Operational Amplifiers

1. Operational Amplifiers Chris Nygren Matt Livianu Brad Schwagler

2. Introduction Background Amplifier Introduction Basic Circuits Review Characteristics of an Ideal Op Amp Types of Op Amps Practical Applications Conclusion Quiz Agenda

3. To introduce the Operational Amplifier by providing background, functionality, applications, and relevance to Mechatronics class projects. Purpose

4. Operational Amplifiers are represented both schematically and realistically below: Active component! Introduction

5. 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 Background

6. Amplifiers • Differential Amplifier • Amplifies difference between inputs Single-ended Amplifier

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+<V-, Vout = LVS • If V+=V-, Vout = 0V

8. 3-stage Op-Amp

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?

10. Negative feedback leads to stable equilibrium Voltage follower (direct feedback) If Vout = V- , then Vout ~ V+ Why are they useful? Closed Loop Transfer Function H(s) = A / (1 + AF) When AF >> 1… H(s) = 1 / F Where: A = Op Amp Open Loop Gain F = Feedback Loop Gain

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.

12. Kirchoff’s Law Voltage Law: The sum of all the voltage drops around the loop = Vin Resistance (Ohms – Ω) Series Parallel Basic Circuits Review V1 + V2 + V3 = Vin

13. Capacitance (Farad – F) Series Parallel Inductance (Henry – H) Series Parallel Basic Circuits Review

14. Zin is infinite Zout is zero Amplification (Gain) Vout / Vin = ∞ Unlimited bandwidth Vout = 0 when Voltage inputs = 0 Ideal Op Amp

15. Ideal Op Amp

16. Comparator (seen earlier) Voltage follower (seen earlier) Signal Modulation Mathematical Operations Filters Voltage-Current signal conversion How are Op-Amps used?

17. Non-inverting Op-Amp www.wikipedia.org Uses: Amplify…straight up

18. Inverting Op-Amp www.wikipedia.org Uses: Analog inverter

19. Comparator V1 Vout V2 www.allaboutcircuits.com Uses: Low-voltage alarms, night light controller

20. Pulse Width Modulator • Output changes when • Vin ~= Vpot • Potentiometer used to vary duty cycle Uses: Motor controllers www.allaboutcircuits.com

21. Summation www.wikipedia.org Uses: Add multiple sensors inputs until a threshold is reached.

22. Difference If all resistors are equal:

23. Integrating Op-Amp www.wikipedia.org Uses: PID Controller

24. Differentiating Op-Amp www.wikipedia.org (where Vin and Vout are functions of time)

25. Filters • Decouple the low-pass RC filter from the load.Uses: Simple audio. Remove frequencies over 20kHz (audible)

26. Cutoff frequency This works because the capacitor needs time to charge. Low-pass Filter (active) www.wikipedia.org

27. High pass filter (active) www.wikipedia.org Band-pass filter cascades both high-pass and low-pass!

28. 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 Measuring current

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 www.allaboutcircuits.com Uses: - In: Sensors (temp, pressure, etc), - Out : Radios (Variable Freq Osc)

30. Op-Amps are often used for Sensor amplification Mathematical operations (sums, difference, inverse) Filters (High/Low/Band pass) Measurement devices Current in –> Voltage out Conclusions

31. 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? Questions?

32. • 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) Practical Tips

33. “Mechatronics”, Sabri Cetinkunt Wikipedia.org Allaboutelectronics.com Bibliography