The LM380 2.5W Audio Power Amplifier is a simple mono Class-AB amplifier IC designed to increase a small audio signal so it can drive a speaker. This article explains the LM380 specifications, pinout, working principle, voltage gain, circuit designs, applications, and common alternatives.

Catalog

LM380 Overview

The LM380 2.5W Audio Power Amplifier is a single-channel analog power amplifier IC developed for consumer and general-purpose audio circuits. It takes a low-level audio signal from a source such as a radio, microphone preamplifier, or audio circuit and increases its voltage and current so that it can drive a low-impedance load such as a speaker.

One important characteristic of the LM380 is its internally fixed voltage gain of 50 V/V, or approximately 34 dB. This reduces the need for external gain-setting resistors and simplifies circuit design. Its input is referenced to ground, while the output automatically biases itself at approximately one-half of the supply voltage. In a normal single-supply speaker circuit, an output coupling capacitor is therefore used to block this DC voltage while allowing the amplified audio signal to reach the speaker.

The LM380 also provides a relatively high 150 kΩ input impedance, which reduces loading on the signal source. Internal short-circuit protection and thermal limiting provide additional protection when the amplifier is overloaded or becomes too hot.

LM380 Features and Specifications

Note: The 2.5 W rating should not be interpreted as the output power available under every condition. Actual speaker power depends mainly on the supply voltage, speaker impedance, allowable distortion, output voltage swing, and device temperature.

LM380 Pinout and Working Principle

LM380 14-Pin PDIP Pinout

LM380 8-Pin PDIP Pinout

LM380 Audio Amplifier Block Diagram

How the LM380 Audio Amplifier Works

The schematic diagram shows that the LM380 contains the main stages needed to convert a small audio input into a higher-power signal capable of driving a speaker. These stages include an input differential amplifier, internal bias network, voltage-amplification circuitry, and a Class-AB output stage.

At the input, the non-inverting and inverting terminals feed a differential transistor stage. Each input has an internal resistance of about 150 kΩ to ground, giving the LM380 its relatively high input impedance. This allows an audio source to drive the amplifier without being heavily loaded. The special input arrangement also permits the input signal to be referenced directly to ground.

The two internal 25 kΩ resistors around the bypass node form part of the bias network. This network establishes the correct internal operating point. As a result, when no audio signal is present, the output automatically settles at approximately:

For example, with an 18 V supply, the quiescent output voltage is approximately 9 V. This midpoint bias allows the amplified AC signal to swing both upward and downward around the operating point without requiring a negative supply. TI specifically identifies this automatic half-supply output centering as a key LM380 characteristic.

After the differential input stage, the signal passes through the internal voltage-amplification section. Internal feedback establishes a fixed closed-loop voltage gain of approximately 50 V/V. The signal then reaches the output transistors, which provide the much higher current needed by a low-impedance speaker.

Because the output pin contains a DC voltage close to half the supply voltage, a large output coupling capacitor is normally placed between VOUT and the speaker. The capacitor blocks this DC component while allowing the changing audio signal to reach the speaker.

The output stage operates in Class AB, where the upper and lower output devices conduct during different portions of the audio waveform with a small overlap around the zero-crossing region. This arrangement provides considerably more output current than the input stage while keeping crossover distortion relatively low. TI classifies the LM380 as a mono Class-AB audio power amplifier.

The schematic also shows internal biasing and protection-related circuitry around the output stage. The LM380 provides short-circuit protection and internal thermal limiting, which help protect the device during excessive current or temperature conditions. These protections do not remove the need for correct speaker loading and adequate heat dissipation.

LM380 Voltage Gain and Input Requirements

The LM380 has an internally fixed voltage gain of 50 V/V, equivalent to approximately 34 dB. Unlike amplifiers that require external feedback resistors to set the gain, the LM380 establishes its gain internally, reducing the number of external components required.

