Application Hints

GAIN CONTROL

To make the LM386 a more versatile amplifier, two pins (1

and 8) are provided for gain control. With pins 1 and 8 open

the 1.35 k

Ω resistor sets the gain at 20 (26 dB). If a capacitor

is put from pin 1 to 8, bypassing the 1.35 k

Ω resistor, the

gain will go up to 200 (46 dB). If a resistor is placed in series

with the capacitor, the gain can be set to any value from 20

to 200. Gain control can also be done by capacitively cou-

pling a resistor (or FET) from pin 1 to ground.

Additional external components can be placed in parallel

with the internal feedback resistors to tailor the gain and fre-

quency response for individual applications. For example,

we can compensate poor speaker bass response by fre-

quency shaping the feedback path. This is done with a series

RC from pin 1 to 5 (paralleling the internal 15 k

Ω resistor).

For 6 dB effective bass boost: R

for good stable operation is R = 10 k

Ω if pin 8 is open. If pins

1 and 8 are bypassed then R as low as 2 k

Ω can be used.

This restriction is because the amplifier is only compensated

for closed-loop gains greater than 9.

INPUT BIASING

The schematic shows that both inputs are biased to ground

witha50k

Ω resistor. The base current of the input transis-

tors is about 250 nA, so the inputs are at about 12.5 mV

when left open. If the dc source resistance driving the LM386

is higher than 250 k

Ω it will contribute very little additional

offset (about 2.5 mV at the input, 50 mV at the output). If the

dc source resistance is less than 10 k

Ω, then shorting the

unused input to ground will keep the offset low (about 2.5 mV

at the input, 50 mV at the output). For dc source resistances

between these values we can eliminate excess offset by put-

ting a resistor from the unused input to ground, equal in

value to the dc source resistance. Of course all offset prob-

lems are eliminated if the input is capacitively coupled.

When using the LM386 with higher gains (bypassing the

1.35 k

Ω resistor between pins 1 and 8) it is necessary to by-

pass the unused input, preventing degradation of gain and

possible instabilities. This is done with a 0.1 µF capacitor or

a short to ground depending on the dc source resistance on

the driven input.

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