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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).