LM386N/D

APPLICATION HINTS

GAIN CONTROL

To make the LM 386 a mo re versatile amplifier, two pins (1and 8) are provided for gain

control. With pins 1 and 8 open the 1.35 kW resistor sets the gain at 20 (26 dB). If a

capacitoris put from pin 1 to 8, by passing the 1.35 kW 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 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 comp ensate 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 kW resistor).

For 6 dB effective bass boost: R . 15 kW, the lowest value for good stable operation is R =

10 kW if pin 8 is open. If pins 1 and 8 are by passed then R as low as 2 kW can be used.

This restriction is because the amplifier is only comp ensated for closed-loop gains greater

than 9.

INPUT BIASING

The schematic shows that both inputs are bi ased to ground with a 50 kW resistor. The

base current of the input transistors is about 250 nA, so the inputs are at about 12.5 mV

when left open. If the dc source resistance driving the D386 is higher than 250 kW 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 kW, 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 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 LM 386 with higher gains (by p assing the 1.35 kW 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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