CHAPTER 11: Op-Amp Applications. pps

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 Z in • 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 V 1 , V 2 , and V 3 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 ° . [...]... 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)... 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... 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... 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 . CHAPTER 11 Op-Amp Applications Objectives Describe and Analyze: • Audio mixers • Integrators • Differentiators • Peak detectors • Comparators • Other applications •. 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. 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