LM386N-3

The LM386N-3 from Texas Instruments is a low-voltage audio power amplifier designed for battery-powered consumer applications. The N-3 variant delivers up to 700mW into an 8Ω speaker from a 9V supply, making it suitable for small portable speakers, radios, and DIY audio projects. The internal gain is set to 20 (26dB) by default, requiring no external components for basic operation. Adding a capacitor between pins 1 and 8 bypasses the internal 1.35kΩ resistor, increasing the gain to 200 (46dB); a series resistor sets any gain between 20 and 200. The inputs are ground-referenced through internal 50kΩ resistors, eliminating the need for external DC biasing. The output automatically biases to half the supply voltage, enabling single-supply operation with an output coupling capacitor. The 4mA quiescent current at 6V means the amplifier draws only 24mW at idle — ideal for battery operation. The LM386 has been in continuous production since the 1970s and remains one of the most popular audio amplifier ICs for hobbyists.

The LM386 uses a Lin topology: a PNP differential input stage, a common-emitter voltage amplifier stage, and a Class-AB complementary push-pull output stage. The two 50kΩ resistors from each input to ground provide ground-referenced biasing — the inputs sit at approximately 12.5mV above ground due to 250nA input bias current. The gain is set by the feedback network between pins 1 and 8: internally, a 1.35kΩ resistor and a 150Ω resistor form the emitter degeneration of the differential pair, setting the gain to 1 + (15kΩ/1.5kΩ) = 20. Bypassing the 1.35kΩ resistor with a capacitor (pin 1 to pin 8) reduces the degeneration to just 150Ω, increasing the gain to 1 + (15kΩ/150Ω) = 200. A series resistor can set intermediate gains. The output stage is a complementary push-pull pair biased into Class-AB through a VBE multiplier, automatically centering the output at VCC/2. The 250µF output coupling capacitor blocks this DC bias from the speaker while passing the AC audio signal. The capacitor forms a high-pass filter with the speaker impedance: fC = 1/(2π×250µF×8Ω) ≈ 80Hz. The bypass pin (7) connects to an internal 50kΩ/10µF node; adding an external 10µF capacitor from pin 7 to ground improves power supply rejection by bypassing the internal node.

LM386N-3 uses the PDIP-8 package with audio power amplifier pinout: Pin 1 = Gain Set (bypass with 10µF to Pin 8 for 200x gain), Pin 2 = Inverting Input (-), Pin 3 = Non-inverting Input (+), Pin 4 = GND, Pin 5 = Output (to speaker via 250µF coupling cap), Pin 6 = VCC (4-12V for N-3), Pin 7 = Bypass (decouple with 10µF to GND for power supply ripple rejection), Pin 8 = Gain Set. Default gain is 20x (26dB) with pins 1 and 8 open. Adding a 10µF cap between pins 1 and 8 increases gain to 200x (46dB). A series resistor (1.2kΩ) with the 10µF cap sets intermediate gains. The Zobel network (0.05µF + 10Ω from Pin 5 to GND) prevents oscillation with inductive speaker loads.

• Basic Amp (Gain=20): Signal → 10kΩ pot → 0.05µF cap → pin 3; pin 2 grounded; 250µF cap from pin 5 to 8Ω speaker; 10µF cap pin 7 to GND
• High Gain (200): Same as above plus 10µF cap between pins 1 and 8; amplifies microphone-level signals directly
• Gain=50: 1.2kΩ resistor + 10µF cap in series between pins 1 and 8; intermediate gain for line-level signals
• Mini Guitar Amp: Guitar → 0.05µF → pin 3; gain=200; pin 5 → 250µF → 8Ω speaker; 9V battery power
• Bass Boost: RC network (0.033µF + 10kΩ) from pin 1 to pin 5 boosts low frequencies; enhances bass response

LM386N-3 is a 325mW low-voltage audio power amplifier in DIP-8 with 4-12V supply and adjustable gain from 20 to 200. The N-1 (250mW) and N-4 (1W) variants scale output power. For modern battery-powered designs, the PAM8302A Class-D amplifier offers 2.5W with 90% efficiency. For higher power, TDA2030A provides up to 18W in a 5-pin TO-220 package.