Linear Integrated circuits interview questions

Linear Integrated circuits

      1.Mention the advantages of integrated circuits.

*Miniaturisation and hence increased equipment density. 
*Cost reduction due to batch processing.

*Increased system reliability due to the elimination of soldered joints. 
*Improved functional performance. 
*Matched devices. 
*Increased operating speeds.
*Reduction in power consumption.

2.Write down the various processes used to fabricate IC's using silicon planar technology.

*Silicon wafer preparation.

* Epitaxial growth

*Oxidation.

*Photolithography.

*Diffusion.

*Ion implantation.

*Isolation.

*Metallisation.

*Assembly processing and packaging.

3.What is the purpose of oxidation?

*SiO2 is an extremely hard protective coating and is unaffected by almost all reagents.

*By selective etching of SiO2, diffusion of impurities through carefully defined windows can be accomplished to fabricate various components.

4.Why aluminium is preferred for metallization?

*It is a good conductor.

*it is easy to deposit aluminium films using vacuum deposition.
 *It makes good mechanical bonds with silicon.
 *It forms a low resistance contact.

5. What are the popular IC packages available?

Metal can package.
 Dual-in-line package. 
Ceramic flat package.

6. Define an operational amplifier.

An operational amplifier is a direct-coupled, high gain amplifier consisting of one or more differential amplifier. By properly selecting the external components, it can be used to perform a variety of mathematical operations.

7.Mention the characteristics of an ideal op-amp.

* Open loop voltage gain is infinity.
 *Input impedance is infinity. 
*Output impedance is zero. 
*Bandwidth is infinity.
 *Zero offset.

8.What happens when the common terminal of V+ and V^- sources is not grounded?

If the common point of the two supplies is not grounded, twice the supply voltage will get applied and it may damage the op-amp.

9.Define input offset voltage.

A small voltage applied to the input terminals to make the output voltage as zero when the two input terminals are grounded is called input offset voltage.

10. Define input offset current. State the reasons for the offset currents at the input
of the op-amp.

The difference between the bias currents at the input terminals of the op-amp is called as input offset current. The input terminals conduct a small value of dc current to bias the input transistors.Since the input transistors cannot be made identical,there exists a difference in bias currents.

11. Define CMRR of an op-amp.

The relative sensitivity of an op-amp to a difference signal as compared to a common -mode signal is called the common -mode rejection ratio. It is expressed in decibels.

CMRR= Ad/Ac

12.What are the applications of current sources?

Transistor current sources are widely used in analog ICs both as biasing elements and as load devices for amplifier stages.

13. Justify the reasons for using current sources in integrated circuits.

*superior insensitivity of circuit performance to power supply variations and temperature.

*more economical than resistors in terms of die area required to provide bias currents of small value.

*When used as load element, the high incremental resistance of current source results in high voltage gain at low supply voltages.

14. What is the advantage of widlar current source over constant current source?

Using constant current source output current of small magnitude(microamp range) is not attainable due to the limitations in chip area. Widlar current source is useful for obtaining small output currents.Sensitivity of widlar current source is less compared to constant current source.

15.Mention the advantages of Wilson current source.

*provides high output resistance.

*offers low sensitivity to transistor base currents.

16.Define sensitivity.

Sensitivity is defined as the percentage or fractional change in output current per percentage or fractional change in power-supply voltage.

17.What are the limitations in a temperature compensated zener-reference source?

A power supply voltage of atleast 7 to 10 V is required to place the diode in the breakdown region and that substantial noise is introduced in the circuit by the avalanching diode.

18.What do you mean by a band-gap referenced biasing circuit?

The biasing sources referenced to V_BE has a negative temperature co-efficient and Vt has a positive temperature co-efficient. Band gap reference circuit is one in which the output current is referenced to a composite voltage that is a weighted sum of Vbe and Vt so that by proper weighting, zero temperature co-efficient can be achieved.

19.In practical op-amps, what is the effect of high frequency on its performance?

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.

20. What is the need for frequency compensation in practical op-amps?

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.

21.Mention the frequency compensation methods.

*Dominant-pole compensation 
*Pole-zero compensation.

22.What are the merits and demerits of Dominant-pole compensation?

*noise immunity of the system is improved.
 *Open-loop bandwidth is reduced.

23.Define slew rate.

The slew rate is defined as the maximum rate of change of output voltage caused

by a step input voltage.An ideal slew rate is infinite which means that op-amp's output voltage should change instantaneously in response to input step voltage.

24.Why IC 741 is not used for high frequency applications?

IC741 has a low slew rate because of the predominance of capacitance present in the circuit at higher frequencies. As frequency increases the output gets distorted due to limited slew rate.

25.What causes slew rate?

There is a capacitor with-in or outside of an op-amp to prevent oscillation.It is this capacitor which prevents the output voltage from responding immediately to a fast changing input.