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, 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.
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 rise- time 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.