Operational Amplifier

1
Operational Amplifier

• Nearly ideal
• High gain
• high input impedance
• low output impedance
• stable
• Simple for circuit construction
• Goals
• Learn basic characteristics of operational
  amplifier.
• Build and characterize amplifier circuits.
• http//www.uoguelph.ca/antoon/gadgets/741/741.htm
  l
• http//ourworld.compuserve.com/homepages/Bill_Bowd
  en/opamp.htm
• http//www.allaboutcircuits.com/vol_3/chpt_8/1.htm
  l

2
Pin Configuration

• Notch
• Counterclockwise numbering
• Input
• 2 V- 3 V
• Output
• 6 Vout
• Power supply
• 4 VDD 7 VCC
• 1, 5, offset adjustment

3
Key Characteristics

• Output saturation voltages
• Output voltage never surpasses power supply
• Linear amplifier
• Nonlinear, saturation
• Not always bad!
• Output impedance limitation
• Finite load driving capability
• Bandwidth limitation - Slew rate
• Finite speed or response time
• Gain dependent
• http//www.amplifier.cd/Tutorial/Slew_Rate/SlewRat
  e.htm
• Offset compensation
• Cascading

4
Circuit

• Two Input Nodes
• What will happen if the output is connected to
  the non-inverting input?

5
Dual Power Supply

• Set voltages, Limit current

Earth Ground Do NOT use
15 to pin 7
-15 to pin 4
Ground Reference Dont let it dangle
6
Open Loop

• R2 ?, R1 0
• Ground non-inverting input
• Signal applied to inverting input
• Differential gain gt 10000
• Driven into saturation
• Measure the output impedance with a load resistor
• Output reduced by 2

7
Waveform
Vsat
8
Unit Gain Amplifier

• Connect output to inverting input
• Do not ground the inverting input
• Signal applied to non-inverting input
• Gain 1
• How fast can the output change?
• Slew rate, ?V / ?t around 0.6 V/?sec
• Are inverting and non-inverting input ports
  symmetric? What will happen when there is a
  feedback from output to non-inverting input?

9
Waveform
Slew Rate
10
Inverting Amplifier

• Feedback R2
• Ground pin 3
• Virtual ground at pin 2
• Input impedance gt R1, R2
• Output impedance lt R1, R2
• Av - R2 / R1

11
Waveform
12
Bode Plot
13
Frequency Response

• Gain vs. frequency plot
• Amplify to DC
• -3dB frequency
• Higher gain, less bandwidth
• Gain bandwidth product

14
Non-Ideal Characteristics

• Offset
• DC input is zero but output is not zero
• Adjust using pins 1 and 5

15
Non-Ideal Characteristics

• Finite input impedance
• Leakage current
• Symmetric input
• Common mode rejection ratio
• Differential input
• Gain
• Common input
• No gain

http//www.mines.edu/Academic/courses/physics/phgn
217/lab8/lect13/sld001.htm
16
Circuit to Build
17
Measurements

• Saturation voltages
• Output impedance
• Slew rate
• Gain vs frequency
• Bandwidth and gain correlation
• In cascaded amplifiers, how to optimize the
  bandwidth?
• Output offset before and after adjustment

18
Variations of Op Amp Circuit

• Non-inverting amplifier

19
Applications of Op Amp

• Microphone pick up
• Audio amplifier
• Active filters
• Log amplifier
• Laser or LED driver

20
Review

• Pin assignments
• Inverting input versus non-inverting input
• Typical saturation voltage, output impedance, and
  slew rate
• How to build an inverting amplifier?
• How to adjust the gain?
• Is the bandwidth gain dependent?
• Gain bandwidth product

21
Challenges

• Can the operational amplifier be operated with a
  single power supply?
• Can you build a summing amplifier or a
  subtracting amplifier?
• What is the instrumentation amplifier? Can you
  build one by using 741s?

22
Circuit Simulations