Page 21 - Op Amps Design, Applications, and Troubleshooting

4 BASIC CONCEPTS OF THE INTEGRATED OPERATIONAL AMPLIFIER

constant current source. If you examine the output circuitry, you can see that it
resembles that of a complementary-symmetry amplifier. The output is pulled clos-
er to the positive supply whenever the upper output transistor conducts harder.
Similarly, if the lower output transistor were to turn on harder, then the output
would be pulled in a negative direction. Also note the low values of resistances in
the output circuit.
The inputs labeled "offset null" are provided to allow compensation for
imperfect circuitry. Use of these inputs is discussed at a later point.

1.1.4 A Survey of Op Amp Applications
Now you know where op amps came from, what they are made of, and a few of
their characteristics. But what uses are there for an op amp in the industry?
Although the following is certainly not an exhaustive list, it does serve to illustrate
the range of op amp applications.

Amplifiers. Op amps are used to amplify signals that range from DC through
the higher radio frequencies (RF). The amplifier can be made to be frequency selec-
tive (i.e., act as a filter) much like the tone control on your favorite stereo system. It
may be used to maintain a constant output in spite of changing input levels. The
output can produce a compressed version of the input to reduce the range needed
to represent a certain signal. The amplifier may respond to microvolt signals origi-
nating in a transducer, which is used to measure temperature, pressure, density,
acceleration, and so on. The gain of the amplifier can be controlled by a digital com-
puter, thus extending the power of the computer into the analog world.

Oscillators. The basic op amp can be connected to operate as an oscillator. The
output of the oscillator may be sinusoidal, square, triangular, rectangular, saw-
tooth, exponential, or other shape. The frequency of oscillation may be stabilized
by a crystal or controlled by a voltage or current from another circuit.
Regulators. Op amps can be used to improve the regulation in power supplies.
The actual output voltage is compared to a reference voltage and the difference is
amplified by an op amp and used to correct the power supply output voltage. Op
amps can also be connected to regulate and/or limit the current in a power supply.

Rectification. Suppose you want to build a half-wave rectifier with a peak
input signal of 150 millivolts. This is not enough to forward bias a standard silicon
diode. On the other hand, an op amp can be configured to provide the characteris-
tics of an ideal diode with 0 forward voltage drop. Thus it can rectify very small
signals.

Computer Interfaces. The op amp is an integral part of many circuits used to
convert analog signals representing real-world quantities (such as temperature,
RPM, pressure, and so forth) into corresponding digital signals that can be manip-
ulated by a computer. Similarly, the op amp is frequently used to convert the digi-
tal output of a computer into an equivalent analog form for use by industrial
devices (such as motors, lights, and solenoids).