Download - L14 ECS - Electronics
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L14 Operational Amplifiers On-line / book Clifford et al. pp. 365-370
At the end of this lecture, you should be able to
explain why we need to amplify signals
calculate the output from an op-amp
explain what negative feedback means
explain how to use an op-amp as a buffer for a sensor
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What will we cover?
2
http://www.omega.com/literature/transactions/volume3/strain.html
http://www.omega.co.uk/ppt/pptsc_lg.asp?ref=ICSS-NHX&Nav=tema06
System with something important to
measure / control
Sensor (strain, temperature, pH,
position, velocity etc.)
Buffer / signal
processing (Opamps)
Cable. Noise.
Good wiring choice
= less noise
A to D
converter
Microprocessor. Digital
logic. DSP. Logging.
Control calculations
D to A
converter
Buffer (Opamps)
Actuator, e.g. heater, motor,
chemical feed unit, fuel injector
etc. High power / 3 phase
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Amplifying signals?
3
http://www.omega.com/literature/transactions/volume3/strain.html
http://www.omega.co.uk/ppt/pptsc_lg.asp?ref=ICSS-NHX&Nav=tema06
Cable. Noise.
Good wiring choice
= less noise
Sensor (strain, temperature,
pH, position, velocity etc.)
1 mV input? 50 mV input?
A to D
converter
1 V output? 5 V output?
Buffer / signal
processing
(Opamps)
What is Gain ? Its how much the input voltage is multiplied by at the output
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'Operational Amplifiers'
Analog operational amplifiers are one of the key building blocks of practically all electronic systems. They convert and condition real-word analog information into signals that can be read and understood by digital systems.
2001/2 world market for opamps was $2 billion. Nearly 4 billion opamps
also see http://www.allaboutcircuits.com/vol_3/chpt_8/1.html
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'Operational Amplifiers'
5 http://www.national.com/company/pressroom/gallery/oa.html
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Differential Op-amp
Input has a very high impedance. Almost no current flows into or out of V+ and V-
Output voltage = A x (V+ - V-)
Any current from the output comes from +Vs and -Vs 6
Vout +Vs
V+
V- -Vs
Input to the opamp Power for the opamp
Output of the opamp
= A (V+-V-)
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Examples?
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Problems?
Need to buy lots of different amplifiers.. 10x, 50x, 100x, 273x etc...
What if theyre not accurate out of the box? We might want 0.1% accuracy e.g.
How can we buy a single type of amplifier and change the circuit to change the gain?
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Negative Feedback
Used in nearly all control systems
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Where we want our hand to be
Signal from brain
Nerves & muscles
Brain
Hand position
Where is our hand really?
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Negative Feedback
Used in nearly all control systems
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Desired value
Signal to actuator
Actuator
Controller
Actual value
Feedback
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Feedback
Biology, Civil eng., Chem Eng - temperature regulation (-ve)
Mechanical engineering - engine control (-ve)
Chemical engineering - reaction control (-ve) [reactions by themselves might be exothermic]
Climate science - ice cap albedo (+ve)
Economy - confidence (+ve)
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Examples
Draw ve feedback circuit, A=100,000 (105)
We ask for 5 V. Initially starts to go for full output, but on the way it hits the right answer.
V+ = 5 V ; Vout=4.99995 ; V-=4.99995
V+ - V- = 5x10-5; Vout =(V+ - V-) x A = 5 V
Works as long as theres no delay. Otherwise it overshoots, then the other way, and oscillates, like control system with gain thats too high.
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Differential Op-amp
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Vout +Vs
V+
V- -Vs
Input to the opamp Power for the opamp
(1) V- = Vout [theyre wired together] (2) Vout = A (V+ - V-) [Op-amp gain rule] (1) (2) Vout = A (V+ - Vout) Vout (A+1) = A V+ Vout / V+ = A/(A+1) : Gain ~ 1 (a buffer)
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Summary Differential op-amps are very common in
electrical things
They produce an output voltage proportional to the difference of their two input voltages
They work on analog signals, often to interface them with microprocessors
Feedback is used to control many processes including op-amp circuits
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http://www.allaboutcircuits.com/vol_3/chpt_8/2.html
http://www.allaboutcircuits.com/vol_3/chpt_8/3.html