1. 2 INTRODUCTION TO OPERATIONAL AMPLIFIERS

2. 3 CHAPTER OBJECTIVES Describe the basic op-amp and its characteristics. Discuss the differential amplifier and its operation. Discuss several op-amp parameters.

3. 4 SYMBOL AND TERMINALS The Operational Amplifier (op-amp) has 2 input terminals (+) and (-). The (+) terminal is called the non- inverting pin. The (-) terminal is called the inverting pin. The typical op-amp operates with 2 dc power supplies, 1 positive and 1 negative.

4. 5 FIGURE 6-1 Op-amp symbols and packages. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

5. 6 THE IDEAL OP-AMP Basic attributes of the Ideal Op-Amp include the following: The IC has infinite voltage gain and input impedance. These characteristics helps in not loading the driving source applied to the IC. The IC has zero output impedance.

6. 7 FIGURE 6-2 Ideal op-amp representation. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. BASIC OP-AMP ATTRIBUTES

7. 8 THE PRACTICAL OP-AMP Although modern IC op-amps approach parameter values that can be treated as ideal in many cases, no practical op-amp can be ideal. Op-amps have both voltage and current limitations. Peak to peak output voltage is usually limited to slightly less than the difference between the 2 supply voltages.

8. 9 THE PRACTICAL OP-AMP. . . Characteristics of a practical op-amp are high voltage gain, high input impedance, and low output impedance, and wide bandwidth.

9. 10 FIGURE 6-3 Practical op-amp representation. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

10. 11 FIGURE 6-10 Basic internal arrangement of an op-amp. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

11. 12 THE DIFFERENTIAL AMPLIFIER The basic Differential Amplifier (diff- amp) circuit that makes up part of an op- amp provides high voltage gain and common mode rejection. Its fundamental to the Op-Amps internal operation. Its assumed that the transistors Q1 and Q2 are identically matched by careful control processes during manufacturing so that their dc emitter currents are the same when there is no input signal.

12. 13 FIGURE 6-4 Basic differential amplifier. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

13. 14 FIGURE 6-5 Basic operation of a differential amplifier (ground is zero volts) showing relative changes in currents and voltages. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

14. 15 MODES OF SIGNAL OPERATION There are 4 typical modes for Signal Operation of an Op-Amp. Single-Ended Input (Single Ended Mode) Differential Input (Differential Mode) Common Mode Input Common Mode Rejection Ratio

15. 16 SINGLE ENDED INPUT (SINGLE-ENDED MODE) In the Single-Ended Mode, 1 input is grounded and the signal voltage is applied only to the other input.

16. 17 FIGURE 6-6 Single-ended operation of a differential amplifier. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

17. 18 DIFFERENTIAL INPUT (DIFFERENTIAL MODE) In the Differential Mode, 2 opposite- polarity (out of phase) signals are applied to the inputs. This type of operation is also referred to as double ended. Unwanted signals (noise) appearing with the same polarity on both input lines are essentially cancelled by the diff-amp and do not appear on the outputs.

18. 19 FIGURE 6-7 Differential operation of a differential amplifier. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

19. 20 COMMON MODE INPUT One of the most important aspects of the operation of a differential amplifier can be seen by considering the common-mode condition where signal voltages of the same phase, frequency, and amplitude are applied to the 2 inputs.

20. 21 FIGURE 6-8 Common-mode operation of a differential amplifier. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

21. 22 COMMON MODE REJECTION RATIO The measure of an amplifiers ability to reject common-mode signals is a parameter called the common mode rejection ratio (CMRR). The Ideal Differential Amplifier provides a very high gain for desired signals (single ended or differential) and zero gain for common mode signals. Practical Diff Amps, however, do exhibit a very small common-mode gain (usually much less than 1), while providing a high differential voltage gain (usually several thousand).

22. 23 COMMON MODE REJECTION RATIO. . . Mathematically, CMRR can express as: EQ 1) CMRR = Av(d) / Acm The higher the CMRR, the better. A very high value of CMRR means that the differential gain Av(d) is high and common-mode gain Acm is low. The CMRR is often expressed in decibels (dB) as EQ 2) CMRR = 20log(Av(d) / Acm )

23. 24 INTERNAL BLOCK DIAGRAM OF AN OP-AMP A typical op-amp is made up of 3 amplifier circuits. The differential amplifier (The input stage for the op-amp). A voltage amplifier (Usually a class A amplifier that provides additional op-amp gain). Push-pull amplifier (A Class B amplifier used for the output stage).

