences between them? Ans. Breakdown of a diode refers to the case when applied reverse bias is large enough to result in a huge surge in the number of carriers causing large reverse current to flow. It can occur in two ways: Avalanche and Zener mechanism. While Avalanche is because of already unbounded electrons colliding with lattice and generating more carriers, in short impact ionization, in Zener the cause is development of a high electric field in the depletion layer which liberates carriers by breaking bonds, in short field emission. So Avalanche is more common in lowly doped diodes and Zener in highly doped ones. Also, with increase in temperature the voltage required for Avalanche increases but for Zener decreases. Q. What are junction capacitances of a diode? Ans. Diffusion capacitance dominates during forward bias and increases linearly with forward current. It occurs due to storage of majority carriers near the depletion layer. Transition capacitance dominates during reverse bias and varies with applied reverse bias as C proportional to V -1/2. It occurs due to storage of minority carriers near depletion layer. Both have values in picofarads and limit the high frequency response of the diode. Q. What is dc and ac model of a diode? Ans. DC model in forward bias has a dc voltage source (equal to cut-in voltage) in series with a small resistance. In reverse bias, we have a huge resistor. The ac model has only a dynamic resistor whose value depends upon forward dc current and temperature. Q. What is FET? Types and uses? Ans. Field effect transistors use an external electric field to modulate flow of carriers in the conducting channel. Two types: JFET and MOSFET. Both have different properties. Mostly they are

4 used as linear amplifier for analog signals. Also in digital electronics, MOSFETs are widely used to design circuitry. CMOS is the most dominant logic family currently. Q. What is difference between FET and BJT? Ans. FETs have smaller transconductance than BJT so usually they have smaller gains. Also FETs have lack of minority carries, so relatively noise free and better switches than BJT. But BJT are more suited for analog signals in amplifiers. Q. What is depletion and enhancement MOSFET? Ans. In depletion mode, MOSFET has a pre-existing channel. Reverse voltage must be applied to nullify this channel. In enhancement mode, MOSFET has no pre-existing channel so an external voltage must be applied to create the channel first and then use the device. Q. What is channel width modulation? Ans. In JFET, with applied reverse bias at the GATE SOURCE terminal, the width of the conduction channel can be altered. With increasing reverse bias, the channel becomes thinner at the drain end, so a wedge shaped. Q. What are the regions of operation of a MOSFET with changing gate bias? Ans. If we assume n-channel enhancement mode MOSFET, then initially no channel exists. So the region just below MOS layers is populated by holes, called accumulation. As gate bias is increased, the electric field develops which pushes the holes downwards and this region is depleted of the mobile carriers and has only uncovered charges, called depletion region. As more bias is applied, the electrons from the substrate will be pulled upwards and more charges will be uncovered. When the concentration reaches equal to that of holes in the bulk, we have inversion region. Now the device is on and ready to use. Q. What is difference between NMOS and PMOS? Ans. NMOS has p-type substrate with electrons as charge carries in the channel. Reverse is the case for PMOS. Both can be either depletion or enhancement mode. But NMOS occupies lesser area, has better mobility or speed and inexpensive. So PMOS is outdated. Q. What is CMOS? Ans. Complementary mosfet has both PMOS and CMOS on the same substrate. It is dominant MOS technology because it allows large scale integration. Also, power dissipation is low, high fanout and small area are some of its advantages. Q. Why in a CMOS inverter we connect PMOS near +V DD and NMOS near ground? What if we interchange them?

5 Ans. PMOS passes logic 1 without attenuation and NMOS passes logic 0 without attenuation of their threshold voltages. If we interchange them, the system is no longer inverter but becomes a buffer which passes the input to the output with attenuation equal to their threshold voltages. Q. What is saturation mode of a FET? How to find whether in saturation or not? Where it works as amplifier? Ans. With a fixed V GS, if we increase V DS the current first increases linearly then levels off at a value of the drain voltage. Thus, initially it was in linear region then in saturation region. To find saturation region or not, we measure the drain to gate voltage and compare with the threshold voltage. It works as an amplifier in saturation region. Q. Which device is said to have pentode characteristics with triode operation? Ans. FET Q. What is ac model of FET? Ans. Dependent current source in parallel with a large output resistance. Q. When was BJT developed? Why is transistor called a transistor? Ans. In late 94 s at Bell Laboratories by Bardeen, Shockley and Brittian. As explained by BJT s inventors, transistor means transfer+resistor. Q. What are operation regions of a BJT? Ans. When the input current is small enough to not bias the base emitter junction, we have cutoff region. When this current increases, we have linear region. But when collector current becomes saturated, we have saturation region. Q. In reverse active region, what are biases at the two junctions? Ans. Emitter junction is reverse and collector has forward. Q. Name some uses of BJT. Ans. Amplifiers, oscillators, digital circuits like TTL logic family. Q. What is overdrive factor? Ans. Ratio of common emitter gains of the active region to that of the saturation region. Q. A BJT behaves as a.. source. Ans. Current controlled current source.

