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048

Parallel Opamps

National Semiconductor application note

+Ub

Some applications notes are real evergreens. This one

originally dates from 1979(!) but has lost nothing of its rel-

evance and is always very interesting when you’re looking

for something like this.

Opamps can only deliver a limited current; typically only

about 10 mA max. When more current is required, several

opamps can be connected in parallel. But this usually does-

n’t work very well because opamps are never 100% equal.

In practice they will fight each other and only get warm,

which was not the intention of course.

11

3

5

10

IC1

1

7

8

IC1.A

IC1.B

IC1.C

4

2

6

9

–Ub

R1

R4

R5

R2

R6

R

R3

L

IC1 = LM324

C1

47n

014065 - 11

7-8/2001

Elektor Electronics

81

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SUMMER CIRCUITS COLLECTION

In the adjacent application note, IC1a is the boss and is

supported by IC1b and IC1c or as many stages as you

require. IC1a delivers, via R1, current to the load R L . The

‘helper-opamps’ are connected to the voltage drop across

R1. This way they will all deliver an identical current,

because the resistors R4 and R5 have all the same values.

Make sure that the whole thing does not oscillate — the

addition of an RC-network R5-C1 across the load can work

wonders.

The circuit can be used with symmetrical or single-

ended power supplies. In the latter case you will have to

connect the negative lead of the power supply to 0 V.

IC1a may also be configured as an amplifier. In this case

you will have to add the dotted resistors. To obtain unity

gain, omit R3 and use a wire link for R2.

(014065-1)

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