Introduction to Op Amps

Transcription

1 Introduction to Op Amps ENGI 242 ELEC 222 Basic Op-Amp The op-amp is a differential amplifier with a very high open loop gain 25k AVOL 500k (much higher for FET inputs) high input impedance 500kΩ ZIN 10MΩ low output impedance 25Ω RO 100Ω April 2004 ENGI 242/ELEC 222 Op Amps 2 Op Amps 1

2 Op-Amp Equivalent Circuit April 2004 ENGI 242/ELEC 222 Op Amps 3 Op-Amp Specifications DC Offset Parameters Even though the input voltage is 0, there will be an output. This is called offset. The following can cause this offset: Input Offset Voltage Output Offset Voltage due to Input Offset Current Total Offset Voltage Due to Input Offset Voltage and Input Offset Current Input Bias Current See lm301.pdf or mc1741c.pdf for sample specification sheets April 2004 ENGI 242/ELEC 222 Op Amps 4 Op Amps 2

3 General Op-Amp Specifications VIO Input Offset Voltage VIO The voltage that must be applied to the input terminals of an op amp to null the output voltage Typical value is 2mV with a max of 6mV When operated open loop, must be nulled or device may saturate April 2004 ENGI 242/ELEC 222 Op Amps 5 General Op-Amp Specifications IIO Input Offset Current The algebraic difference between the two input currents These are base currents and are usually nulled Typical value IIO 20 na with a max of 200nA April 2004 ENGI 242/ELEC 222 Op Amps 6 Op Amps 3

4 Technique to Null VO Short Input terminals to ground Connect potentiometer between compensation pins with wiper to VEE Potentiometer is usually a 10 turn device Connect meter to output and adjust potentiometer for VO = 0 April 2004 ENGI 242/ELEC 222 Op Amps 7 General Op-Amp Specifications CMRR VO A D = VIN V OCM A CM = VCM A CMRR = 20 log A Common Mode Rejection Ratio The ratio of the differential voltage gain (AD) to the common mode gain (ACM) ACM is the ratio between the differential input voltage (VINCM) applied common mode, and the common mode output voltage (VOCM) it can exceed minimum is 70db with a typical value of 90 db in properly designed circuit, it may exceed 110db D CM April 2004 ENGI 242/ELEC 222 Op Amps 8 Op Amps 4

5 General Op-Amp Specifications Input Bias Current The average of the currents that flow into the inverting and noninverting terminals Typical values rage from 7nA to 80 na Differential Input Resistance Also know as the input resistance Resistance seen looking into the input terminals of the device Runs from a low of 2MΩ for an LM741 to a high of Ω for FET input devices Output resistance Resistance between the output terminal ad ground Typical values are 75Ω or less Input Capacitance The equivalent capacitance measured at either the inverting or noninverting terminal with the other terminal connected to ground May not be on all spec sheets Typical value for LM741 is 1.4pF I B = I + I 2 B+ B- April 2004 ENGI 242/ELEC 222 Op Amps 9 General Op-Amp Specifications Power Supply Range May be differential or single ended Max is ± 22V Output Voltage Swing Range of output voltage Depends on power supply voltage used (typically about 85% to 90%) Usually about ±13.5V for a power supply voltage of ±15V Slew Rate The maximum rate of change in the output voltage in response to an input change Depends greatly on device, higher is better (output resonds faster to input changes) For LM741 it is.5v/µs while for the LM318 it is 70V /µs Gain Bandwidth Product The bandwidth of the device when the open loop voltage gain is 1 April 2004 ENGI 242/ELEC 222 Op Amps 10 Op Amps 5

6 Op Amp Equivalent Circuit April 2004 ENGI 242/ELEC 222 Op Amps 11 Op-Amp Gain Op-Amps have a very high gain. They can be connected open- or closed loop. Open-loop (AVOL) refers to a configuration where there is no feedback from output back to the input AVOL may exceed 10,000 Closed-loop (AVCL) configuration reduces the gain In order to control the gain of an op-amp it must have negative feedback Negative feedback will reduce the gain and improve many characteristics of the op-amp April 2004 ENGI 242/ELEC 222 Op Amps 12 Op Amps 6

7 Typical Op Amp Frequency Response April 2004 ENGI 242/ELEC 222 Op Amps 13 Change in AV with Feedback April 2004 ENGI 242/ELEC 222 Op Amps 14 Op Amps 7

8 Virtual Ground Since ZIN is very high, we assume no current can flow into any lead of the op amp When the noninverting input pin is at ground, the inverting input pin is at 0V The equivalent circuit. April 2004 ENGI 242/ELEC 222 Op Amps 15 Practical Op-Amp Circuits Typical Op-amp circuit configurations include the: Unity Gain Buffer (Voltage Follower) Inverting Amplifier Noninverting Amplifier Summing Amplifier Integrator Differentiator Note: the integrator and differentiator are considered active filters April 2004 ENGI 242/ELEC 222 Op Amps 16 Op Amps 8

9 Unity Gain Buffer (Follower) A V = V V V V O = 1 A V = 1 O 1 April 2004 ENGI 242/ELEC 222 Op Amps 17 Inverting Op Amp The input is applied to the inverting (-) input the non-inverting input (+) is grounded RF is the feedback resistor, and is connected from the output to the inverting input This is called negative feedback April 2004 ENGI 242/ELEC 222 Op Amps 18 Op Amps 9

