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 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 V out is the accumulated history of V in

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 V out proportional to how fast V in 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 mV pp 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 D 1 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

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.