## LINEAR INTEGRATED CIRCUITS 70 INTERVIEW QUESTIONS AND SOLUTIONS 2

1.Explain in detail the fabrication of ICs using silicon planar technology.

*Silicon wafer preparation.
* Epitaxial growth
*Oxidation.
*Photolithography.
*Diffusion.
*Ion implantation.
*Isolation.
*Metallisation.
*Assembly processing and packaging.
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2.Design an active load for an emitter-coupled pair(differential amplifier) and perform a detailed analysis to find its differential mode gain and the output resistance.

Output voltage, Vo=Vcc -VBE(on) + 2VA(eff) tanh(Vid / 2Vt)

Gain , Avd = 1 / ( Vt/Van + Vt/Vap )

Output resistance,Ro = ronpn II ropnp

3.Design a Widlar current source and obtain the expression for output current.Also prove that widlar current source has better sensitivity than constant current source.

For Widlar current source, Vt ln( Id/ Ic2 ) = Ic2 R2
Sensitivity is defined as the percentage or fractional change in output current per percentage or fractional change in power-supply voltage. For constant current source sensitivity is unity because the output current is directly proportional to supply voltage. The sensitivity of a widlar current source is better compared to constant current source because the output current has a logarithmic dependence on power supply voltage.

4. Explain the supply independent biasing technique using VBE as the reference voltage.Also, find the dependence of its output current on temperature.

The output current is given by, Iout = Vbe1 / R2 = (Vt /R2) ln ( Iref / Is1 )

(Circuit diagram, self-biasing Vbe reference circuit, start-up circuit to avoid zero-current state)
Temperature co-efficient, TCf = £Vbe1/ Vbe1 ST dRJ R dT

5.Explain supply independent biasing using zener-referenced bias circuit. Also,design a temperature compensated zener-reference source.

The output current is given by, Iout = Vz / R2

(Circuit diagram, self-biasing zener bias reference circuit, temperature compensated zener reference source )

6.Obtain the frequency response of an open-loop op-amp and discuss about the methods of frequency compensation .

The open-loop gain of op-amp decreases at higher frequencies due to the presence of parasitic capacitance. The closed-loop gain increases at higher frequencies and leads to instability. Frequency compensation is needed when large bandwidth and lower closed loop gain is desired. Compensating networks are used to control the phase shift and hence to improve the stability.

Frequency compensation methods:
*Dominant-pole compensation
*Pole-zero compensation.

7.Mention some of the linear applications of op – amps :
Adder, subtracter, voltage -to- current converter, current -to- voltage converters, instrumentation amplifier, analog computation ,power amplifier, etc are some of the linear op-amp circuits.

8.Mention some of the non – linear applications of op-amps:
Rectifier, peak detector, clipper, clamper, sample and hold circuit, log amplifier, anti -log amplifier, multiplier are some of the non – linear op-amp circuits.

9.What are the areas of application of non-linear op- amp circuits?

* industrial instrumentation
* Communication
* Signal processing

10.What is the need for an instrumentation amplifier?
In a number of industrial and consumer applications, the measurement of physical quantities is usually done with the help of transducers. The output of transducer has to be amplified So that it can drive the indicator or display system. This function is performed by an instrumentation amplifier.

11.List the features of instrumentation amplifier:
* high gain accuracy
* high CMRR
* high gain stability with low temperature co-efficient
* low dc offset
* low output impedance

12.What are the applications of V-I converter?
* Low voltage dc and ac voltmeter
* L E D
* Zener diode tester

13.What do you mean by a precision diode?
The major limitation of ordinary diode is that it cannot rectify voltages below the cut – in voltage of the diode. A circuit designed by placing a diode in the feedback loop of an op – amp is called the precision diode and it is capable of rectifying input signals of the order of millivolt.

14.Write down the applications of precision diode.
* Half – wave rectifier
* Full – Wave rectifier
* Peak – value detector
* Clipper
* Clamper

15.List the applications of Log amplifiers:
* Analog computation may require functions such as lnx, log x, sin hx etc. These functions can be performed by log amplifiers
* Log amplifier can perform direct dB display on digital voltmeter and spectrum analyzer
* Log amplifier can be used to compress the dynamic range of a signal

16.What are the limitations of the basic differentiator circuit?
* At high frequency, a differentiator may become unstable and break into oscillations
* The input impedance decreases with increase in frequency , thereby making the circuit sensitive to high frequency noise.

