Operational Amplifier: Characteristics and Open-Loop Op-Amp

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1 Lesson: Operational Amplifier: Characteristics and Open- Loop Op- Amp Lesson Developer: Dr. Arun Vir Singh College/Department: Shivaji College, University of Delhi Institute of Lifelong Learning, Delhi University Page 1

2 Table of Contents 1.1 Introduction 1.2 The Operational Amplifier Block Diagram 1.3 Operational Amplifier Symbol and Packages Terminal Connections 1.4. Characteristics of Op-Amp Characteristics of an Ideal Op-Amp Ideal Op-Amp Transfer Curve Characteristics of a Practical Op-Amp: 1.5 Op Amp Characteristics that add Error to DC Applications Input Bias Current Input Offset Current Input Offset Voltage Drift 1.6. Op-Amp Characteristics that add Error to AC Applications Frequency Response Band Width Unity Gain Band Width Slew Rate Rise Time 1.7 Parameters of Op-Amp 1.8 Open loop Op-Amp Differential Configuration Inverting Configuration Noninverting Configuration Frequency Response Summary Exercise Glossary References Institute of Lifelong Learning, Delhi University Page 2

3 1.1 Introduction One of the most versatile and widely used electronic devices in linear applications is the operational amplifier, commonly abbreviated as the Op Amp. Operational amplifiers had their origins in computers. The word operational in operational amplifier originally stood for mathematical operation. Mathematical operations like addition, subtraction, multiplication, integration and differentiation can be done with the help of op-amp, even differential equations can also be solved. With the addition of suitable external components op-amp can be used for a variety of applications, such as ac and dc signal applications, active filters, oscillators, comparators, power supplies and others. Op-amps are very popular because of they are low in cost and allow you to build without needing to know about their complex internal circuitry. It is available as a single integrated circuit package. In this lesson characteristic, parameter of op-amp, and op-amp in open-loop configuration will be discussed. 1.2 Operational Amplifier An operational amplifier is a direct-coupled high gain amplifier consisting of one or more differential amplifiers usually followed by a level translator and output stage. Differential amplifier can be used in any one of the following configuration. (i) Dual input, balanced-output differential amplifier (ii) Dual input, unbalanced-output differential amplifier (iii) Single-input, balanced-output differential amplifier (iv) Single input, unbalanced-output differential amplifier When both input signals are used the configuration is said to be dual input; otherwise it is single input configuration. When the output is measured at one of the collectors with respect to ground the configuration is said to an unbalanced output configuration. In case when output voltage is measured between two collectors, then it is called a balanced output, because both collectors are at the same dc potential with respect to ground. Differential amplifier with a differential input and, usually, a single-ended output is generally called op-amp Block Diagram Op-amp is a multistage amplifier and its block diagram is illustrated in figure 1. Fig.1 Block diagram of a typical op-amp. Developed by :ILLL (i) Input Stage Differential Amplifier It is a dual-input, balanced output differential amplifier. This stage provides very high input impedance, as well as very large voltage gain. Institute of Lifelong Learning, Delhi University Page 3

4 (ii) Intermediate Stage This stage is usually another differential amplifier, which is driven by output of first stage. This is dual input, unbalanced (single ended) output. (iii)level Shifting Stage Since it is a direct coupled device, so the dc voltage at the output of intermediate stage is well above the ground potential. Therefore, generally, the level shifting circuit is used after intermediate stage to shift the dc level at the output of intermediate stage to zero volts with respects to ground. (iv)output Stage: Push Pull Amplifier This stage increases the output voltage swing and raises the current supplying capability. It also provides the low output impedance. 1.3 Operational Amplifier and Packages Symbol The operational amplifier schematic symbol with two input terminals and one output terminal is illustrated in figure 2(a). A is the voltage gain of op-amp. The input terminals are designated by the (+) and (-) notations. The (+) is noninverting input terminal and (-) is inverting input terminal., and are node voltages assigned to noninverting, inverting and output terminals. These are measured with respect to ground. Op-amps operate with two dc supply voltages, one positive (+V CC ) and the other negative (-V EE ). Most widely used symbol is shown in figure 2(b). In this figure dc voltage terminals are not shown for simplicity, but are understood to be there. In figure 2(c) three basic IC 2(a) 2(b) (a) (b) (c) Fig.2(c) Fig:2. Op-amp schematic symbols and packages. Developed by :ILLL packages are shown.( i) Flat pack package, (ii) Metal can package and, (iii) The dual-inline package (DIP), and Pin no.1 is indicated by a dot or notch. Institute of Lifelong Learning, Delhi University Page 4

