Operational Amplifiers

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

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

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

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 / FWhere: 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

V1 Vout

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

)( RRV

RRRRRRV

Vout

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)