The voltage-gain relationship can be expressed as:

For the LM380:

Av = 50

Therefore:

Vout = 50Vin

For example, an input signal of 50 mV peak would ideally require an output signal of:

Vout = 50 x 0.05 = 2.5V peak

This relationship applies while the amplifier remains within its available output-voltage and current limits. If the required output swing becomes greater than the LM380 can provide from the selected supply voltage and load, the waveform begins to clip, causing increased distortion.

In decibels, voltage gain is calculated as:

 Av(dB) = 20log₁₀ (Av)

For a gain of 50:

20log₁₀(50) = 34dB

The LM380 also has a typical input impedance of approximately 150 kΩ, allowing it to accept signals from many relatively high-impedance audio sources without severe loading.

The input signal should be kept within a level that does not demand more output voltage or power than the amplifier can provide. For this reason, a volume potentiometer is commonly placed before the LM380 input. It attenuates excessive source signals rather than changing the LM380's internal gain.

For normal single-supply operation, the input signal is generally AC-coupled when the preceding source has an unwanted DC offset. The LM380's input stage is designed to accept ground-referenced input signals, while the IC internally establishes the bias conditions needed for amplification.

A useful design distinction is that 34 dB is the amplifier's voltage gain, not its output-power rating. The actual speaker power is determined by the available output voltage and current, speaker impedance, supply voltage, distortion limit, and thermal conditions. The LM380's specified output capability reaches up to about 2.5 W under appropriate operating conditions rather than at every input or load condition.

LM380 Circuit Design

The LM380 can be used in several audio circuits. The external resistors, capacitors, switches, and transformers control how the signal enters, leaves, or feeds back into the IC.

LM380 Phono Amplifier

This circuit amplifies the signal from a crystal cartridge and sends it to an 8 Ω speaker. The 25 kΩ control adjusts the volume, while the 10 kΩ control and 0.05 µF capacitor adjust the tone.

The LM380 runs from an 18 V supply and amplifies the audio signal. The 500 µF output capacitor blocks DC voltage and passes the audio signal to the speaker. The 2.7 Ω resistor and 0.1 µF capacitor help keep the amplifier stable.

LM380 Bridge Amplifier

This circuit uses two LM380 amplifiers to drive one speaker. The two outputs operate in opposite phase, so the speaker receives a larger voltage swing than with one LM380.

This bridge connection can produce more output power, but it also increases current and heat. The feedback components help keep the two amplifier signals properly controlled.

LM380 Intercom Circuit

This circuit uses the LM380 in a simple two-way intercom. The TALK/LISTEN switch changes which speaker receives or sends the voice signal.

The transformer helps match the speaker signal to the LM380 input. The IC then amplifies the weak voice signal and sends it through the 50 µF output capacitor to the remote speaker.

LM380 Phase-Shift Oscillator

This circuit uses the LM380 to generate an audio tone instead of amplifying an external signal. Three RC sections feed part of the output signal back to the inverting input.

The feedback produces the phase shift needed for oscillation. With the shown resistor and capacitor values, the circuit generates a tone around 4 kHz or higher. The 250 µF capacitor passes the AC output to the speaker while blocking DC.

Common LM380 Applications

• Small speaker amplifiers

• Portable radios

• Phono amplifiers

• Intercom systems

• Alarm and warning sound systems

• TV sound systems

• AM/FM radio audio stages

• Teaching and educational audio circuits

• Line-driver circuits

• Small audio signal monitors, etc.

LM380 Comparison and Alternatives

Mechanical Dimensions

Manufacturer

Texas Instruments (TI) supports the production of the LM380 audio power amplifier through its established analog semiconductor manufacturing, assembly, testing, and quality-control systems. TI operates a global network of wafer fabrication and assembly-and-test facilities, allowing it to control major stages of production from processed semiconductor wafers through packaging and final electrical testing. During manufacturing, TI uses process controls, statistical process control, qualification testing, functional verification, and reliability testing to help ensure devices meet their electrical specifications and quality requirements.