Operational Amplifiers

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# Operational Amplifiers - PowerPoint PPT Presentation

Operational Amplifiers. Chris Nygren Matt Livianu Brad Schwagler. Introduction Background Amplifier Introduction Basic Circuits Review Characteristics of an Ideal Op Amp Types of Op Amps Practical Applications Conclusion Quiz. Agenda. To introduce the Operational Amplifier by

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## Operational Amplifiers

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### Operational Amplifiers

Chris Nygren

Matt Livianu

Introduction

Background

Amplifier Introduction

Basic Circuits Review

Characteristics of an Ideal Op Amp

Types of Op Amps

Practical Applications

Conclusion

Quiz

Agenda
To introduce the Operational Amplifier by

providing background, functionality,

applications, and relevance to Mechatronics

class projects.

Purpose

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
Amplifiers
• Differential Amplifier
• Amplifies difference between inputs

Single-ended Amplifier

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
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?
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

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.
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

Series

Parallel

Inductance (Henry – H)

Series

Parallel

Basic Circuits Review
Zin is infinite

Zout is zero

Amplification (Gain) Vout / Vin = ∞

Unlimited bandwidth

Vout = 0 when Voltage inputs = 0

Ideal Op Amp
Comparator (seen earlier)

Voltage follower (seen earlier)

Signal Modulation

Mathematical Operations

Filters

Voltage-Current signal conversion

How are Op-Amps used?
Non-inverting Op-Amp

www.wikipedia.org

Uses: Amplify…straight up

Inverting Op-Amp

www.wikipedia.org

Comparator

V1

Vout

V2

Uses: Low-voltage alarms, night light controller

Pulse Width Modulator
• Output changes when
• Vin ~= Vpot
• Potentiometer used to vary duty cycle

Uses: Motor controllers

Summation

www.wikipedia.org

Uses: Add multiple sensors inputs until a threshold is reached.

Difference

If all resistors are equal:

Integrating Op-Amp

www.wikipedia.org

Uses: PID Controller

Differentiating Op-Amp

www.wikipedia.org

(where Vin and Vout are functions of time)

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

This works because the capacitor needs time to charge.

Low-pass Filter (active)

www.wikipedia.org

High pass filter (active)

www.wikipedia.org

Band-pass filter cascades both high-pass and low-pass!

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
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)

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
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?
• 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
“Mechatronics”, Sabri Cetinkunt

Wikipedia.org