1. Introduction to Op Amps
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April 2004
Introduction to Op Amps
ENGI 242
ELEC 222
Op Amps

2. 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 k ≤ ZIN ≤ 10M
low output impedance  ≤ RO ≤ 100
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3. Op-Amp Equivalent Circuit
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ENGI 242/ELEC 222 Op Amps

4. 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
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5. 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
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6. General Op-Amp Specifications IIO
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April 2004
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
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Op Amps

7. 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

8. General Op-Amp Specifications CMRR
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
April 2004
ENGI 242/ELEC 222 Op Amps

9. 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 2M for an LM741 to a high of 1012 for FET input devices
Output resistance
Resistance between the output terminal ad ground
Typical values are 75 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
April 2004
ENGI 242/ELEC 222 Op Amps

10. 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/s while for the LM318 it is 70V /s
Gain Bandwidth Product
The bandwidth of the device when the open loop voltage gain is 1
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11. Op Amp Equivalent Circuit
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12. 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
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13. Typical Op Amp Frequency Response
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14. Change in AV with Feedback
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15. Virtual Ground
Since ZIN is very high, we assume no current can flow into any lead of the op amp
When the non-inverting input pin is at ground, the inverting input pin is at 0V
The equivalent circuit.
April 2004
ENGI 242/ELEC 222 Op Amps

16. 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
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17. Unity Gain Buffer (Follower)
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18. 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
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19. Inverting Op Amp
We assume that no current enters the inverting terminal
II- < 100nA
VD  0V
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20. Inverting Op-Amp Gain
Closed Loop Gain is controlled by the external resistors: RF and R1
For Unity Gain: AV is -1 and RF = R1
The minus sign denotes a 180 degree phase shift between input and output
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21. Inverting Op Amp Compensated for Ibias
R is used to compensate for difference in IBIAS+ and IBIAS-
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22. Inverting Op-Amp A
This configuration achieves high gain with a smaller range of resistor values than the basic inverter V- V+
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23. Inverting Amplifier with High Zin
Use a Unity Gain Buffer to obtain a very high input resistance with an inverting amplifier
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24. Inverting Amplifier for Low RL
Use a Unity Gain Buffer to obtain a very high input resistance to drive a low impedance load
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25. Noninverting Amplifier
V- = V+ = vi
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26. Noninverting Op Amp Compensated for IBIAS
Rbias is used to compensate for difference in IBIAS+ and IBIAS-
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27. Differential (Difference) AmplifierV1 V2 A
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28. Differential Amplifier Output
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29. Instrumentation Amplifier Buffered Input
R1 = R2, RF1 = RF2
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30. Instrumentation Amplifier
R1 = R2, RF1 = RF2
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31. 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
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32. Non-inverting Summing Amplifier
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Non-inverting Summing Amplifier
Perform a source transformation for each input
Sum the current sources and find RTH for the resistances
VIN+ = IT RTH
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Op Amps

33. The output is the integral of the input
Integrator
The output is the integral of the input
This circuit is a low-pass filter circuit, and is used and sensor conditioning circuits
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34. Differentiator The differentiator takes the derivative of the input
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April 2004
Differentiator
The differentiator takes the derivative of the input
This circuit is a high-pass filter circuits
April 2004
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Op Amps

35. 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
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36. Comparator Operation Example
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37. LM 311 Comparator
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38. Window Comparator
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39. Determine the Output
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40. Block Diagram of 555
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41. Astable Multivibrator
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42. 555 Used as an Astable Multivibrator
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43. Schmidt Trigger 7414
A Schmidt trigger (a comparator with Hysteresis) is a bistable digital (two-state) 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
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44. Hysteresis April 2004 ENGI 242/ELEC 222 Op Amps