Important Questions and Answers: Linear Integrated Circuits : Basics of Operational Amplifiers

BASICS OF OPERATIONAL AMPLIFIERS

1. Define an Integrated circuit.

 An integrated circuit(IC) is a miniature, low cost electronic circuit consisting of active and passive components fabricated together on a single crystal of silicon. The active components are transistors and diodes and passive components are resistors and capacitors.

2. Mention the advantages of integrated circuits over discrete components.

*Miniaturization 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.

3. Define sheet resistance.

Sheet resistance is defined as the resistance in ohms /square offered by the diffused

4. What is the use of buried n+ layer in monolithic IC transistor?

The buried n+ layer provides a low resistance path in the active collector region for the flow of current

5. What are the two common methods for obtaining integrated capacitors?

• Monolithic junction capacitor

• Thin-flim capacitor

6. What is active load? Where it is used and why? 

The active load realized using current source in place of the passive load in the collector arm of differential amplifier makes it possible to achieve high voltage gain without requiring large power supply voltage.

7. Why open loop OP-AMP configurations are not used in linear Applications?

The open loop gain of the op-amp is not a constant and it varies with changing the temperature and variations in power supply. Also the bandwidth of the open loop op-amp is negligibly small. For this reasons open loop OP-AMP configurations are not used in linear applications.

8. Define virtual ground of a OP-Amp?

A virtual ground is a ground which acts like a ground. It is a point that is at the fixed ground potential (0v), though it is not practically connected to the actual ground or common terminal of the circuit.

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

Using constant current source output current of small magnitude(micro amp 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.

14. Mention the advantages of Wilson current source.

(i) Provides high output resistance.

(ii) Offers low sensitivity to transistor base current

15. Define sensitivity

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

16. What are the limitations in a temperature compensated Zener-reference source?

A power supply voltage of at least 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

17. 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.

18. 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.

19. 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.

20. 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.

21. What causes slew rate?

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

22. 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.

 23. What is an integrated circuit?

IC is a miniature low cost electronic circuit consisting of active and passive components that are irreparably joined together on a single crystal chip of silicon.

24. What is current mirror?

A constant current source (current mirror) uses a transistor in the active mode of operation where the collector current is relatively independent of the collector voltage.

25. Define slew rate and CMRR.

Slew rate is the maximum rate of change of output voltage caused by a step input voltage.

CMRR is defined as the ratio of differential mode gain to common mode gain.

26. Why are active loads preferred than passive loads in the input stage of an operational amplifier?

A large value of resistance requires large chip area.

27. Name the different methods used in fabrication of integrated resistors.

Diffused resistor, Epitaxial resistor, pinched resistor & Thin film resistor.

28. What are the two requirements to be met for a good current source?

Transistors should be matched in order to have same Vbe.

29. State the limitations of discrete circuits.

Operating speed is low due to parasitic capacitance effect.

Power consumption is more.

30. What is meant by monolithic IC?

An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small plate ("chip") of semiconductor material, normally silicon.

31. What are the characteristics of an ideal op-amp?

• Infinite open-loop gain G = Vout / 'v

• Infinite input impedance Rin, and so zero input current.

• Zero input offset voltage.

• Zero output impedance.

• Infinite bandwidth with zero phase shift and infinite slew rate.

32. A differential amplifier has a differential voltage gain of 2000 and common mode gain of 0.2. Determine CMRR in db.

CMRR= 20 LOG [2000/0.2] =80 dB

33. Mention two advantages of active load over passive load in an operational amplifier.

34. Define input bias current and input offset current of an operational amplifier.

Input Bias Current:

Ideally, no current flows into the input terminals of an op amp. In practice, there are always two input bias currents, IB+ and IB-

Input bias current IB=IB+ (-) IB- / 2

Input Offset Current:

One of the practical op –amp limitations that the input bias current for the two inputs may be slightly different. Even though the inputs are designed to be symmetrical, slight differences which occur in the manufacturing process may give slightly different bias currents. This offset current is typically on the order of a tenth of the input bias current, with 10nA being a representative offset current for a 741.

Input Offset Current Ios= |IB+ (-) IB-|

35. The output of an operational amplifier is 5V peak sine wave whose slew rate is 0.5V/μs. Find the maximum allowable frequency of the signal.

f_m =S/2πV_m

Given S=0.5V/µs V _m=5V

fm=0.5*10^-6/2π*5=1.59*10^-08

36. Find the maximum frequency for sine wave output voltage 10V peak to Peak with an op-amp whose slew rate is 1V/µs.

f_m =S/2πV_m

Given S=1V/µs V _m=10V

fm=1*10^-6/2π*10=1.59*10^-08

37. Differentiate the ideal and practical characteristics of an op-amp.

Ideal operational amplifier are characterized by

• Infinite gain

• Infinite input resistance

• Zero output resistance (order of 10’s of ohms)

• Infinite bandwidth (practically restricted by slew rate)

• Linear irrespective of entire analog signal range No offsets and, so on.

Practical operational amplifier are characterized by

• Input resistance is of the of mega ohm Order due to differential stage at the front end

• Output resistance order of tens of ohms.

• Practically bandwidth of Opamp restricted by slew rate.

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

The biasing sources referenced to VBE has a negative temperature coefficient 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 coefficient can be achieved.

