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

ME 6405 Introduction to Mechatronics

Operational Amplifiers

Chris Nygren

Matt Livianu

Brad Schwagler


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


Purpose

To introduce the Operational Amplifier by

providing background, functionality,

applications, and relevance to Mechatronics

class projects.


Introduction

• Operational Amplifiers are represented both schematically and realistically below:– Active component!


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


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


3-stage Op-Amp


• 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+

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


1. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.

2. The inputs draw no current.


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


Basic Circuits Review

• Capacitance (Farad – F)– Series

– Parallel

• Inductance (Henry – H)– Series

– Parallel


Ideal Op Amp

• Zin is infinite

• Zout is zero

• Amplification (Gain) Vout / Vin = ∞

• Unlimited bandwidth

• Vout = 0 when Voltage inputs = 0


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


How are Op-Amps used?

• Comparator (seen earlier)

• Voltage follower (seen earlier)

• Signal Modulation

• Mathematical Operations

• Filters

• Voltage-Current signal conversion


Non-inverting Op-Amp

Uses: Amplify…straight up

www.wikipedia.org


Inverting Op-Amp

Uses: Analog inverter

www.wikipedia.org


Comparator

www.allaboutcircuits.com

Uses: Low-voltage alarms, night light controller

V2

V1Vout


Pulse Width Modulator

• Output changes when – Vin ~= Vpot

• Potentiometer used to vary duty cycle

www.allaboutcircuits.com Uses: Motor controllers


Summation

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

www.wikipedia.org


Difference

1

31

124

4132

)( R

RV

RRR

RRRVVout

12 VVVout

If all resistors are equal:


Integrating Op-Amp

Uses: PID Controller

www.wikipedia.org


Differentiating Op-Amp

(where Vin and Vout are functions of time)

www.wikipedia.org


Filters

• Decouple the low-pass RC filter from the load.

Uses: Simple audio. Remove frequencies over 20kHz (audible)


Low-pass Filter (active)

• Cutoff frequency

• This works because the capacitor needs time to charge.

www.wikipedia.org


High pass filter (active)

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

www.wikipedia.org


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


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


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


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?


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)


Bibliography

• “Mechatronics”, Sabri Cetinkunt

• Wikipedia.org

• Allaboutelectronics.com

Documents

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