10 Inverting Op Amp We assume that no current enters the inverting terminal II- < 100nA VD 0V O IN F A V = = - S IN 1 A V V I R V I R RF = - R 1 April 2004 ENGI 242/ELEC 222 Op Amps 19 Inverting Op-Amp Gain Closed Loop Gain is controlled by the external resistors: RF and R1 O IN F A V = = - S IN 1 F A V = - R 1 V I R V I R R For Unity Gain: AV is -1 and RF = R1 R R F A V = - = -1 1 The minus sign denotes a 180 degree phase shift between input and output April 2004 ENGI 242/ELEC 222 Op Amps 20 Op Amps 10

11 Inverting Op Amp Compensated for Ibias R is used to compensate for difference in IBIAS+ and IBIAS- A V R = - R F 1 April 2004 ENGI 242/ELEC 222 Op Amps 21 Inverting Op-Amp This configuration achieves high gain with a smaller range of resistor values than the basic inverter A V- V+ R 2+ R F R 2R F A V = - + R 1 R 1R 3 April 2004 ENGI 242/ELEC 222 Op Amps 22 Op Amps 11

12 Inverting Amplifier with High Zin Use a Unity Gain Buffer to obtain a very high input resistance with an inverting amplifier April 2004 ENGI 242/ELEC 222 Op Amps 23 Inverting Amplifier for Low RL Use a Unity Gain Buffer to obtain a very high input resistance to drive a low impedance load April 2004 ENGI 242/ELEC 222 Op Amps 24 Op Amps 12

13 Noninverting Amplifier R 2 V O = V in 1 + R1 V O R 2 A V = = 1 + V in R1 V- = V+ = vi April 2004 ENGI 242/ELEC 222 Op Amps 25 Noninverting Op Amp Compensated for IBIAS Rbias is used to compensate for difference in IBIAS+ and IBIAS- April 2004 ENGI 242/ELEC 222 Op Amps 26 Op Amps 13

14 Differential (Difference) Amplifier V1 V2 A A V R - V - V R O 2 A V = - = April 2004 ENGI 242/ELEC 222 Op Amps 27 Differential Amplifier Output April 2004 ENGI 242/ELEC 222 Op Amps 28 Op Amps 14

15 Instrumentation Amplifier Buffered Input R1 = R2, RF1 = RF2 A V = April 2004 ENGI 242/ELEC 222 Op Amps 29 - R R F 1 Instrumentation Amplifier R1 = R2, RF1 = RF2 R F R A A V = R 1 RB April 2004 ENGI 242/ELEC 222 Op Amps 30 Op Amps 15

16 Inverting Summing Amplifier By applying KCL to the multiple inputs, we can consider the contribution of each source individually IF + I- = I1 + I2 + I3 but I- 0 IF = I1 + I2 + I3 VO = -IF RF RF RF RF V O = - V 1 + V 2 + V3 R1 R2 R3 V 1 V 2 V3 V O = - R F + + R1 R2 R3 April 2004 ENGI 242/ELEC 222 Op Amps 31 Non-inverting Summing Amplifier Perform a source transformation for each input Sum the current sources and find RTH for the resistances VIN+ = IT RTH V 1 V 2 V 3 V IN + = + + R R1 R2 R3 where R = R // R // R VIN + V = R + R R IN V O RF A V = = 1 + V IN + RIN ( ) TH TH O IN F April 2004 ENGI 242/ELEC 222 Op Amps 32 Op Amps 16

17 Integrator The output is the integral of the input This circuit is a low-pass filter circuit, and is used and sensor conditioning circuits 1 (t) = v (t)dt RC vo 1 April 2004 ENGI 242/ELEC 222 Op Amps 33 Differentiator The differentiator takes the derivative of the input This circuit is a high-pass filter circuits dv1(t) vo(t) = RC dt April 2004 ENGI 242/ELEC 222 Op Amps 34 Op Amps 17

18 Comparator High Gain Op Amp Operated Open Loop Designed to compare an input to a reference voltage Gives output (digital level) to indicate if input is above or below reference Circuit designed to give VOSAT and VOSAT only April 2004 ENGI 242/ELEC 222 Op Amps 35 Comparator Operation Example April 2004 ENGI 242/ELEC 222 Op Amps 36 Op Amps 18

19 LM 311 Comparator April 2004 ENGI 242/ELEC 222 Op Amps 37 Window Comparator April 2004 ENGI 242/ELEC 222 Op Amps 38 Op Amps 19

20 Determine the Output April 2004 ENGI 242/ELEC 222 Op Amps 39 Block Diagram of 555 April 2004 ENGI 242/ELEC 222 Op Amps 40 Op Amps 20

21 Astable Multivibrator April 2004 ENGI 242/ELEC 222 Op Amps Used as an Astable Multivibrator April 2004 ENGI 242/ELEC 222 Op Amps 42 Op Amps 21

22 Schmidt Trigger 7414 A Schmidt trigger (a comparator with Hysteresis) is a bistable digital (twostate) device It accepts virtually any analog input and provides a logic 0 or 1 output A typical use is to take distorted digital signals (due to RC time constant of transmission line) and provide a used to square-wave output Can be used to eliminate noise near reference point that would cause problems in analog comparators April 2004 ENGI 242/ELEC 222 Op Amps 43 Hysteresis April 2004 ENGI 242/ELEC 222 Op Amps 44 Op Amps 22