aid stability. In all cases, the op amp will behave predictably

only if the supplies are properly bypassed, ground loops are

controlled and high-frequency feedback is derived directly

from the output terminal, as recommended earlier.

So-called capacitive loads are not always capacitive. A

high-Q capacitor in combination with long leads can present

a series-resonant load to the op amp. In practice, this is not

usually a problem; but the situation should be kept in mind.

Large capacitive loads (including series-resonant) can be

accommodated by isolating the feedback amplifier from the

load as shown above. The inductor gives low output imped-

ance at lower frequencies while providing an isolating im-

pedance at high frequencies. The resistor kills the Q of se-

ries resonant circuits formed by capacitive loads. A low

inductance, carbon-composition resistor is recommended.

Optimum values of L and R depend upon the feedback gain

and expected nature of the load, but are not critical. A 4 µH

inductor is obtained with 14 turns of number 18 wire, close

spaced, around a one-inch-diameter form.

The LM12 can be made stable for all loads with a large ca-

pacitor on the output, as shown above. This compensation

gives the lowest possible closed-loop output impedance at

high frequencies and the best load-transient response. It is

appropriate for such applications as voltage regulators.

A feedback capacitor, C

pin of the IC. The output capacitor, C

output terminal with short leads. Single-point grounding to

avoid dc and ac ground loops is advised.

The impedance, Z

to the load capacitor. About 3-inches of number-18 wire

(70 nH) gives good stability and 18-inches (400 nH) begins

to degrade load-transient response. The minimum load ca-

pacitance is 47 µF, if a solid-tantalum capacitor with an

equivalent series resistance (ESR) of 0.1

Ω is used. Electro-

lytic capacitors work as well, although capacitance may have

to be increased to 200 µF to bring ESR below 0.1

Ω.

Loop stability is not the only concern when op amps are op-

erated with reactive loads. With time-varying signals, power

dissipation can also increase markedly. This is particularly

true

with

the

combination

of

capacitive

loads

and

high-frequency excitation.

INPUT COMPENSATION

The LM12 is prone to low-amplitude oscillation bursts com-

ing out of saturation if the high-frequency loop gain is near

unity. The voltage follower connection is most susceptible.

This glitching can be eliminated at the expense of

small-signal bandwidth using input compensation. Input

compensation can also be used in combination with LR load

isolation to improve capacitive load stability.

An example of a voltage follower with input compensation is

shown here. The R

with R

greatly affecting response below 100 kHz. A lead capacitor,

C

quency. Proper operation requires that the output impedance

of the circuitry driving the follower be well under 1 k

Ω at fre-

quencies up to a few hundred kilohertz.

Extending input compensation to the integrator connection is

shown here. Both the follower and this integrator will handle

1 µF capacitive loading without LR output isolation.

CURRENT DRIVE

This circuit provides an output current proportional to the in-

put voltage. Current drive is sometimes preferred for servo

motors because it aids in stabilizing the servo loop by reduc-

ing phase lag caused by motor inductance. In applications

requiring high output resistance, such as operational power

supplies running in the current mode, matching of the feed-

back resistors to 0.01% is required. Alternately, an adjust-

able resistor can be used for trimming.

DS008704-7

DS008704-8

DS008704-9

DS008704-10

DS008704-11

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