Download Ch 11

CHAPTER 11
Op-Amp
Applications
Objectives
Describe and Analyze:
• Audio mixers
• Integrators
• Differentiators
• Peak detectors
• Comparators
• Other applications
• Troubleshooting
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, lowpower, 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.
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.
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.
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.
An Adder Circuit
V1, V2, and V3 represent (are the analog of)
three numbers that need to be added.
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.
Audio Mixers
<insert figure 11-10 here>
The input resistors would be adjustable.
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°.
Integrators
Vout is the accumulated history of Vin
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 risetime and fall-time of the edges of the input pulse.
Differentiators
Vout proportional to how fast Vin changes
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.
Single-Supply Op-Amps
For signals, circuit (a) looks like circuit (b)
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.
Precision Rectifiers
D1 prevents saturation, allowing use at higher
frequencies.
Peak Detector
Another way to use a capacitor for memory
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.
Comparators
The LM311
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.
The Schmitt Trigger
The switching threshold changes when the output switches.
The Schmitt Trigger
Implementation of a Schmitt Trigger
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.
Window Detector
<insert figure 11-34 here>
Troubleshooting
There 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.