5 Value Addition : Did you know? Differential amplifier and its various configurations. Body text: Double input balanced output Double input unbalanced output Single input balanced output Single input unbalanced output It is basically two common emitter amplifier connected back to back. V in1 is applied to base of transistor T 1 and V in2 is applied to the base of transistor T 2. +V CC and -V EE are the power supply voltages of equal magnitude. It can be used to amplify both ac and dc signals. Suggested reading Op-Amps and Linear Integrated Circuits : Ramakant A. Gayakwad,4 th Edition. Electronics Devices and Circuit Theory by Robert. L Boylestad and L. Nashelsky,8 th Edition. Institute of Lifelong Learning, Delhi University Page 5

6 1.3.1 Terminal Connections. It is a 8 pin IC. Most of the manufacturers use the pin designation depicted in figure 3. Fig: 3. Terminal Connections for DIP packages Developed by:illl Pin 1. This is used in offset null arrangement. See pin 5 (Fig. 6). Pin 2. It is called inverting input (-) terminal because an ac signal (or dc voltage) applied to this terminal produces an 180 o out of phase (or opposite polarity).voltage at this pin is designated by symbol ( ). Signals at input and output terminals are as shown in figure 4. Fig:4. Inverted input Developed by:illl Pin 3: The (+) input is the noninverting input terminal and voltage at this pin is designated by symbol (. An ac signal (or dc voltage) applied to this terminal produces an in phase (or same polarity or without inversion) signal at output. The waveforms at the noninverting and output terminals are shown in figure 5. Institute of Lifelong Learning, Delhi University Page 6

7 Fig:5. Noninverted input, Developed by:illl Pin 4 (-V): This is the negative supply voltage terminal also referred to as -V EE. Supply voltage for the 741 is -5 volts (minimum) to -18 volts (maximum). Pin 5 (Offset Null): Pins 1 and 5 are use for offset null arrangement. As shown in the figure 6, a 10-kΩ is connected between pins 1 and 5, and wiper of the potentiometer is connected to the negative supply -V EE. By varying the potentiometer, the offset voltage (V of ) (output without any input applied) can be reduced to zero. For 741C offset voltage adjustment range is ± 15 mv. Fig:6. Offset null arrangement, Developed by: ILLL Pin 6 (Output): Output voltage ( is measured with respect to ground. depends upon the difference in the voltage ( ) between inverting and non inverting pins. Maximum current drawn from this terminal is in the range of 5 to 10 ma. There are also limits on the output terminal s voltage levels. These limits are set by the supply voltages and the op-amp s output transistor. Upper limit of is called positive saturation voltage,( ) and the lower limit is called,( ). For example, with a power supply of ± 15V, =13V and =13V. Therefore, is restricted a 26 V peak to peak swing undistorted sine wave for ac input signals. Both these current and voltage values limits place a minimum value on the load resistance R L of 2 KΩ. Pin 7 (+V): This is the positive supply voltage terminal also referred to as V CC. Supply-voltage is in the range of +5 volts (minimum) to +18 volts (maximum). Institute of Lifelong Learning, Delhi University Page 7