6 Q. What is an operational amplifier? Ans. It is a multi stage, direct coupled high gain amplifier which is used to implement a variety of mathematical functions. It is non linear in open loop but linear in closed loop. Q. An op amp is source. Ans. Voltage controlled voltage. Q. What is difference between feedback amplifiers and oscillators? Ans. Feedback amps use negative feedback but oscillators use positive feedback. Feedback amplifiers produce ac output which can be either voltage or current. Oscillators produce sinusoidal output ithout any ac input. Q. What is difference between oscillators and multi vibrators? Ans. Oscillators produce sinusoidal output but multivibrators can produce sinusoidal, sawtooth, triangular or square waveforms with control over their duty cycles. Q. What is Schmitt trigger? Does it generate waveforms? Ans. It is basically regenerative comparator which uses a high gain op amp to convert ac inout to square wave output. It does not generate waveforms but only converts to square wave. Q. What is CMRR? Should it be high? Ans. Common mode rejection ratio is the ratio of differential to common mode gain of a differential amplifier. It should be as large as possible so that the output has lesser amount of common mode voltage. Q. What is Slew Rate? Ans. The maximum rate of change of the output signal with time. It should be as large as possible. But if the output changes faster than the mentioned slew rate, distortions will be seen. Q. name some uses of op amps. Ans. Adder, Subtractor, integrator, differentiator, log and anti log amplifiers, Multivibrators, timers.

7 SIGNALS & SYSTEMS Q. What is a signal? Ans. Any naturally or artificially occurring phenomenon which has information associated with it can be called as signal. Information can be measures in terms of the signal power, energy or probability of its occurrence. Ex: Average rainfall, oscillations of a pendulum. Q. Why do we need Fourier transform? Ans. Every device or system is frequency selective. That is it passes different frequencies with varying gains and phase shifts. So we need to analyze frequency characteristics of a signal, hence Fourier series or transform. Q. What is difference between Laplace and Fourier transform? Ans. Both are used for frequency domain analysis. But Laplace transform is more versatile as it considers damping also. S= SIGMA + J.OMEGA So, Fourier transform is same as Laplace when evaluated along the imaginary axis. Q. What is CAUSAL system? Ans. Those systems where the output depends only upon the present or past inputs but not on the future. Mathematically, speaking h(t) = 0 for t<0. Thus, output starts only at or after the input has been applied. Physically, causal systems can be realized. Q. What is energy spectral density? Ans. The distribution of energy of a signal with respect to its frequency spectrum. Q. Is Dirac function an energy or power signal? Ans. In continuous time, it is neither as its energy is infinite but power is zero. In discrete domain, it is energy signal. Q. What is parameter of measurement for an impulse input, step input and ramp input? Ans. Area, Step magnitude and slope respectively. Q. Is a stable system always causal or vice versa. Tell the answer in terms of Region of convergence properties. Ans. Not necessarily. Causal systems have rightwards ROC in Laplace or outwards ROC in z transform. Stable systems must have ROC including imaginary axis in Laplace domain or unit circle in Z transform.

8 Q. What is difference between unilateral and bilateral Laplace transform? Ans. In Bilateral Laplace transform, the integration used to evaluate the function has limits from infinity to +infinity but in unilateral we have limit from zero- to +infinity. zero is used to include impulse functions and its derivatives occurring at the origin. Also, bilateral transform is not unique expression wise so we need to associate the ROC with it but unilateral Laplace is always unique. So need to associate ROC. Q. What is initial value and final value theorems? Any precautions while using these theorems? Ans. Both are used on unilateral Laplace only. Initial value is used to find the starting value of a function and final value to find the end value. Expressions... Initial value can be used only when degree of numerator is less than degree of denominator. Final value can be used only when all the poles lie in left half except a first order pole at the origin itself. Q. What is DFT? Ans. It is a modified version of DTFT used with algorithms on a computer. We calculate 2 n point DFT using algorithms like FFT. Q. Why is convolution used to find output? Ans. LTI systems have the property that output = input * impulse response and this can be shown mathematically. Q. What is auto-correlation? Ans. F(t) *F(-t) gives auto correlation which is a measure and expression of total power of a signal. It also measures the similarity of a signal with shifted version of itself. The Fourier transform of auto correlation gives energy/power spectral density. Q. Tell some properties of power/energy spectral density. Ans. It depicts the frequency distribution of power/energy. Real, even and positive function of frequency. Its integral with frequency gives total power/energy as the case may be. Q. What is convolution of an even function and an odd function? What is differentiation of an odd function? ANS. ODD, EVEN. Q. what is odd half wave symmetry?

