PARALLEL OPERATION

Output drive beyond the capability of one power amplifier

can be provided as shown here. The power op amps are

wired as followers and connected in parallel with the outputs

coupled

through

equalization

resistors.

A

standard,

high-voltage op amp is used to provide voltage gain. Overall

feedback compensates for the voltage dropped across the

equalization resistors.

With parallel operation, there may be an increase in un-

loaded supply current related to the offset voltage across the

equalization resistors. More output buffers, with individual

equalization resistors, may be added to meet even higher

drive requirements.

This connection allows increased output capability without

requiring a separate control amplifier. The output buffer, A

provides load current through R

the main amplifier, A

ers can be added.

Current sharing among paralleled amplifiers can be affected

by gain error as the power-bandwidth limit is approached. In

the first circuit, the operating current increase will depend

upon the matching of high-frequency characteristics. In the

second circuit, however, the entire input error of A

across R

power limiting to be activated as the slew limit is ap-

proached. This will not damage the LM12. It can be avoided

in both cases by connecting A

restricting bandwidth with C

SINGLE-SUPPLY OPERATION

Although op amps are usually operated from dual supplies,

single-supply operation is practical. This bridge amplifier

supplies bi-directional current drive to a servo motor while

operating from a single positive supply. The output is easily

converted to voltage drive by shorting R

to the output of A

Either input may be grounded, with bi-directional drive pro-

vided to the other. It is also possible to connect one input to

a positive reference, with the input signal varying about this

voltage. If the reference voltage is above 5V, R

not required.

HIGH VOLTAGE AMPLIFIERS

The voltage swing delivered to the load can be doubled by

using the bridge connection shown here. Output clamping to

the supplies can be provided by using a bridge-rectifier as-

sembly.

DS008704-12

DS008704-13

DS008704-14

DS008704-15

www.national.com

7