8 Pin 8 (N/C): The 'N/C' stands for 'Not Connected'. Nothing is connected at this pin.it is just there to make it a standard 8-pin package Characteristics of Op-Amps The op-amp is used in amplifier circuits to amplify dc or ac signals or combinations of them. The output voltage of op-amp can be obtained by taking the product of amplifier voltage gain (A) and differential input voltage (V id ). However, the output voltage may have added an error component. This error is due to the differences between an ideal op-amp and a real op-amp Ideal Op-Amp Characteristics 1) Infinite voltage gain A. 2) Infinite input resistance ( so that almost no current flows into the input terminals of the device. In practical terms this implies that it does not load the preceding stage/driving stage. 3) Zero output impedance.this implies that the output voltage is independent of the load connected to the output. Output can derive an infinite number of other devices. 4) Zero output offset voltage. i.e, zero output voltage when input voltage is zero. 5) Infinite bandwidth. This means, any frequency signal from 0 to infinity Hz can be amplified without attenuation 6) Infinite common mode rejection ratio. 7) Infinite slew rate. Equivalent circuit diagram is shown in figure 7. The input voltage, noninverting and inverting terminals and the output voltage is appears between ( [1] Where is the voltage at noninverting terminal and at inverting terminal. The ideal device can never be made. Any device has limitations, and the op-amp is no exception. Institute of Lifelong Learning, Delhi University Page 8

9 Fig: 7 Ideal op-amp representations. Developed by: ILLL Ideal Voltage Transfer Curve Op-amp output voltage is given by expression By assuming output offset voltage value equal to zero, variation of output voltage as a function of V id is represented in figure 8. This curve is called an ideal voltage transfer curve. It can be seen from this curve that output voltage is directly proportional to the differential voltage only until it reaches the saturation voltages and there after output voltage remains constant or saturated as shown in figure 8. Fig:8. Ideal voltage transfer curve. Developed by: ILLL The saturated voltages ( ) are specified by an output voltage swing rating of the op-amp for given values of power supply voltages, Characteristics of a Practical Op-Amp 1) Very high voltage gain (open-loop)a ( ). 2) Very high input impedance (2MΩ). 3) A very low output impedance Ω 4) Band width 1 MHz. 5) Output voltage / offset voltage (2mV) when input voltage is zero. 6) Finite common mode rejection ratio. 7) Finite slew rate Institute of Lifelong Learning, Delhi University Page 9

10 Fig: 9. Practical op-amp representation Developed by: ILLL Value Addition: Did you Know? Op-Amps Through the Ages. Institute of Lifelong Learning, Delhi University Page 10

11 Body Test: In 1940s, concept of operational amplifier emerged from the extensive development of analog computers. Philbrick K2-W was the most famous op-amp of the vacuum tube era. Bob Widlar designed the first op-amp 702 ( wide-lar ). (using only nine transistors). In 1963 Fairchild Semiconductors manufactured µa702. It was the first commercial op-amp. Drawbacks: It had low voltage gain (3600), unequal power supply (V CC =+12V, V EE =-6 V, and low input impedance 40kΩ.When gets shorted, it had the a tendency to burn out. Because of these reasons it was not universally accepted. Despite all these drawbacks this device was the best in its day. In 1965, µa709 was introduced by Fairchild Semiconductors with (i) symmetrical supply voltages (V CC =+15V, V EE =-15V), (ii) voltage gain of 4500 and (iii) much higher input impedance (400kΩ). It had higher gain and a larger bandwidth, lower input current. It is also regarded as 1 st generation op amp. Limitations: (i) No short circuit protection (ii) Requires an external frequency compensating network.(iii) Output voltage can be latch up to some value then fails to respond to changes in input signal applied. Dave Fullagar developed the 741, in IC op amp (µa 741) was introduced in 1968 by Fairchild. It is 2 nd generation op amp. LM101, LM307 by National Semiconductor, MC 1558 by Motorola, and Raytheon Semiconductor's RC4558 are examples of 2 nd generation op-amp These are free from the limitations of 1 st generation op amp. They have improved electrical specifications and exhibits superior performance over preceding generation IC. The first FET input op amp was the CA3130 made by RCA. In July 1975, National Semiconductor came out with the J-FET type LF355. Now precision op amps with BI-FET with laser trimming technology are also available. Examples are MC340001/MC 34002/MC34004 single, dual, and quad op-amps. In dated sequence, the op-amp developed like this: 1963-µA702, 1965-µA709, 1967-LM101/LH101, 1968-µA741, 1974-RC4558/LM324, 1975-CA3130/LF355, and in 1976 the TL084 Suggested reading : Op-Amps and Linear Integrated Circuits : Ramakant A. Gayakwad Institute of Lifelong Learning, Delhi University Page 11