9 Ans. if by shifting the signal by half its time period we obtain negative values or mirror image in x- axis, we have an odd half wave symmetry. its Fourier transform will have only odd numbered harmonics but no even numbered harmonics or dc term. Q. what do you understand by control systems? CONTROL SYSTEMS Ans. a system which can be made to produce desired outputs for a given input by varying its parameters is called control systems. Q. Difference open loop and closed loop. Ans. Open loop systems do not have any part of the output sampled and feedback to the input. But the closed loop systems have feedback. This is done to facilitate comparison between the output and input and hence, measure error. Q. What is static and dynamic error coefficients? Ans. Static error coefficients give only the steady state error value but do not give any indication as to how the error varies with the input over a period of time. Thats why we need dynamic error coefiicients by using long division method and then inverse laplace transform. Q. What is difference between absolute stability and relative stability? Ans. Absolute stability gives the stability in terms of location of poles but doesnot tell the degree of stability. But relative stability gives the stability measure in terms of degree of stability. For example, Routh array and root locus will give absolute stability And bode plot will give relative stability. Nyquist plot gives both. Q. What is Gain margin and Phase margin? Ans. Both are used to ascertain the degree of stability. Gain margin is the excess gain that given to the system to make it go the verge of instability. Phase margin is the excess phase that can be given to make the system go to the verge of instability. Mathematically, GM = (expression) PM = (expression) Q. What ig GM = 0 db and if GM = 0? Ans. IF GM = 0 db, system is marginally stable. GM =0 means it can be stable or conditionally stable is also stable.

10 Q. What is difference between transfer function method and state space method? Ans. The overall response of a system is sum of zero input and zero state response. The transfer function considers zero input response as zero but state space considers both as non zero. Transfer function considers initial conditions as zero. Q. What is controllability and observability? Ans. Controllability means the state variables can be altered by changing the input. Observability means the output can be altered by changing any one of the state variables. To check controllability and observability we create their respective matrices and check for a non zero determinant. Expressions for matrices... Q. What is the purpose of drawing root locus? Ans. Root locus gives the idea of variation of closed loop system poles variations with respect to open loop dc gain. Thus, we can choose a particular value of gain to position poles at desired location and hence modulate system properties. Q. What are advantages of using controllers and compensators? Ans. Controllers are used with time domain analysis and are used to alter damping ratio and location of poles. Compensators are used with frequency domain analysis and used to alter the phase characteristics of a system. Q. Name different controllers. Ans. Proportional, Integral, Differential, PI, PD and PID. Q. Why is integrator used in state space but not differentiator? Ans. Integrator is /s so it minimises the magnitude of high frequency noise but differentiator or s will augment the high frequency noise. Q. If in a second order system, damping ratio is changed keeping natural frequency constant, what is the locus? If we reverse the situation, then the locus of the poles will be... Ans. If only damping ratio is varied we will have a circular shape. If only natural frequency is varied we will have a straight line. Q. What is the advantage of using Mason s gain? Ans. It makes signal flow graph which are concise representation of block diagram and hence calculation of transfer function becomes easier. But Mason s gain has some limitations like if the graph determinant becomes 0.

11 DIGITAL ELECTRONICS Q. What is difference between analog and digital signals? Ans. analog signals can have any value in a given range but digital signals take only some fixed values. Also storage of digital data is easier as compared to analog signals. Digital signals can be easily processed by computer as they are quantized. Q. What is hazard? How can it be removed? Ans. Hazards are unwanted logic signals generated during the operation of a digital circuit. They are like transients but not steady state and occur due to timing mismatch or delays. They are of various types like static-0 hazard (in 2 level and-or gates), static-1 hazard(two level or-gates), dynamic hazard (in multilevel combinational circuits) and essential hazards(in sequential circuits). The static and dynamic hazards can be removed by using redundant gates but not the essential hazards. Q. What is a logic family? Ans. A group of similar circuits manufactured by using a specific circuit configuration and with compatible voltage levels which can be used to implement logic functions. Ex: RTL, TTL, ECL, CMOS. Q. Which is fastest logic family and why? What are its voltage levels? Ans. ECL is fastest with delays of 1 ns. Because in ECL the transistor is not allowed to go in saturation region so swing time is reduced but power dissipation is increased. It is a [positive logic family with logic-0 at -1.7V and logic-1 at -0.8V. Q. What are stages of a TTL circuit and what is its basic gate? Ans. Basic gate is NAND gate. TTL has three stages: a) Multi emitter input transistor b) Phase shifter c) Totem Pole output. Q. Comment upon Totem pole. Ans. Totem pole is series connection of a BJT and a diode. It ensures that at one time only one of the two output transistors is ON thus, reducing power dissipation. Also since the transistors are connected in Common collector mode thus, output impedance is small resulting in fast charging times of capacitive loads. The disadvantage of totem pole is that it does not facilitate wired logic. So we have to use open collector modes. Q. What is Schottky TTL?