17.Write down the condition for good differentiation
For good differentiation, the time period of the input signal must be greater than or equal to Rf C1
T > R f C1
Where, Rf is the feedback resistance
Cf is the input capacitance

18.What is a comparator?
A comparator is a circuit which compares a signal voltage applied at one input of an op-amp with a known reference voltage at the other input. It is an open loop op – amp with output + Vsat .

19.What are the applications of comparator?
* Zero crossing detector
* Window detector
* Time marker generator
* Phase detector

20.What is a Schmitt trigger?
Schmitt trigger is a regenerative comparator. It converts sinusoidal input into a square wave output. The output of Schmitt trigger swings between upper and lower threshold voltages, which are the reference voltages of the input waveform.

21.What is a multivibrator?
Multivibrators are a group of regenerative circuits that are used extensively in timing applications. It is a wave shaping circuit which gives symmetric or asymmetric square output. It has two states either stable or quasi- stable depending on the type of multivibrator.

22.What do you mean by monostable multivibrator?
Monostable multivibrator is one which generates a single pulse of specified duration in response to each external trigger signal. It has only one stable state. Application of a trigger causes a change to the quasi-stable state.An external trigger signal generated due to charging and discharging of the capacitor produces the transition to the original stable state.

23.What is an astable multivibrator?
Astable multivibrator is a free running oscillator having two quasi-stable states. Thus, there is oscillations between these two states and no external signal are required to produce the change in state.

24.What is a bistable multivibrator?
Bistable multivibrator is one that maintains a given output voltage level unless an external trigger is applied . Application of an external trigger signal causes a change of state, and this output level is maintained indefinitely until an second trigger is applied . Thus, it requires two external triggers before it returns to its initial state

25.What are the requirements for producing sustained oscillations in feedback circuits?
For sustained oscillations,
* The total phase shift around the loop must be zero at the desired frequency of oscillation, fo. ie, LAB =0 (or) 360°
* At fo, the magnitude of the loop gain | A fS | should be equal to unity

26.Mention any two audio frequency oscillators :
* RC phase shift oscillator
* Wein bridge oscillator

27.What are the characteristics of a comparator?
* Speed of operation
* Accuracy
* Compatibility of the output

28.What is a filter?
Filter is a frequency selective circuit that passes signal of specified band of frequencies and attenuates the signals of frequencies outside the band

29.What are the demerits of passive filters?
Passive filters works well for high frequencies. But at audio frequencies, the inductors become problematic, as they become large, heavy and expensive.For low frequency applications, more number of turns of wire must be used which in turn adds to the series resistance degrading inductor’s performance ie, low Q, resulting in high power dissipation.

30.What are the advantages of active filters?
Active filters used op- amp as the active element and resistors and capacitors as passive elements.
*By enclosing a capacitor in the feed back loop , inductor less active fulters can be obtained

*Op-amp used in non – inverting configuration offers high input impedance and low output impedance, thus improving the load drive capacity.

31.Mention some commonly used active filters :
* Low pass filter
* High pass filter
* Band pass filter
* Band reject filter.

32.Discuss the need for an instrumentation amplifier? Give a detailed analysis for the same.

In a number of industrial and consumer applications, the measurement of physical quantities is usually done with the help of transducers. The output of transducer has to be amplified So that it can drive the indicator or display system. This function is performed by an instrumentation amplifier.
Circuit diagram, instrumentation amplifier with transducer bridge,
Analysis, Expression for out put voltage.

33.Explain the operation of the Schmitt trigger.
Schmitt trigger is a regenerative comparator. It converts sinusoidal input into a square wave output. The output of Schmitt trigger swings between upper and lower threshold voltages, which are the reference voltages of the input waveform.
Circuit diagram, Analysis.
Expression for upper and lower threshold voltages with and without Vref. Hysterisis width. Waveforms.

34.Discuss in detail the operation of Astable multivibrator.
Astable multivibrator is a free running oscillator having two quasi-stable states. Thus, there is oscillations between these two states and no external signal are required to produce the change in state.
Expression for time period , T = 2RC ln(l + P)/(l-P)
Waveforms.
Circuit for asymmetric square wave generator.