12 1.5 Op-Amp Parameters that add Error Components to DC Applications. Op- amp characteristics that add error components to the dc output voltage are Input bias current The input bias current is the average of base currents and (dc currents) entering through both the input terminals of the op-amp is depicted in figure 10. or Fig:10. Input bias current. The range of is from 1µA or more for general purpose op-amp, to 1pA for op-amp that have field effect transistors (FET) at the input. Its value for 741C is 500 na and for precision 741C is ± 7 na. For ideal amplifier its value is Input offset current It is defined as the algebraic difference between the input bias currents entering the input terminals and expressed as an absolute value. Its value for 741C is 200 na and for precision 741C is ± 6 na. Its value decreases with the increase in matching between two input terminals. In case of ideal op amp the two input bias currents are equal, and thus their difference is zero Input offset voltage For practical op-amp, a small dc voltage, (error), appears at the output when no Fig:11. Input offset voltage. Institute of Lifelong Learning, Delhi University Page 12

13 differential input voltage is applied. To make output to zero volts, a differential dc voltage that must be applied between the inputs terminals is called the input offset voltage, as shown in the figure 11.It is given by the equation, It could be positive or negative. Typically it lies in the range of 1 mv to 5 mv or less. For precision 741C has =150μV (maximum). In the ideal case, it is Drift Bias current, offset voltage and offset current changes with temperature. This is called drift. Offset current drift is expressed in na/ o C and offset voltage drift in mv/ o C. These indicate the change in offset for each degree Celsius change in temperature. 1.6 Op-Amp Characteristics that add Error Components to AC Applications. For ac amplification coupling capacitors eliminates dc output voltage error. In ac applications characteristics listed in sec.1.5 are often unimportant. In ac applications ac output voltage can be divided into two categories (i) small signal (output voltage below 1V pp ) and (ii) large signals (output voltages above 1V pp ) If small, ac output signals are present, the important op amp characteristics that limits performance are noise and frequency response. If large signals are expected then an op amp characteristics called slew rate limiting determine whether distortion will be introduced by op amp and may further limit frequency response Frequency Response General purpose op amps are internally compensated ( a 30 pf capacitor is installed by the manufacturers).voltage gain of op amp is related to frequency of the input signal by a curve called frequency curve. A typical curve is shown in figure 12. Fig:12. Frequency response curve : Developed by :ILLL Institute of Lifelong Learning, Delhi University Page 13

14 Right hand axis represents gain in decibels (db) and left hand axis represents voltage gain. Maximum gain shown is 2x10 5 (106dB). Break frequency is shown by point A. Voltage gain at this point is x10 5 =140,000. Its value is 5 Hz. Point B and C shows how gain drops by a factor of 10 as frequency increases by a factor of 10. Or it decreases by 20dB for an increase in frequency of 1 decade. This explains why frequency curve from A to D is described as rolling of 20 db/ decade. This can be explained by considering the impedance offered by the internally compensated capacitor,. If frequency increases by 10, the capacitor reactance decreases by 10. Thus, the voltage gain of op amp goes down by 10 as the frequency of input signal is increased by Band Width The bandwidth of ac amplifiers is the frequency range between the points where the gain is 3 db less than the maximum gain or the maximum gain. In general, the bandwidth equals the upper critical frequency (f H ) to the lower critical frequency ( f L ). The curve shown in figure 12 has single break frequency and its value is 5Hz. It is defined as the frequency at which the gain is 3dB down from its value at 0 Hz Unity gain bandwidth It can be seen from frequency response curve [Fig.12] that the product of the gain and frequency is constant at any point along the curve. Example: (i) At point B Gain= 60dB=1000 and f= 1K GBWP = 1000 x 1000Hz = 1,000,000=1 MHz (i) At point C Gain= 40dB=1000 and f= 10K GBWP = 1000 x 1000Hz = 1,000,000=1 MHz (iii) At point D Gain= (0dB) or unity and f=1mhz GBWP = =1MHz This is called unity gain bandwidth. UGBW= =1MHz. It is defined as the bandwidth at unit gain If frequency response curve is not available then one can calculate bandwidth by using the specification provided by manufacturers, called transit response time (unity gain ). For 741 its value is 0.35μs. Using this value bandwidth can be calculated by using the relation Bandwidth= 0.35/ rise time. Institute of Lifelong Learning, Delhi University Page 14