24. 25 OP-AMP DATA SHEET PARAMETERS Input Offset Voltage (VOS) is the differential dc voltage required between the inputs to force the differential output to zero volts. Typically values of input offset voltage are in the range of 2mV or less. In the ideal case, it is 0V.

25. 26 FIGURE 6-11 Illustration of input offset voltage, VOS. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

26. 27 OP-AMP DATA SHEET PARAMETERS. . . Input Offset Voltage drift is the parameter related to VOS that specifies how much change occurs in the input offset voltage for each degree change in temperature. Typical values range anywhere from about 5V per degree Celsius to about 50V per degree Celsius. Usually, an op-amp with a higher value of input offset voltage exhibits a higher drift.

27. 28 OP-AMP DATA SHEET PARAMETERS. . . Input Bias Current is the dc current required by the inputs of the amplifier to properly operate the first stage. By definition, the input bias current is the average of the both input currents and is calculated as. EQ 3) IBIAS = I1 + I2 / 2

28. 29 FIGURE 6-12 Input bias current is the average of the two op-amp input currents. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

29. 30 OP-AMP DATA SHEET PARAMETERS. . . The 2 basic ways of specifying the input impedance of an op-amp are the differential and common mode. The differential input impedance is the total resistance between the inverting and non-inverting inputs. Differential input impedance is measured by determining the change in bias current for given change in differential input voltage.

30. 31 OP-AMP DATA SHEET PARAMETERS. . . The Common mode input impedance is the resistance between each input and ground and is measured by determining the change in bias current for a given change in common-mode input voltage. Input Offset Current (IOS) is the difference of the input bias currents, expressed as an absolute value EQ 4) IOS = ABS(I1 I2)

31. 32 FIGURE 6-13 Op-amp input impedance. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

32. 33 FIGURE 6-14 Effect of input offset current. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

33. 34 OP-AMP DATA SHEET PARAMETERS. . . Output impedance is the resistance viewed from the output terminal of the op-amp. Common-mode input voltage is the range of input voltages which, when applied to both inputs, will not cause clipping or other output distortion. Many op-amps have common-mode ranges of no more than +/- 10V with dc supply voltages of +/- 15V. Other op-amps outputs can go as high as the supply voltages (known as rail to rail)

34. 35 FIGURE 6-15 Op-amp output impedance. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

35. 36 OP-AMP DATA SHEET PARAMETERS. . . The open loop voltage gain, (AOL), of an op- amp is the internal voltage gain of the device and represents the ratio of output voltage to input voltage when there are no external components. The open loop voltage gain is set entirely by the internal design. Open loop voltage gain can range up to 200,000 and is not a well-controlled parameters. Also known as the large-signal voltage gain.

36. 37 OP-AMP DATA SHEET PARAMETERS. . . CMRR is the measure of an op-amps ability to reject common-mode signals. An infinite value of CMRR means that the output is zero when the same signal is applied to both inputs (common-mode). An infinite CMRR is never achieved in practice, but a good op-amp does have very high CMRR. A high CMRR enables the op-amp to virtually eliminate 60Hz power supply ripple an noise voltage interference signals from the output.

37. 38 OP-AMP DATA SHEET PARAMETERS. . . The maximum rate of change of the output voltage in response to a step input voltage is the slew rate. The slew rate is dependent upon the high frequency response of the amplifier stages within the op-amp. The slew rate is expressed mathematically as EQ 5) Slew rate =VOUT / t where VOUT = +Vmax (-Vmax) The unit of slew rate is V/s.

38. 39 FIGURE 6-16 Slew rate measurement. Thomas L. Floyd and David Buchla Fundamentals of Analog Circuits Copyright 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

39. 40 OP-AMP DATA SHEET PARAMETERS. . . Frequency Response; The internal amplifier stages that make up an op-amp have voltage gains limited by junction capacitances. The op-amp has no internal coupling capacitor, therefore the low frequency response extends down to dc (0Hz).