LM386N/D
APPLICATION HINTS
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 kW resistor sets the gain at 20 (26 dB). If a
capacitoris put from pin 1 to 8, bypassing 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 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 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 bypassed then R as low as 2 kW 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 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
LM386
with higher gains (bypassing the 1.35 kW 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.
3/6
TGS [ Tiger Electronic Co.,Ltd ]