15 1.6.4 Slew Rate Slew rate is defined as the maximum rate of change of output voltage. Its unit is V/µs. Mathematically it is given by the relation For 741 op-amps, the maximum slew rate is 0.5V/µs. This means that the output voltage can change a maximum of 0.5 V in 1µs. The slowest slew rate is specified at unity gain. Slew rate limiting is caused by a limitation in the internal circuitry of an op-amp to derive capacitive loads. It also depends upon as amplifier gain, and compensating capacitor. Op-amp with slew rate s greater than 100/μs are referred to as high speed operational amplifiers. Cause of Slew Rate Limiting At high frequencies the current available to charge and discharge the compensating capacitance becomes exhausted and slew rate limiting occurs. This shows up as a distortion in the output. The rate of change of voltage across the compensating capacitor is given by where I =15μA, is the maximum current furnished by op-amp to the capacitor C of capacity (30 pf). This means that output of an op-amp can change no faster than 0.5V in a μsec. Example: If a large step input voltage is applied to 741 op-amp. We do not get the sudden step in the output. Instead we get an exponential output wave (dotted) as shown in figure 13 Institute of Lifelong Learning, Delhi University Page 15

16 Fig.13: Step input and exponential (distorted) output. Slew Rate Limiting of Sine Waves For a sinusoidal input signal. Maximum rate of change of output is given by The rate of change depends on both its frequency and the peak amplitude. But maximum rate of change is slew rate, hence It defines how fast the output voltage of op-amp can change in response to change in input frequency. If this rate is lesser than the op amp s slew rate, the op amp output V o will not be distorted because output V o follows V i as shown in figure 14 (a) and if this rate is greater than the op amp s slew rate, the op amp output V o will be distorted as shown in figure 14 (b). That is, output V o tries to follow V i but cannot do so because of slew rate limiting. In this case output is triangular wave instead of sinusoidal.. (a) (b) Fig.14: (a) Sine wave initial slope is less than SR (b) slope is > SR and output. Developed by :ILLL distorted The maximum frequency f max at which undistorted output voltage with a peak value of V p is determined by the slew rate in accordance with Institute of Lifelong Learning, Delhi University Page 16

17 This is called the full power response. It is the maximum frequency of a large amplitude sinusoidal wave that the op-amp can amplify without distortion Rise time Rise time is defined as time required for the output voltage to rise from 10% to 90 % of its final value. Rise time for 741 is 0.35μs. For an output of 20 mv, it would take 0.35μs for the output voltage to change from 2 mv to 18mV. Value addition :Did you Know? What are noncompensated op-amps. Body Text: Some op-amps require external compensating components are called noncompensated op-amps as shown below. V1 3 + A V2 2-3 R C 1 C 2 R, C 1 and C 2 are the external components. By taking different values of these components open-loop bandwidth/frequency response can be changed or tailored. 741 is internally compensated op-amp. Suggested reading: Op-Amps and Linear Integrated Circuits : Ramakant A. Gayakwad,4 th Edition 1.7 Parameters of Op-Amp Differential Input : It is the difference of the signal applied to noninverting and inverting input terminals. [1] Common Input ( : Same input is applied to both input terminals as shown in figure 15. It can be defined as the average of the sum of the two input signals. [2] Institute of Lifelong Learning, Delhi University Page 17

18 For 741C its range is ±13V. Fig 15: Common-mode configuration Output Voltage: Signals applied to inverting and noninverting input terminals in general have both inphase and out-of phase components, and output in this case can be expressed as [3] Where is differential gain and gain in common-mode. Example: Calculate, and by assuming = =1V (i) Same Polarity (same phase signal) So for same phase input signals, only common-mode operation occurs in output. [4] (ii) Opposite polarity: [5] This shows that when the inputs are out of phase only differential mode operation occurs and output the differential gain times twice the input signal applied to one of the inputs (no common mode operation occurs element), Common-Mode Rejection Ratio (CMRR). Differential amplifiers have a ability to cancel out or reject certain types of undesired signals. Such undesired signals are called noise. The important point is that these signals are not to be amplified. Noise signals appear equally at both inputs of the op-amp. It means that any undesired (noise) signals that appears in polarity or common to both the input terminals, will be largely rejected, or cancelled out at the differential amplifier Institute of Lifelong Learning, Delhi University Page 18