12 Ans. Schottky diode connected between base and collector of transistors of TTL logic circuits results in Schottky TTL circuit is the fastest in TTL family with delays of 2ns. It doesnot allow saturation of BJTs connected. Q. What is noise margin? Which logic family has highest noise marginans. Ans. Noise margin refers to the maximum acceptable value of dc that can be added despite changing the logic levels at the output of the next digital block. It has two values, each corresponding to one logic level. High threshold logic family which uses zener diodes has highest noise margin. Q. What is fanout? Which has highest fanout? Ans. Fanout refers to the maximum number of similar logic gates that can driven by the output of one such logic circuit. CMOS has highest fanout. Q. Why is multiplexer called universal circuit and not demultiplexer? Ans. Mux has all the basic gates namely NOT, AND and OR. But demultiplexer has only NOT and AND. Q. Make an astable multivibrator using an universal gate. Ans. Odd number of NAND gates cascaded with shorted inputs will achieve this task. Q. Difference between RAM and ROM. Ans. RAM is volatile i.e. in case of interrupted power supply the data is lost. But ROM is non-volatile. So BIOS related information are usually stored in ROM but user stores in RAM. RAM cannot be used to implement Boolean functions but ROM can be. RAM is of two types SRAM (costlier, faster, requires continuous power and no refreshing) and DRAM (made with MOSFET, requires refreshing). ROM can be erased by using ultra-violet rays or electrical signals. Q. What is advantage and disadvantage of karnaugh maps? Ans. K-maps are useful to graphically form a pattern of minterms or maxterms and hence recombine to minimize the expression. They use grey codes or unit distance codes. But they are prone human errors and cannot be programmed into algorithms unlike Quine-McCluskey method. Q. If we use BCD codes to represent numbers from 0 to 99, how many codes are wasted? Ans. We require 8 bits so 2 8 =256 codes are possible. But we will use only 0 to 99 i.e. 100 codes. So, = 156 codes are wasted. Q. In a three input XOR gate, how many input combinations will give a logic1? Ans. 2 3 /2 = 4 combinations will give logic 1 and rest 4 will give logic 0.

13 Q. How many Boolean functions can be formed using 3 variables? Ans. 2 8 = 256 functions will be formed. Q. What is positive logic and negative logic family? ANS. POSITIVE LOGIC HAS HIGHER VOLTAGE FOR LOGIC-1 AND LOWER VOLTAGE FOR LOGIC-0. REVERSE IS THE CASE FOR NEGATIVE LOGIC. Q. DIFFERNCE BETWEEN COMBINATIONAL AND SEQUENTIAL CIRCUITS. ANS. Combinational circuits do not have any feedback unlike sequential. Combinational circuits have output depending upon only present inputs but in sequential circuits outputs depend on present inputs as well as past outputs. Combinational circuits have do not employ clock inputs but sequential circuits may (synchronous) or may not (asynchronous) use clock inputs. Examples of combinational circuits are multiplexers, decoders, etc. And of sequential circuits are latches, flip flops and counters. Q. What is counter and its types and how can they be used? Ans. Counters are basically used to count the clock pulses used in a given time duration. They can be synchronous or asynchronous depending upon how the clock input is derived. Synchronous counters can be designed for any count sequence and are relatively free of decoding errors unlike asynchronous ones which can count only up or down. Synchronous counters are faster also. Counters can be used to pulse width measurement, range measurement, frequency measurement, frequency division, etc... Q. What is racing and race around? Ans. Racing refers to a situation when the propagation delay of one gate determines the output of a circuit from two equally likely possible outcomes. Example is a NAND latch when both the inputs are 1 and previous outputs were also 1. But race around occurs in a level triggered JK latch when both the inputs are 1 and the propagation delay of the circuit is less than pulse width. So to avoid race around the propagation delay should be more than pulse width and less than the clock time period or we can use Master Slave configuration. Q. What is shift register and its uses? Ans. Shift register is used to shift the data from one flip flop to the next at the occurrence of a clock pulse. We use D Flip-Flop. It used to store and shift data. Q. Name the analog to digital converters and arrange them in increasing order of their speed. Which is most accurate?

14 Ans. In the increasing order of their speed: Dual slope, Ramp type integrating, Successive approximation type and Flash converter. The first one is slowest and most accurate. Dear Students, if you have any query / questions, then contact our team for support. Wishing you all the best...