VOLTAGE REGULATORS

An op amp can be used as a positive or negative regulator.

Unlike most regulators, it can sink current to absorb energy

dumped back into the output. This positive regulator has a

0–50V output range.

Dual supplies are not required to use an op amp as a voltage

regulator if zero output is not required. This 4V to 50V regu-

lator operates from a single supply. Should the op amp not

be able to absorb enough energy to control an overvoltage

condition, a SCR will crowbar the output.

REMOTE SENSING

Remote sensing as shown above allows the op amp to cor-

rect for dc drops in cables connecting the load. Even so,

cable drop will affect transient response. Degradation can be

minimized by using twisted, heavy-gauge wires on the out-

put line. Normally, common and one input are connected to-

gether at the sending end.

AUDIO AMPLIFIERS

A power amplifier suitable for use in high-quality audio equip-

ment is shown above. Harmonic distortion is about

0.01-percent. Intermodulation distortion (60 Hz/7 kHz, 4:1)

measured 0.015-percent. Transient response and saturation

recovery are clean, and the 9 V/µs slew rate of the LM12 vir-

tually eliminates transient intermodulation distortion. Using

separate amplifiers to drive low- and high-frequency speak-

ers gets rid of high-level crossover networks and attenua-

tors. Further, it prevents clipping on the low-frequency chan-

nel from distorting the high frequencies.

DETERMINING MAXIMUM DISSIPATION

It is a simple matter to establish power requirements for an

op amp driving a resistive load at frequencies well below

10 Hz. Maximum dissipation occurs when the output is at

one-half the supply voltage with high-line conditions. The in-

dividual output transistors must be rated to handle this power

continuously at the maximum expected case temperature.

The power rating is limited by the maximum junction tem-

perature as determined by

T

where T

of the package bottom, P

tion and

θ

age of the output transistor. Recommended maximum junc-

tion temperatures are 200˚C within the power transistor and

150˚C for the control circuitry.

If there is ripple on the supply bus, it is valid to use the aver-

age value in worst-case calculations as long as the peak rat-

ing of the power transistor is not exceeded at the ripple peak.

With 120 Hz ripple, this is 1.5 times the continuous power

rating.

Dissipation requirements are not so easily established with

time varying output signals, especially with reactive loads.

Both peak and continuous dissipation ratings must be taken

into account, and these depend on the signal waveform as

well as load characteristics.

With a sine wave output, analysis is fairly straightforward.

dissipation of both output transistors is

DS008704-22

DS008704-23

DS008704-24

DS008704-25

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