19 output. A measure of this rejection of signals is given by the ratio of differential voltage gain to the common mode gain and is given as [6] where, and is output common-mode voltage, input common mode voltage (the voltage common to both inputs and ground). Value of CMMR is very high because of large value of and small value of so CMMR is expressed in decibels. [7] For 741C, CMMR is 90 db. For 741 precision op-amp, CMMR is 120dB. Example: Given that A d =2 10 5,and CMMR =90 db. Calculate A CM Output impedance, (R o ): Its value for 741C is 75Ω and is the measure of the equivalent resistance between output terminal and ground. Input Capacitance( ). It is the equivalent capacitance measured at either of two input terminals with other terminal grounded. For 741C its value is 1.4 pf. Input Resistance (R i ). This is the differential input resistance that can be seen by looking into either of two input terminals with other terminal grounded. Its value is 2MΩ for 741C. For FET input op-amp R i =1000GΩ Large Signal Voltage Gain: When magnitude of output signal is much larger than the input signal, then the voltage gain is called large-signal voltage gain. It is defined as value is 2x10 5. where is output voltage and, is differential input voltage. For 741C, its Supply Current. It is the current drawn by the op-amp when it is connected to power supply. For 741C op-amp the supply current is 2.8 ma. Output Short Circuit Current. (I sc ) It is defined as the maximum output current that can be delivered by op-amp to load. Power Consumption: Power consumed by the 741C is 85 mw. Power supply rejection ratio (PSRR): This is defined as the change in op-amp input offset voltage caused by variation in supply voltage, Institute of Lifelong Learning, Delhi University Page 19

20 Its value lies in the range of 10-5 to 7x10-5 V 1.8 Open-Loop Gain of Op-Amp Amplifier The term open loop means that no connection exists between output and input terminals of an amplifier. That is, the output signal is not fed back/mixed in any form as part of the input signal. There are three open-loop op-amp configurations Differential Amplifier Open-loop differential amplifier is illustrated in figure 16. Input signals and are applied to the noninverting (+) and inverting (-) input terminals. and could be either ac or dc voltage signals. A is the open-loop gain of the amplifier. Fig.16. Open-loop differential amplifier Output voltage is given by the expression V 1 =V in1, and V 2 = V in2, Substituting for and in eq.[8] [8] Thus the output voltage is equal to the voltage gain (A) times the difference between the two input voltages. Polarity of output voltage depends on the polarity of V id. It may be positive or it may be negative, hence the output. [10] Institute of Lifelong Learning, Delhi University Page 20

21 Fig:17. Input ( same polarity,v in1 < V in2 )and output wave forms for open-loop differential amplifier. Source: Self For positive, and for negative, Figure 17 shows output wave forms obtained using p-spice probe, for sinusoidal inputs of same polarity,v in1 < V in2. is negative. Output wave is saturated at. Figure 18 shows sinusoidal inputs wave forms of opposite polarity) and output wave forms obtained using p-spice probe. In this case V id is positive, hence the Institute of Lifelong Learning, Delhi University Page 21

22 Fig:18. Input ( opposite polarity,)and output wave forms for open-loop differential amplifier. Source: Self Output wave form is saturated at Inverting Amplifier In this configuration only one input is applied to inverting (-) input terminal and noninverting (+) terminal is grounded as shown in figure 19. Since and. From Eq.[8] Fig:19. Open-loop inverting amplifier [13] The negative sign indicates that the output voltage is out of phase with respect to input by 180 o or opposite in opposite polarity. Thus the input voltage is amplified by gain A and is also inverted at the output. Hence, the name inverting amplifier Noninverting Amplifier Institute of Lifelong Learning, Delhi University Page 22

