1. Chapter 14: Operational Amplifiers

2. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Basic Op-Amp The electronic amplifier: is an electronic device that increases the power of a signal. Operational amplifier or op-amp, is a very high gain differential amplifier (that amplifies the difference between two voltages) with a high input impedance (typically a few meg-Ohms) and low output impedance (less than 100  ). Note the op-amp has two inputs and one output. 2

3. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky differential amplifier

4. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Op-Amp Gain Op-Amps have a very high gain. They can be connected open-loop or closed-loop. Open-loopOpen-loop refers to a configuration where there is no feedback from output back to the input. In the open-loop configuration the gain can exceed 10,000. Closed-loop negative feedbackClosed-loop configuration reduces the gain. In order to control the gain of an op-amp it must have feedback. This feedback is a negative feedback. A negative feedback reduces the gain and improves many characteristics of the op-amp. 4

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12. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky CMRR common-mode rejection ratio One rating that is unique to op-amps is CMRR or common-mode rejection ratio. Because the op-amp has two inputs that are opposite in phase (inverting input and the non-inverting input) any signal that is common to both inputs will be cancelled. Op-amp CMRR is a measure of the ability to cancel out common-mode signals. 12

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20. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Inverting Op-Amp inverting (–) inputThe signal input is applied to the inverting (–) input non-inverting input (+)The non-inverting input (+) is grounded feedback resistor negative feedbackThe resistor R f is the feedback resistor. It is connected from the output to the negative (inverting) input. This is negative feedback. 20

21. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Inverting Op-Amp Gain Gain can be determined from external resistors: R f and R 1 Unity gain—voltage gain is 1 The negative sign denotes a 180  phase shift between input and output. Constant Gain—R f is a multiple of R 1 21

22. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Virtual Ground virtual ground An understanding of the concept of virtual ground provides a better understanding of how an op- amp operates. The non-inverting input pin is at ground. The inverting input pin is also at 0 V for an AC signal. The op-amp has such high input impedance that even with a high gain there is no current from inverting input pin, therefore there is no voltage from inverting pin to ground—all of the current is through R f. 22

23. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Practical Op-Amp Circuits Inverting amplifier Noninverting amplifier Unity follower Summing amplifier IntegratorDifferentiator 23

24. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Inverting/Noninverting Op-Amps Inverting Amplifier Noninverting Amplifier 24

25. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Unity Follower 25

26. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Summing Amplifier Because the op-amp has a high input impedance, the multiple inputs are treated as separate inputs. 26

27. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Integrator The output is the integral of the input. Integration is the operation of summing the area under a waveform or curve over a period of time. This circuit is useful in low- pass filter circuits and sensor conditioning circuits. 27

28. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Differentiator The differentiator takes the derivative of the input. This circuit is useful in high-pass filter circuits. 28

29. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Op-Amp Specifications—DC Offset Parameters Input offset voltage Input offset current Input offset voltage and input offset current Input bias current offset Even when the input voltage is zero, there can be an output offset. The following can cause this offset: 29

30. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Input Offset Voltage (V IO ) The specification sheet for an op-amp indicate an input offset voltage (V IO ). The effect of this input offset voltage on the output can be calculated with 30

31. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Output Offset Voltage Due to Input Offset Current (I IO ) The input offset Current (I IO ) is specified in the specifications for the op-amp. The effect on the output can be calculated using: If there is a difference between the dc bias currents for the same applied input, then this also causes an output offset voltage: 31

32. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Total Offset Due to V IO and I IO Op-amps may have an output offset voltage due to both factors V IO and I IO. The total output offset voltage will be the sum of the effects of both: 32

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34. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

35. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Input Bias Current (I IB ) input bias current A parameter that is related to input offset current (I IO ) is called input bias current (I IB ) The separate input bias currents are: The total input bias current is the average: 35

36. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky An op-amp is a wide-bandwidth amplifier. The following affect the bandwidth of the op-amp: Gain Slew rate Frequency Parameters 36

37. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky An op-amp is designed to be a high-gain, and wide-bandwidth amplifier. This operation tends to be unstable (oscillate) due to positive feedback. To ensure stable operation, op-amps are built with internal compensation circuitry, which also causes the very high open-loop gain to reduce with increasing frequency. This gain reduction is referred to as roll-off. A number of circuit improvements result from this gain reduction. 1.More stable amplifier voltage gain 2.Incresing the input impedance of the circuit over that of the op-amp alone. 3. Decreasing the circuit output impedance from that of the op-amp alone. 4. Increasing the frequency response of the circuit over that of the op-amp alone.

38. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky a measure of the width of a range of frequencies Bandwidth As the frequency of the input signal increases the open-loop gain drops off until it finally reaches the value of 1 (unity). A frequency at which the gain becomes 1, since the frequency band from 0 Hz to the unity-gain frequency is also a bandwidth. Op-amp specifications provide a description of the gain versus bandwidth.

39. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Slew Rate (SR) Slew rate (SR) Slew rate (SR) is the maximum rate at which an op-amp can change output without distortion. The SR rating is given in the specification sheets as V/  s rating. 39

40. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Maximum Signal Frequency The slew rate determines the highest frequency of the op-amp without distortion. where V P is the peak voltage 40

41. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky General Op-Amp Specifications Other ratings for op-amp found on specification sheets are: Absolute Ratings Electrical Characteristics Performance 41

42. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Absolute Ratings These are common maximum ratings for the op-amp. 42

43. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Electrical Characteristics Note: These ratings are for specific circuit conditions, and they often include minimum, maximum and typical values. 43

44. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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48. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

49. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

50. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky What is the range of the voltage-gain adjustment in the circuit:

51. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

52. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Op-Amp Applications Constant-gain multiplier Voltage summing Voltage buffer Controlled sources 52

53. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Constant-Gain Amplifier Inverting Version more… 53

54. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Constant-Gain Amplifier Noninverting Version 54

55. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Multiple-Stage Gains 55 The total gain (3-stages) is given by: or

56. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Summing [Formula 14.3] The output is the sum of individual signals times the gain: 56

57. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Subtraction

58. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Buffer Realistically these circuits are designed using equal resistors (R 1 = R f ) to avoid problems with offset voltages. unity gain amplifier Any amplifier with no gain or loss is called a unity gain amplifier. The advantages of using a unity gain amplifier: Very high input impedance Very low output impedance 58

59. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Controlled Sources Voltage-controlled voltage source Voltage-controlled voltage source Voltage-controlled current source Voltage-controlled current source Current-controlled voltage source Current-controlled voltage source Current-controlled current source Current-controlled current source 59

60. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage-Controlled Voltage Source The output voltage is the gain times the input voltage. What makes an op-amp different from other amplifiers is its impedance characteristics and gain calculations that depend solely on external resistors. Noninverting Amplifier Version more… 60

61. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage-Controlled Voltage Source The output voltage is the gain times the input voltage. What makes an op-amp different from other amplifiers is its impedance characteristics and gain calculations that depend solely on external resistors. Inverting Amplifier Version 61

62. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage-Controlled Current Source The output current is: 62

63. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Current-Controlled Voltage Source This is simply another way of applying the op-amp operation. Whether the input is a current determined by V in /R 1 or as I 1 : or 63

64. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Current-Controlled Current Source This circuit may appear more complicated than the others but it is really the same thing. 64