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ME 6405 Introduction to Mechatronics
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 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
• Differential Amplifier– Amplifies difference
between inputs
Single-ended Amplifier
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
• 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?
ME 6405 Introduction to Mechatronics
• 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
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
• Resistance (Ohms – Ω)– Series
– Parallel
V1 + V2 + V3 = Vin
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
Uses: Amplify…straight up
www.wikipedia.org
ME 6405 Introduction to Mechatronics
Inverting Op-Amp
Uses: Analog inverter
www.wikipedia.org
ME 6405 Introduction to Mechatronics
Comparator
www.allaboutcircuits.com
Uses: Low-voltage alarms, night light controller
V2
V1Vout
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
Uses: Add multiple sensors inputs until a threshold is reached.
www.wikipedia.org
ME 6405 Introduction to Mechatronics
Difference
1
31
124
4132
)( R
RV
RRR
RRRVVout
12 VVVout
If all resistors are equal:
ME 6405 Introduction to Mechatronics
Integrating Op-Amp
Uses: PID Controller
www.wikipedia.org
ME 6405 Introduction to Mechatronics
Differentiating Op-Amp
(where Vin and Vout are functions of time)
www.wikipedia.org
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
• This works because the capacitor needs time to charge.
www.wikipedia.org
ME 6405 Introduction to Mechatronics
High pass filter (active)
Band-pass filter cascades both high-pass and low-pass!
www.wikipedia.org
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
• 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
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