39. Define thermal drift.

The bias current, offset current & offset voltage change with temperature. A circuit carefully nulled at 25oC may not remain so when the temperature raises to 35oC.This is called thermal drift. Often, offset current drift is expressed in nA/ oC and offset voltage drift in mV/ 0C.

40. Define supply voltage rejection ratio (SVRR)

The change in OPAMP’s input off et voltage due to variations in supply voltage is called the supply voltage rejection ratio. It is also called Power Supply Rejection Ratio (PSRR) or Power Supply Sensitivity (PSS)

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41. Define an operational amplifier.

An operational amplifier is a direct-coupled, hi h ga 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.

42. 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.

43. 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.

44. 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

45. What is frequency response of Op-amp?

The plot showing the variations in magnitude and phase angle of the gain due to change in frequency is called frequency response of Op-amp. The plot is used to find the bandwidth and cut-off frequencies of Op-amp.

46. Define Unity Gain Bandwidth of Op-amp.

For a certain frequency of the input signal, the gain of the Op-amp reduces to 0 dB. This means 20 log |AOL (f) | is 0dB i.e. |AOL (f) | = 1. Such a frequency is called gain cross over frequency or unity gain bandwidth (UGB).

47. 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 opamp’s output voltage should change instantaneously in response to input step voltage.

PART-B

1. Explain the method of improving CMRR. 

2. (a) Write a note on Widlar biasing circuit.

 (b) Explain the various circuits used to provide constant current bias in a differential amplifier. 

3. Explain the method of improving the CMRR using active load. 

4. Explain the operation of differential amplifier and give its differential gain, common mode gain and CMRR. 

5. Draw and explain the basic band gap reference circuit. 

6. Derive the slew rate equation for an op-amp. 

7. Explain in detail about the frequency compensation applied to operational amplifiers.

8. (a) Explain the method of improving the slew rate of an op-amp. 

(b) Draw and explain briefly the equivalent circuit of an op-amp. 

9. Draw and explain the internal block diagram of an op-amp. 

10. Define and explain slew rate. What is full-power bandwidth? Also explain the methods adopted to improve slew rate [10] 

11. Define output off-set voltage. Explain methods of nullify offset voltage 

12. (i) Define CMRR. Draw the circuit of an Op-amp differential amplifier and give the expression for CMRR. 

(ii) Define Slew Rate. Explain the cause of slew rate and derive an expression for Slew rate for an op-amp voltage follower.

13. Draw the circuit diagram of the output stage of the IC 741 OP AMP and explain its operation with clearly indicating the protection mechanisms indicated.

14. With neat circuit diagram explain the operation of

(i) Voltage reference circuit using temperature compensation

(ii) Voltage reference circuit using avalanche diode reference  16. (i) List and explain the non-ideal DC characteristics of an operational amplifier.

(ii) Explain the AC characteristics of an operational amplifier

17. Compare different configurations of Differential Amplifier.

18. For a dual input, balanced output differential amplifier, Rc = 2.2kΩ, Re =4.7kΩ, Rs1 = Rs2 = 50 Ω. The supply voltages are ± 10V. The hfe for the transistor is 50. Assume silicon transistors and hie = 1.4kΩ. Determine the operating point values, differential gain common mode gain and CMRR. 

19. State the advantages of Integrated circuits over discrete components.

20. Explain the working of BJT Differential Amplifier with Active Load. 

21. Write down the characteristics and respective values of an ideal operational amplifier.  

22. Explain the internal circuit diagrams of IC 741.Discuss its AC and DC performance Characteristics.

23. With simple schematic of differential amplifier explain the function of operational amplifier?  

24. With the neat diagram, explain the input side of the internal circuit diagram of IC741. . 

25. What is the need for the frequency compensation in an OPAMP? With a suitable illustration, explain the pole-zero frequency compensation technique.

26. Draw the circuit diagram of a basic current mirror and explain its operation

27. With a schematic diagram, explain the effect of RE on CMRR in differential amplifier. 

28. Discuss about the methods of improve the CMRR.. 

29. For the DC level shifter shown in Fig 1, Determine the level shift between input and output voltages.

30. Obtain the level shift Vo for the circuit shown in Fig. 2.

31. A differential Amplifier has i) CMRR=1000 and ii)CMRR = 10,000.The first set of inputs is  V₁=100µV and V₂=-100µV.The second set of input is  V₁=1100µV and V₂= 900µV.Calculate the percentage difference in output voltage obtained for the two sets of input voltages and also comment on this. (PO2)

32. Design a i) constant current source ii) Widlar current source for generating a constant current of Io=10µA.Assume Vcc=10V, Vbe=0.7 V, β=125, VT=25Mv.Find R1.Justify which circuit can be used for generating small current?

33. Design a differential Amplifier for a differential gain of 5000 and CMRR 100.If the inputs are 290µV and 250 µV, Find the output voltage.

34. An operational amplifier has a slew rate of 2V/µs. If the peak output is 15 V, What is the power bandwidth?

35, An operational amplifier has a slew rate of 35V/µs. How long will it take for the output to change from 0 to 15V?

36.The output of an opamp voltage follower is a triangular wave for a square wave input of frequency 2MHz and 8Vpp amplitude. What is the slew rate of opamp? 

37. Design a simple current source to provide an output current of 150µA.Assume Vcc=5V,Vbe=0.6 V, β=125.

38.A peak to peak input signal of 400mV has to produce a peak to peak undistorted output voltage of 3V with a rise time of 4µs.Can IC741 be used for such application.? Justify.