CHAPTER 11: Op-Amp Applications. pps

• 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 “loa

CHAPTER 11

Op-Amp

Applications

• 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, power, single-supply, etc There’s an op-amp for every niche in linear circuits

low-• It’s typically cheaper to use an op-amp than to build

a circuit with transistor Plus you get better

performance

• 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

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

Audio Mixers

<insert figure 11-10 here>

The input resistors would be adjustable

• 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°

Vout is the accumulated history of Vin

• 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

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

• 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

The LM311

• 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

• That function can be implemented with two

comparators in a window detector circuit.

Window Detector

<insert figure 11-34 here>

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