Lecture 5: The ideal operational amplifier

Transcription

1 Lecture 5: The ideal operational amplifier g The ideal operational amplifier n Termals n Basic ideal opamp properties g Opamp families g Operational amplifier circuits n Comparator and buffer n Invertg and nonvertg amplifier n Summg and differential amplifier n Integratg and differentiatg amplifier n Currentvoltage conversion icardo GutierrezOsuna 1

2 The ideal opamp g Primary opamp termals n Invertg put n Nonvertg put n Output n Power supply From [Car91] icardo GutierrezOsuna 2

3 Ideal opamp characteristics g The ideal opamp is characterized by seven properties n Knowledge of these properties is sufficient to design and analyze a large number of useful circuits g Basic opamp properties n Infite openloop voltage ga n Infite put impedance n Zero put impedance n Zero noise contribution n Zero DC put offset n Infite bandwidth n Differential puts that stick together icardo GutierrezOsuna 3

4 Ideal OpAmp Properties g Property No.1: Infite OpenLoop Ga n OpenLoop Ga A vol is the ga of the opamp with positive or negative feedback n In the ideal opamp A vol is fite g Typical values range from 20,000 to 200,000 real devices g Property No.2: Infite Input Impedance n Input impedance is the ratio of put voltage to put current n When Z is fite, the put current I =0 g Highgrade opamps can have put impedance the TΩ range n Z = I Some lowgrade opamps, on the other hand, can have ma put currents icardo GutierrezOsuna 4

5 Ideal OpAmp Properties g Property No. 3: Zero Output Impedance n The ideal opamp acts as a perfect ternal voltage source with no ternal resistance g This ternal resistance is series with the load, reducg the put voltage available to the load g eal opamps have putimpedance the 10020Ω range n Example 1 0 = icardo GutierrezOsuna 5

6 Ideal OpAmp Properties g Property No.4: Zero Noise Contribution n In the ideal opamp, zero noise voltage is produced ternally g This is, any noise at the put must have been at the put as well n Practical opamp are affected by several noise sources, such as resistive and semiconductor noise g These effects can have considerable effects low signallevel applications g Property No. 5: Zero put Offset n The put offset is the put voltage of an amplifier when both puts are grounded n The ideal opamp has zero put offset, but real opamps have some amount of put offset voltage 0 icardo GutierrezOsuna 6

7 Ideal OpAmp Properties g Property No. 6: Infite Bandwidth n The ideal opamp will amplify all signals from DC to the highest AC frequencies n In real opamps, the bandwidth is rather limited g This limitation is specified by the GaBandwidth product (GB), which is equal to the frequency where the amplifier ga becomes unity g Some opamps, such as the 741 family, have very limited bandwidth of up to a few KHz g Property No. 7: Differential Inputs Stick Together n In the ideal opamp, a voltage applied to one put also appears at the other put icardo GutierrezOsuna 7

8 Operational amplifier types g GeneralPurpose OpAmps n These devices are designed for a very wide range of applications g These opamps have limited bandwidth but return have very good stability (they are called frequency compensated) n Noncompensated opamps have wider frequency response but have a tendency to oscillate g oltage Comparators n These are devices that have no negative feedback networks and therefore saturate with very low (µ) put signal voltages g Used to compare signal levels of the puts g Low Input Current OpAmps n Opamps with very low (picoamp) put currents, as opposed to µa or ma put currents found other devices g Low Noise OpAmps n Optimized to reduce ternal noise g Typically employed the first stages of amplification circuits g Low Power OpAmps n Optimized for low power consumption g These devices can operate at low powersupply voltages (I.e., ±1.5DC) g Low Drift OpAmps n Internally compensated to mimize drift caused by temperature g Typically employed strumentation circuits with lowlevel put signals icardo GutierrezOsuna 8

9 Operational amplifier types g Wide Bandwidth OpAmps n These devices have a very high GB product (i.e., 100MHz) compared to 741type opamps (0.31.2MHz) g These devices are sometimes called video opamps g Sgle DC Supply OpAmps n Devices that operate from a monopolar DC power supply voltage g Higholtage OpAmps n Devices that operate at high DC power supply voltages (i.e. ±44DC) compared to most other opamps (±6 to ±22) g Multiple Devices n Those that have more than one opamp the same package (i.e., dual or quad opamps) g Instrumentation OpAmps n These are DC differential amplifiers made with 23 ternal opamps g oltage ga is commonly set with external resistors icardo GutierrezOsuna 9

10 Families of operational amplifiers From [Car91] icardo GutierrezOsuna 10

11 Opamp practical circuits g oltage comparator CC time = CC sign ( ) CC g oltage follower n What is the ma use of this circuit? g Bufferg = icardo GutierrezOsuna 11

12 Invertg and nonvertg amplifiers g Nonvertg amplifier 2 1 = g Invertg amplifier 2 1 = 2 1 icardo GutierrezOsuna 12

13 Summg and differential amplifier g Summg amplifier 1 1 f 2 N 2 N = 1 f 1 2 f 2 L N f N g Differential amplifier = 2 1 ( ) icardo GutierrezOsuna 13

14 Integratg and differentiatg amplifier g Integratg amplifier C 1 1 = = dt j C C g Differentiatg amplifier C = 1 j C = d C dt icardo GutierrezOsuna 14

15 Current to voltage conversion g Currenttovoltage I = I g oltage to current 2 I L = I L L icardo GutierrezOsuna 15

16 eferences [Car91] [Whi96] [Elg98] [PAW91] J. J. Carr, 1991, Designer s Handbook of Instrumentation and Control Circuits, Academic Press, San Diego, CA. J. C. Whitaker, 1996, The Electronics Handbook, CC Press P. Elgar, 1998, Sensors for Measurement and Control, Addison Wesley Longman, Essex, UK.. PallasAreny and J. G. Webster, 1991, Sensors and Signal Conditiong, Wiley, New York icardo GutierrezOsuna 16