35. Discuss in detail the operation of Monostable multivibrator.

Monostable multivibrator is one which generates a single pulse of specified duration in response to each external trigger signal. It has only one stable state. Application of a trigger causes a change to the quasi-stable state.An external trigger signal generated due to charging and discharging of the capacitor produces the transition to the original stable state.
Circuit diagram, Analysis.
Expression for time period , T = RC In (1 + Vd / Vsat) / (1 – p )
Waveforms.

36. What are the requirements for producing sustained oscillations in feedback circuits?Discuss any two audio frequency oscillators.

For sustained oscillations,
* The total phase shift around the loop must be zero at the desired frequency of oscillation, fo. ie, LAB =0 (or) 360°
* At fo, the magnitude of the loop gain | A | | should be equal to unity.

RC phase shift oscillator: Circuit diagram,Derive the condition for frequency of oscillation.
Gain,Av >= – 29
Wein bridge oscillator : Circuit diagram,Derive the condition for frequency of oscillation.
Gain, Av = 3

37.Mention some areas where PLL is widely used:

*satellite communication systems
*FM communication systems *Computers.

38.List the basic building blocks of PLL:

*Phase detector/comparator
*Low pass filter
*Error amplifier
*Voltage controlled oscillator

39.What are the three stages through which PLL operates?

*Free running
*Capture
*Locked/ tracking

40.Define lock-in range of a PLL

The range of frequencies over which the PLL can maintain lock with the incoming signal is called the lock-in range or tracking range.It is expressed as a percentage of the VCO free running frequency.

41.Define capture range of PLL

The range of frequencies over which the PLL can acquire lock with an input signal is called the capture range. It is expressed as a percentage of the VCO free running frequency.

42.Define Pull-in time.

The total time taken by the PLL to establish lok is called pull-in time.It depends on the initial phase and frequency difference between the two signals as well as on the overall loop gain and loop filter characteristics.

43.For perfect lock, what should be the phase relation between the incoming signal and VCO output signal?
The VCO output should be 90 degrees out of phase with respect to the input
signal.

44.Give the classification of phase detector:

*Analog phase detector
*Digital phase detector

45.What is a switch type phase detector?

An electronic switch is opened and closed by signal coming from VCO and the input signal is chopped at a repetition rate determined by the VCO frequency.This type of phase detector is called a half wave detector since the phase information for only one half of the input signal is detected and averaged.

46.What are the problems associated with switch type phase detector?

*The output voltage Ve is proportional to the input signal amplitude.This is undesirable because it makes phase detector gain and loop gain dependent on the input signal amplitude.

*The output is proportional to coscp making it non linear.

47.What is a voltage controlled oscillator?

Voltage controlled oscillator is a free running multivibrator operating at a set frequency called the free running frequency.This frequency can be shifted to either side by applying a dc control voltage and the frequency deviation is proportional to the dc control voltage.

48.On what parameters does the free running frequency of VCO depend on?

*External timing resistor,RT *External timing capacitor,CT *The dc control voltage Vc.

49.Give the expression for the VCO free running frequency.

fo = 0.25 / Rt Ct

50.Define Voltage to Frequency conversion factor.
Voltage to Frequency conversion factor is defined as, Kv = Af0/AVc= 8f0/Vcc where, A Vc is the modulation voltage required to produce the frequency shift A f0

51.What is the purpose of having a low pass filter in PLL?

*It removes the high frequency components and noise.

*Controls the dynamic characteristics of the PLL such as capture range, lock-in range,band-width and transient response.

*The charge on the filter capacitor gives a short- time memory to the PLL.

52.Discuss the effect of having large capture range.

The PLL cannot acquire a signal outside the capture range, but once captured, it will hold on till the frequency goes beyond the lock-in range.Thus , to increase the ability of lock range,large capture range is required.But, a large capture range will make the PLL more susceptible to noise and undesirable signal.

53.Mention some typical applications of PLL:
• Frequency multiplication/division
• Frequency translation
• AM detection
• FM demodulation
• FSK demodulation.

54.What is a compander IC? Give some examples.

The term companding means compressing and expanding.In a communication system, the audio signal is compressed in the transmitter and expanded in the receiver. Examples : LM 2704- LM 2707 ; NE 570/571.

55.What are the merits of companding?

*The compression process reduces the dynamic range of the signal before it is transmitted.
*Companding preserves the signal to noise ratio of the original signal and avoids non linear distortion of the signal when the input amplitude is large.
*It also reduces buzz,bias and low level audio tones caused by mild interference.

