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

What will we cover?

2

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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

Amplifying signals?

3

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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

'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

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-)

Examples?

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?

Negative Feedback

Used in nearly all control systems

9

Where we want our hand to be

Signal from brain

Nerves & muscles

Brain

Hand position

Where is our hand really?

Negative Feedback

Used in nearly all control systems

10

Desired value

Signal to actuator

Actuator

Controller

Actual value

Feedback

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)

11

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.

Differential Op-amp

13

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)

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

14

http://www.allaboutcircuits.com/vol_3/chpt_8/2.html

http://www.allaboutcircuits.com/vol_3/chpt_8/3.html