SL386

System Logic

Semiconductor

SLS

APPLICATION INFORMATION

GAIN CONTROL

To make the SL386 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

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

feeback 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 boots: R

≅15 KΩ, the lowest value 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 the

9.

INPUT BIASING

The schematic shows that both inputs are biased to

ground with a 50 K

Ω resistor. The base current of the

input transistors is about 250 nA, so the inputs are at

at out 12.5 mV when left open. If the dc source

resistance oriving the IL386 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 putting a resistor from the

unesed input to ground, equal in value to the dc

source resistance. Of course all affset problems are

eliminated if the input is capacitively coupled.

When using the IL386 with higher gains (by pessing

the 1.35 K

Ω 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 0.1

µF capacitor or a short to ground

depending on the dc source resistance on the driven

input.

SCHEMATIC DIAGRAM