Transcript Operational Amplifiers - Georgia Institute of Technology

Operational Amplifiers
Chris Nygren
Matt Livianu
Brad Schwagler
ME 6405 Introduction to Mechatronics
Agenda
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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 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
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 = 0V
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
V1 + V2 + V3 = 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
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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?
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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
V1
Vout
V2
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
Vout 
V 2 R3  R1 R4 V1R3

( R4  R2 ) R1
R1
If all resistors are equal:
Vout  V2  V1
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
20kHz (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:
- In: Sensors (temp, pressure, etc),
- Out : Radios (Variable Freq Osc)
• 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
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 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)
ME 6405 Introduction to Mechatronics
Bibliography
• “Mechatronics”, Sabri Cetinkunt
• Wikipedia.org
• Allaboutelectronics.com
ME 6405 Introduction to Mechatronics