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Operational Amplifiers Georgia Institute of Technology PowerPoint Presentation

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On : Jan 08, 2015

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  • Slide 1 - Operational Amplifiers Chris Nygren Matt Livianu Brad Schwagler
  • Slide 2 - Agenda Introduction Background Amplifier Introduction Basic Circuits Review Characteristics of an Ideal Op Amp Types of Op Amps Practical Applications Conclusion Quiz
  • Slide 3 - Purpose To introduce the Operational Amplifier by providing background, functionality, applications, and relevance to Mechatronics class projects.
  • Slide 4 - Introduction Operational Amplifiers are represented both schematically and realistically below: Active component!
  • Slide 5 - Background Originally invented in early 1940s using vacuum tube technology Initial purpose was to execute math operations in analog electronic calculating machines Shrunk in size with invention of transistor Most now made on integrated circuit (IC) Only most demanding applications use discrete components Huge variety of applications, low cost, and ease of mass production make them extremely popular
  • Slide 6 - Amplifiers Differential Amplifier Amplifies difference between inputs Single-ended Amplifier
  • Slide 7 - Operational Amplifier Output gain high A ~= 106 Tiny difference in the input voltages result in a very large output voltage Output limited by supply voltages Comparator If V+>V-, Vout = HVS If V+
  • Slide 8 - 3-stage Op-Amp
  • Slide 9 - Sensor signals are often too weak or too noisy Op Amps ideally increase the signal amplitude without affecting its other properties Why are they useful?
  • Slide 10 - Negative feedback leads to stable equilibrium Voltage follower (direct feedback) If Vout = V- , then Vout ~ V+ Closed Loop Transfer Function H(s) = A / (1 + AF) When AF >> 1… H(s) = 1 / F Why are they useful? Where: A = Op Amp Open Loop Gain F = Feedback Loop Gain
  • Slide 11 - Op Amp Golden Rules The output attempts to do whatever is necessary to make the voltage difference between the inputs zero. The inputs draw no current.
  • Slide 12 - Basic Circuits Review Kirchoff’s Law Voltage Law: The sum of all the voltage drops around the loop = Vin Resistance (Ohms – Ω) Series Parallel V1 + V2 + V3 = Vin
  • Slide 13 - Basic Circuits Review Capacitance (Farad – F) Series Parallel Inductance (Henry – H) Series Parallel
  • Slide 14 - Ideal Op Amp Zin is infinite Zout is zero Amplification (Gain) Vout / Vin = ∞ Unlimited bandwidth Vout = 0 when Voltage inputs = 0
  • Slide 15 - Ideal Op Amp
  • Slide 16 - How are Op-Amps used? Comparator (seen earlier) Voltage follower (seen earlier) Signal Modulation Mathematical Operations Filters Voltage-Current signal conversion
  • Slide 17 - Non-inverting Op-Amp Uses: Amplify…straight up www.wikipedia.org
  • Slide 18 - Inverting Op-Amp                                                                         Uses: Analog inverter www.wikipedia.org
  • Slide 19 - Comparator www.allaboutcircuits.com Uses: Low-voltage alarms, night light controller V2 V1 Vout
  • Slide 20 - Pulse Width Modulator Output changes when Vin ~= Vpot Potentiometer used to vary duty cycle www.allaboutcircuits.com Uses: Motor controllers
  • Slide 21 - Summation Uses: Add multiple sensors inputs until a threshold is reached. www.wikipedia.org
  • Slide 22 - Difference If all resistors are equal:
  • Slide 23 - Integrating Op-Amp Uses: PID Controller www.wikipedia.org
  • Slide 24 - Differentiating Op-Amp (where Vin and Vout are functions of time) www.wikipedia.org
  • Slide 25 - Filters Decouple the low-pass RC filter from the load. Uses: Simple audio. Remove frequencies over 20kHz (audible)
  • Slide 26 - Low-pass Filter (active) Cutoff frequency This works because the capacitor needs time to charge. www.wikipedia.org
  • Slide 27 - High pass filter (active) Band-pass filter cascades both high-pass and low-pass! www.wikipedia.org
  • Slide 28 - Measuring current Current (I) better than voltage (V) for measurement Voltage suffers losses due to resistances in path Low impedance is better for resisting noise So how do we generate a constant current source? Transconductance Amplifier
  • Slide 29 - Transconductance Amp Precision 250Ω resistor 1V / 250 Ω = 4mA 5V / 250 Ω = 20mA RLoad doesn’t matter, just as long as op-amp has high enough voltage rails Uses: - In: Sensors (temp, pressure, etc), - Out : Radios (Variable Freq Osc) www.allaboutcircuits.com
  • Slide 30 - Conclusions Op-Amps are often used for Sensor amplification Mathematical operations (sums, difference, inverse) Filters (High/Low/Band pass) Measurement devices Current in –> Voltage out
  • Slide 31 - Questions? Does an Op-Amp amplify current or voltage? Can you use an Op-Amp as a buffer? If so, How? Why should you care about the device bandwidth rating? What is the most common Op Amp chip? What is an ‘active’ component? Is an Op Amp an active or a passive component? What is the advantage of an active vs. passive filter?
  • Slide 32 - Practical Tips • Try to use single supply op-amps in order to minimize need for a 10V difference from power supply • Good low resistance, twisted, and shielded wire should be used when a sensor is located far away from the op-amp circuit. • Minimize current draw in sensor circuits to reduce thermal drift • Filter power into op-amp circuits using capacitors • Design op-amp circuits so output cannot be negative in order to protect 68HC11 A/D port. • Isolate op-amp circuit output with unity gain op-amp if connected to an actuator. • Make sure bandwidth of op-amp is adequate • Use trimmer potentiometers to balance resistors in differential op-amp circuits • Samples of op-amps can be obtained from National Semiconductor (http://www.national.com)
  • Slide 33 - Bibliography “Mechatronics”, Sabri Cetinkunt Wikipedia.org Allaboutelectronics.com
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