23 Figure 20 shows the open-loop noninverting amplifier. In this configuration input is applied to noninverting (+) terminal and inverting (-) input terminal is connected to ground. Fig: 20. Noninverting amplifier From figure and.substituting for and in eq.[8] This means that the output voltage is equal to the product of input voltage and open-loop gain A of op-amp. Output voltage is positive, it reveals that it is in phase with the input signal Frequency Response and Bandwidth Frequency response curve for open-loop operational amplifier is shown in shown in figure 12 (sec and 1.6.2) has single break frequency and its value is 5Hz. Bandwidth equals the upper critical frequency (f H ) to the lower critical frequency ( f L ). Or In this case and Bandwidth of open-loop op-amp is 5Hz. After the break frequency gain drops by a factor of 10 as frequency increases by a factor of 10. Or it decreases by 20dB for an increase in frequency of 1 decade. Unity gain bandwidth of open-loop op-amp (UGBW) is equal to =1MHz. [14] Value addition : FAQ Limiting value of input voltages in open-loop configurations of op-amp. Body Text For an open-loop configuration (i) (ii) Supply voltage ±15 V. For a ±15 V supply,the saturated voltage are approximately ±13 V. Max differential voltage + Institute of Lifelong Learning, Delhi University Page 23

24 Max differential voltage - Hence input voltage greater than ±65μV produces saturated output voltages. This value of input is the limiting value. Suggested Reading: Op-Amps and Linear Integrated Circuits : Ramakant A. Gayakwad,4 th Electronic Devices by Thomas L. Floyd,6 th Edition. Edition. Summary: After completing this section, you should be able to Identify the schematic symbol and IC package terminals Define and discuss op-amp parameters Name the op-amp characteristics that add dc and ac error component to the output voltage. Understand Slew rate Define and calculate common-mode rejection ratio (CMRR) Explain frequency response Explain the three open-loop op-amp configuration Predict the op-amp gain of op-amp at any frequency if unity gain bandwidth is known to you. To explain various configuration of open-loop op-amp. To draw input and output wave forms. Exercises Question Number Type of question 1 Multiple choice questions 1.The op-amp can amplify (a) a.c signals only (b) d.c signals only (c)both a.c and d.c signals (d) Neither d.c nor a.c signals 2. With zero volts on both input,an op-amp ideally should have an output voltage (a) Zero (b) Equal to the positive supply voltage (c) Equal to negative supply voltage (d) Equal to CMMR. 3. When the initial slope of a sine wave is greater than the slew rate of op-amp, (a) Linear operation occurs (b) Distortion occurs (c) Op- amp works best (d) Voltage gain is Maximum. 4. A 741 C has (a) A voltage gain of 100,000 (b) An input impedance of 2MΩ (c) An output impedance of 75 Ω (d) All of above 5. Frequency response is a graph showing relationship between voltage gain and Institute of Lifelong Learning, Delhi University Page 24

25 (a) Frequency (ii) Input voltage (III) Power gain (iv) Output voltage Correct answers (1). C (2). A (3). B (4). D (5). A Question Number Type of question 2 Fill in the blanks 1.The common-mode gain is The 741 C has a unity gain frequency of Hz. 3. The initial slope of sine wave is directly is proportional to Common mode gain of the op-amp is equal to for an op-amp having CMMR value equal to 10 5 and differential gain. 5. Differential amplifier with a input and, usually, a output is generally called an op-amp. Correct Answers 1. very low 2. 1 mega 3. Frequency 4. unity 5. differential, single-ended Institute of Lifelong Learning, Delhi University Page 25

26 Question Number Type of question 3 Subjective questions 1. What is op-amp? 2. What is practical op-amp? Draw its equivalent circuit? 3. Draw the block diagram of op-amp and describe its various blocks. 4. Define common-mode rejection. 5. For a given value of open-loop differential gain, does a higher common-mode gain result in a higher or lower CMRR? 6. What is slew rate? What causes the slew rate? 7. List the parameters that are important for ac applications? 8. Define the following parameters: input offset voltage, input resistance. 10. Why are open-loop op-amp configurations not suitable for linear applications? 11. Discuss why open-loop op-amp configurations are not used in linear applications. 12. Explain how is op-amp used as a differential amplifier? Derive the expression of voltage gain. Question Number Type of question 4 Unsolved questions 1. What is the band width of an op-amp if rise time is 0.25μf 2.Determine the frequency of an op-amp having value of UGBW =1 MHz and A d =100V/mV. 3.Calculate the CMRR in (db) for V id =1mV,V o =120mV and V CM =1mV, V ocm =20 μv 4. Find the maximum frequency for a sine wave output voltage of 10V peak with an op- amp slew rate value is 1V/μs. 5. An op-amp with the following specifications: A=2x10 5, power supply voltage=±15v, and output voltage swing=±13v is used in open-loop noninverting configuration. Find output voltage for the following input voltages (i) -10μV (ii) +10μV (iii) -1mV and (iv) 20 mv rms open-loop op-amp is used in inverting configuration, with following specifications (i) A=10 5, (ii) output voltage swing=±10v and (iii) power supply voltage=±12v. Determine output voltage for input voltage (i) -5 μv and (ii) 1mV rms. Draw input and output wave forms. Institute of Lifelong Learning, Delhi University Page 26

