Operational Amplifiers Chris Nygren Matt Livianu Brad Schwagler

Operational Amplifiers Chris Nygren Matt Livianu Brad Schwagler ME 6405 Introduction to Mechatronics

Agenda • • • Introduction Background Amplifier Introduction Basic Circuits Review Characteristics of an Ideal Op Amp Types of Op Amps Practical Applications Conclusion Quiz ME 6405 Introduction to Mechatronics

Purpose To introduce the Operational Amplifier by providing background, functionality, applications, and relevance to Mechatronics class projects. ME 6405 Introduction to Mechatronics

Introduction • Operational Amplifiers are represented both schematically and realistically below: – Active component! ME 6405 Introduction to Mechatronics

Background • Originally invented in early 1940 s 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 ME 6405 Introduction to Mechatronics

Amplifiers Single-ended Amplifier • Differential Amplifier – Amplifies difference between inputs ME 6405 Introduction to Mechatronics

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 = 0 V ME 6405 Introduction to Mechatronics

3 -stage Op-Amp ME 6405 Introduction to Mechatronics

Why are they useful? • Sensor signals are often too weak or too noisy – Op Amps ideally increase the signal amplitude without affecting its other properties ME 6405 Introduction to Mechatronics

Why are they useful? • 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… Where: A = Op Amp Open Loop Gain H(s) = 1 / F F = Feedback Loop Gain ME 6405 Introduction to Mechatronics

1. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero. 2. The inputs draw no current. ME 6405 Introduction to Mechatronics

Basic Circuits Review • Kirchoff’s Law – Voltage Law: The sum of all the voltage drops around the loop = Vin V 1 + V 2 + V 3 = Vin • Resistance (Ohms – Ω) – Series – Parallel ME 6405 Introduction to Mechatronics

Basic Circuits Review • Capacitance (Farad – F) – Series – Parallel • Inductance (Henry – H) – Series – Parallel ME 6405 Introduction to Mechatronics

Ideal Op Amp • • • Zin is infinite Zout is zero Amplification (Gain) Vout / Vin = ∞ Unlimited bandwidth Vout = 0 when Voltage inputs = 0 ME 6405 Introduction to Mechatronics

Ideal Op Amp Ideal Op-Amp Typical Op-Amp Input Resistance infinity 106 (bipolar) 109 - 1012 (FET) Input Current 0 10 -12 – 10 -8 A Output Resistance 0 100 – 1000 Operational Gain infinity 105 - 109 Common Mode Gain 0 10 -5 Bandwidth infinity Attenuates and phases at high frequencies (depends on slew rate) Temperature independent Bandwidth and gain ME 6405 Introduction to Mechatronics

How are Op-Amps used? • • • Comparator (seen earlier) Voltage follower (seen earlier) Signal Modulation Mathematical Operations Filters Voltage-Current signal conversion ME 6405 Introduction to Mechatronics

Non-inverting Op-Amp www. wikipedia. org Uses: Amplify…straight up ME 6405 Introduction to Mechatronics

Inverting Op-Amp www. wikipedia. org Uses: Analog inverter ME 6405 Introduction to Mechatronics

Comparator V 1 Vout V 2 www. allaboutcircuits. com Uses: Low-voltage alarms, night light controller ME 6405 Introduction to Mechatronics

Pulse Width Modulator • Output changes when – Vin ~= Vpot • Potentiometer used to vary duty cycle www. allaboutcircuits. com Uses: Motor controllers ME 6405 Introduction to Mechatronics

Summation www. wikipedia. org Uses: Add multiple sensors inputs until a threshold is reached. ME 6405 Introduction to Mechatronics

Difference If all resistors are equal: ME 6405 Introduction to Mechatronics

Integrating Op-Amp www. wikipedia. org Uses: PID Controller ME 6405 Introduction to Mechatronics

Differentiating Op-Amp www. wikipedia. org (where Vin and Vout are functions of time) ME 6405 Introduction to Mechatronics

Filters • Decouple the low-pass RC filter from the load. Uses: Simple audio. Remove frequencies over 20 k. Hz (audible) ME 6405 Introduction to Mechatronics

Low-pass Filter (active) • Cutoff frequency www. wikipedia. org • This works because the capacitor needs time to charge. ME 6405 Introduction to Mechatronics

High pass filter (active) www. wikipedia. org Band-pass filter cascades both high-pass and low-pass! ME 6405 Introduction to Mechatronics

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 ME 6405 Introduction to Mechatronics

Transconductance Amp • • www. allaboutcircuits. com Uses: Precision 250Ω resistor 1 V / 250 Ω = 4 m. A 5 V / 250 Ω = 20 m. A RLoad doesn’t matter, just as long as op-amp has high enough voltage rails - In: Sensors (temp, pressure, etc), - Out : Radios (Variable Freq Osc) ME 6405 Introduction to Mechatronics

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 ME 6405 Introduction to Mechatronics

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? ME 6405 Introduction to Mechatronics

Practical Tips • Try to use single supply op-amps in order to minimize need for a 10 V 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 68 HC 11 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) ME 6405 Introduction to Mechatronics

Bibliography • “Mechatronics”, Sabri Cetinkunt • Wikipedia. org • Allaboutelectronics. com ME 6405 Introduction to Mechatronics