56.List the applications of OTA:

OTA can be used in
• programmable gain voltage amplifier
• sample and hold circuits
• voltage controlled state variable filter
• current controlled relaxation oscillator.

57. In detail dicuss the applications of PLL:

AM detection
FM demodulation
FSK demodulation
Frequency multiplication/division.

2. List out the direct type ADCs.

1. Flash (comparator) type converter
2. Counter type converter
3. Tracking or servo converter
4. Successive approximation type converter

3. List out some integrating type converters.

59. What is integrating type converter?

An ADC converter that perform conversion in an indirect manner by first changing the analog I/P signal to a linear function of time or frequency and then to a digital code is known as integrating type A/D converter.

60. Explain in brief the principle of operation of successive Approximation ADC.

The circuit of successive approximation ADC consists of a successive approximation register (SAR), to find the required value of each bit by trial & error.With the arrival of START command, SAR sets the MSB bit to 1. The O/P is converted into an analog signal & it is compared with I/P signal. This O/P is low or High. This process continues until all bits are checked.

i. The integrating type of ADC’s do not need a sample/Hold circuit at the input.
ii. It is possible to transmit frequency even in noisy environment or in an isolated form.

62. Where are the successive approximation type ADC’s used?

The Successive approximation ADCs are used in applications such as data loggers & instrumentation where conversion speed is important.

63. What is the main drawback of a dual-slop ADC?

The dual slope ADC has long conversion time. This is the main drawback of dual slope ADC.

It provides excellent noise rejection of ac signals whose periods are integral multiples of the integration time T.

65. Define conversion time.

It is defined as the total time required to convert an analog signal into its digital output. It depends on the conversion technique used & the propagation delay of circuit components.The conversion time of a successive approximation type ADC is given by
T(n+1) where T—clock period
Tc—conversion time
n— no. of bits

66. Define resolution of a data converter.

The resolution of a converter is the smallest change in voltage which may be produced at the output or input of the converter.
Resolution (in volts)= VFS/2n-1=1 LSB increment. The resolution of an ADC is defined as the smallest change in analog input for a one bit change at the output.

67. Define accuracy of converter.
Absolute accuracy:
It is the maximum deviation between the actual converter output & the ideal converter output.
Relative accuracy:
It is the maximum deviation after gain & offset errors have been removed. The accuracy of a converter is also specified in form of LSB increments or % of full scale voltage.

68. What is settling time?

It represents the time it takes for the output to settle within a specified band i^LSB of its final value following a code change at the input (usually a full scale change). It depends upon the switching time of the logic circuitry due to internal parasitic capacitance & inductances. Settling time ranges from 100ns. 10^s depending on word length & type circuit used.

69. Explain in brief stability of a converter:

The performance of converter changes with temperature age & power supply variation . So all the relevant parameters such as offset, gain, linearity error & monotonicity must be specified over the full temperature & power supply ranges to have better stability performances.

70. What is meant by linearity?

The linearity of an ADC/DAC is an important measure of its accuracy & tells us how close the converter output is to its ideal transfer characteristics. The linearity error is usually expressed as a fraction of LSB increment or percentage of full-scale voltage. A good converter exhibits a linearity error of less than +V2LSB.

71. What is monotonic DAC?

A monotonic DAC is one whose analog output increases for an increase in digital input.

72. What is multiplying DAC?

A digital to analog converter which uses a varying reference voltage VR is called a multiplying DAC(MDAC). If the reference voltage of a DAC, Vr is a sine wave give by
V(t)=Vm Cos 2nft Then, Vo(t)=VomCos(2nft + 180°)

73.What is a sample and hold circuit? Where it is used?

A sample and hold circuit is one which samples an input signal and holds on to its last sampled value until the input is sampled again. This circuit is mainly used in digital interfacing, analog to digital systems, and pulse code modulation systems

74.Define sample period and hold period.

The time during which the voltage across the capacitor in sample and hold circuit is equal to the input voltage is called sample period.The time period during which the voltage across the capacitor is held constant is called hold period.

75.What is meant by delta modulation?

Delta modulation is a technique capable of performing analog signal quantisation with smaller bandwidth requirements. Here, the binary output representing the most recent sampled amplitude will be determined on the basis of previous sampled amplitude levels.