CHAPTER 11
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Transcript of CHAPTER 11
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CHAPTER 11
Op-AmpApplications
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ObjectivesDescribe and Analyze:• Audio mixers• Integrators• Differentiators• Peak detectors• Comparators• Other applications• Troubleshooting
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Introduction• There are many applications for op-amps; they’re
the building blocks (gain blocks) of most analog circuits.
• There are many types of op-amps: high-speed, low-power, single-supply, etc. There’s an op-amp for every niche in linear circuits.
• It’s typically cheaper to use an op-amp than to build a circuit with transistor. Plus you get better performance.
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Loading• Some signal sources, such as crystal microphones,
have a high internal resistance. To amplify the signal from such a source, the amplifier’s input must be high impedance to avoid “loading down” the signal.
• Loading down means that the internal resistance of the signal source and the input impedance of the amplifier form a voltage divider. So the signal that actually gets to the input is much less than what the source is generating.
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Circuits with High Zin
• To prevent the loading down of a signal source, an amplifier must have an input impedance that is much higher (10 times or more) than the source resistance.
• A noninverting op-amp amplifier will do the job nicely.
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Arithmetic Circuits• The term operational amplifier goes back to the days when
op-amp circuits were used to carry out mathematical operations inside an analog computer.
• Before digital computers, analog computers could “do the math” by adding, subtracting, multiplying, and dividing voltages that represented numbers.
• Op-amps can even do the calculus operations of integration and differentiation.
• All those operations are still done by op-amps, but not in computers. They’re done in circuits like digital-to-analog and analog-to-digital converters.
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An Adder Circuit
V1, V2, and V3 represent (are the analog of)
three numbers that need to be added.
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Audio Mixers• When music is being recorded, the sound is usually
picked up by several microphones; maybe one for each instrument. The output of each microphone is recorded on a separate track, and combined later by a sound engineer into the final version.
• The combining of the different sound tracks is called mixing.
• During mixing, the sound engineer needs to adjust the volume coming from each track. That is done with potentiometers in a mixer circuit.
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Audio Mixers<insert figure 11-10 here>
The input resistors would be adjustable.
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Integrators• In some applications it is necessary for the circuit to have
“memory” of a signal. An example is the error signal in a control system. Not only do you need to compensate for the current error, you need to compensate for errors that have accumulated over time.
• Integration is the process of accumulating a signal over time. If you integrate a sinewave from 0° to 180°, you get a voltage proportional to the “area” under the sine curve. But if you integrate that same sinewave from 0° to 360° you will get zero. This is because the positive area from 0° to 180° cancels out the negative area from 180° to 360°.
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Integrators
Vout is the accumulated history of Vin
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Differentiators• How fast something changes is often important. Think of fuel
in a tank or pressure in a boiler. If you know the present level, the rate of change lets you predict where it will be in the future.
• Differentiation is the process of determining how fast something is changing.
• If you differentiate a pulse, you first get a voltage spike, then zero volts, then a voltage spike in the opposite direction. The amplitudes of the spikes are proportional to the rise- time and fall-time of the edges of the input pulse.
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Differentiators
Vout proportional to how fast Vin changes
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Single-Supply Op-Amps
• It’s usually cheaper (and more reliable) to have one power supply voltage instead of two.
• If you need to add an op-amp circuit to a digital system, it would be convenient if all the op-amp needed was +5 Volts and ground.
• In battery-powered equipment, the ability to work with 9 Volts and ground would be convenient.
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Single-Supply Op-Amps
For signals, circuit (a) looks like circuit (b)
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Precision Rectifiers• Precision rectifiers are often called ideal-diode
circuits. An ideal diode, if one existed, would conduct current in the forward direction with a diode drop of zero volts.
• A real diode requires 0.7 Volts to conduct. So if you need to rectify a 100 mVpp AC signal, a real diode can’t do it.
• By placing a real diode in the feedback loop of an op-amp, it can be made to work like an ideal diode.
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Precision Rectifiers
D1 prevents saturation, allowing use at higher frequencies.
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Peak Detector
Another way to use a capacitor for memory
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Comparators• The output of a comparator is high or low, depending on
which of its two inputs “sees” a higher voltage.• Comparators need to be:
– Fast: output can switch high or low very quickly– High-Gain: very small V across inputs to switch – Stable: output should not “chatter” with equal
voltages on the inputs• For good performance, use a chip designed to be a
comparator instead of an open-loop op-amp.
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Comparators
The LM311
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Hysteresis• We need to prevent a comparator’s output from
oscillating high and low (chattering) when the two inputs are very close. To do that requires hysteresis.
• Hysteresis means that the V required to make the output switch from low to high is different from the V required to make the output switch from high to low.
• Hysteresis in a comparator is done with a Schmitt Trigger circuit at its input.
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The Schmitt Trigger
The switching threshold changes when the output switches.
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The Schmitt Trigger
Implementation of a Schmitt Trigger
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Window Detector• If you are monitoring pressure in a boiler, it
may not be necessary to know the exact pressure. What is important to know is if the pressure is too low (no heat) or too high (danger of explosion).
• That function can be implemented with two comparators in a window detector circuit.
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Window Detector
<insert figure 11-34 here>
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TroubleshootingThere are too many applications to give specific advice
on each one. So just remember:
• Current in or out the input pins is negligible.• Voltage between the two inputs is essentially zero
unless the op-amp is saturated.• Output of a comparator is either high or low (or off if it
has an output enable).• Always check the DC levels.