27 Correct answers and solutions 1.Ans. 1.4 HHz Sol: Band width=0.35/rise time =0.35/.25 μs= Hz 2. Ans:10Hz Sol: 3.Ans: CMMR=75.56dB Sol:, =75.56dB 4.Ans:15.92 khz Sol:, 5.Ans (i)=-2v, (ii) +2 V,(iii) (iv) Clipped sine wave. Sol: (i) =-2V (ii) + 2V, (iii) =- 200V, but output will be saturated at (iv) =4000Vrms. So output will clipped at +13 V. 6. Ans: (i) +0.5V, (ii) Saturated output at ±10V Sol: (i) Vo=-[-5μV 10 5 ]=0.5V (ii) Vo=-[1mVrms 10 5 ]=-10 2 Vrms, Saturated output at V=±10V 0.5V 0 t Sol: (i) Input in shown by dotted line and output by solid line -5μV +1.41mV +10V Sol: Output is shown with red line and input with black curve mV -10V GLOSSARY: I.C: Integrated Circuit: A device that contains all components ( diodes,resistors, transistors, capacitors) on a chip. Differential Amplifier: Amplifiers which amplify the difference of input signals. Institute of Lifelong Learning, Delhi University Page 27

28 Direct Coupling: Using a direct wire connection between two stages of amplifier instead of a coupling capacitor. Op-Amp: A high gain dc amplifier, usable in the frequency from 0 to 1MHz. Linear Applications: Those circuits in which output follows input and output never saturates. Push Pull Amplifier: It is a power amplifier in which two transistors are place back to back one as current source, and other one as sink. It is frequently employed in output stages of electronic circuit. FET: Field Effect Transistor J-FET: Junction Field Effect Transistor Node: A junction where two or more components meet. Impedance: It is the ratio of voltage to current Attenuation: It is a general term it refers to any reduction in the intensity of a signal. Loading: A device will make no power demands on the input signal source which is connected at its input. Noise: Noise is an undesired signal that affects the quality of a desired signal. Voltage Gain: Ratio of out voltage or current and input voltage or current. Slew Rate: The maximum rate that the output voltage can change. It causes for high frequency large signal operation Decade: A change in frequency of 10 is called decade Frequency Response: The graph of voltage gain versus frequency. Break Frequency: The frequency at which gain is gain is of maximum gain. Unity Gain Frequency: The frequency where the voltage gain of an op-amp is 1. Bandwidth: It the frequency range over which the voltage gain of the amplifier is 70.7% or -3dB of its maximum output value. Compensating Capacitor: A capacitor inside an op-amp that prevents oscillations. It produces low critical frequency and decreases gain at the rate of 20dB per decade above the mid band. Open-Loop gain: Gain of amplifier without external feedback. Noninverting amplifier: A amplifier in which the input is applied to noninverting terminal. Institute of Lifelong Learning, Delhi University Page 28

29 Inverting amplifier: A amplifier in which the input is applied to inverting terminal. References/ Bibliography/ Further Reading Source: Op-Amps and linear Integrated Circuits : Ramakant A. Gayakwad,4 th Edition. Electronic Principles :A.P. Malvino, 6 th Edition. Electronics Devices and Circuit Theory by Robert. L Boylestad and L. Nashelsky.8 th Edition. Electronic Devices by Thomas L. Floyd,6 th Edition. Institute of Lifelong Learning, Delhi University Page 29

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