LM386

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

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www.unisonic.com.tw

QW-R107-007.J

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APPLICATION NOTES

GAIN CONTROL

To make the LM386 a more versatile amplifier, two pins(1and 8) are provided for gain control. With pins 1 and 8

open the 1.35 kΩ resistor sets the gain at 20 (26dB), If a capacitor is put from pin 1 to 8, bypassing the 1.35 kΩ

resistor, the gain will go up to 200(46dB). 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 coupling 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

frequency response for individual applications. For example we can compensate poor speaker bass response by

frequency shaping the feedback path. This is done with a series RC from pin 1 to 5 (paralleling the internal 15 kΩ

resistor). For 6 dB effective bass boost: R=15 kΩ, the lowest value for good stable operation is R=10 kΩ, if pin 8 is

open, If pins 1 and 8 are bypassed then R as low as 2 kΩ 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 with a 50 kΩ resistor. The base current of the input

transistors is about 250nA, so the inputs are at about 12.5mW when left open. If the dc source resistance driving the

LM386

is higher than 250 kΩ it will contribute very little additional offset (about 2.5mW at the input, 50mW at the

output).If the dc source resistance is less than 10 kΩ, then shorting the unused input to ground will keep the offset

low (about 2.5mV at the input, 50 mV at the output). For dc source resistances between these values we can

eliminate excess offset by putting a resistor from the unused input to ground, equal in value to the dc source

resistance. Of course all offset problems are eliminated if the input is capacitively coupled.

When using the LM386 with higher gains (bypassing the 1.35 kΩ resistor between pins 1 and 8) it is necessary to

bypass the unused input, preventing degradation of gain and possible instabilities. This is done with a 0.1μF

capacitor or a short to ground depending on the dc source resistance on the driven input.