15-06-2012, 01:36 PM
LM386 Low Voltage Audio Power Amplifier
LM386 Low Voltage Audio Power Amplifie.pdf (Size: 303.61 KB / Downloads: 31)
General Description
The LM386 is a power amplifier designed for use in low voltage
consumer applications. The gain is internally set to 20 to
keep external part count low, but the addition of an external
resistor and capacitor between pins 1 and 8 will increase the
gain to any value from 20 to 200.
The inputs are ground referenced while the output automatically
biases to one-half the supply voltage. The quiescent
power drain is only 24 milliwatts when operating from a 6 volt
supply, making the LM386 ideal for battery operation.
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 kW resistor sets the gain at 20 (26 dB). If a capacitor
is 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.
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 LM386
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.