Bridge Power Amplifier with LM380

The bridge power amplifier is a common configuration used to increase the output power of audio signals. One popular integrated circuit (IC) used for this purpose is the LM380, a low-distortion power amplifier with a fixed gain of 34 dB. This article explores how to design a bridge amplifier using two LM380 ICs and discusses its applications in audio amplification systems.

What is a Bridge Amplifier?

In a standard amplifier, the load (e.g., a speaker) is connected between the amplifier's output and ground. In a bridge amplifier, two amplifiers are connected to drive the load, but instead of grounding one side of the load, each side is connected to the output of one of the two amplifiers. One amplifier drives the positive half of the signal, and the other drives the negative half. This push-pull arrangement effectively doubles the voltage across the load, which increases the output power.

Key Benefits of a Bridge Amplifier:

Increased Output Power: The voltage across the load is doubled, and power is proportional to the square of the voltage, resulting in up to four times the power compared to a single-ended configuration.
No Need for Output Capacitors: In bridge amplifiers, there’s no need for large output capacitors, which are often required in single-ended designs to block DC from reaching the load.

The LM380 Power Amplifier

The LM380 is a popular audio power amplifier IC designed for delivering output power up to 2.5 watts with minimal external components. Its key features include:

Low distortion: The LM380 is designed to provide clear, low-distortion audio signals.
Wide supply voltage range: It operates on a single power supply of 10V to 22V.
Internal thermal shutdown: This protects the IC from overheating, ensuring safe and reliable operation.
Fixed gain of 34 dB: Simplifies design by removing the need for external gain-setting resistors.

Designing a Bridge Amplifier with LM380

To build a bridge amplifier with LM380, two ICs are used in a complementary configuration. Here's how you can connect them:

bridge power amplifier LM380 circuit diagram

Components Required:

2 × LM380 ICs
Power supply (12V to 18V recommended)
Resistors (for biasing)
Capacitors (for power supply decoupling)
Loudspeaker (8 ohms or 4 ohms)

Schematic Overview:

Input Signal: The same input signal is fed into both LM380 ICs, but one IC is driven in the inverted phase. This can be achieved using a phase inverter circuit such as an operational amplifier in an inverting configuration.
Bridge Output: The speaker is connected across the two outputs of the LM380s—one output driving the positive side of the speaker, and the other driving the negative side. This allows the speaker to receive double the voltage swing of a single amplifier, resulting in higher output power.
Power Supply: Both LM380s are powered by the same supply voltage. Ensure proper decoupling capacitors are placed close to the ICs to prevent power supply noise from affecting the audio quality.

Step-by-Step Assembly:

Step 1: Setup Power Supply
Connect the power supply (12V to 18V) to both LM380 ICs, making sure to include decoupling capacitors (typically 0.1 µF) between the power pins and ground to stabilize the voltage.
Step 2: Phase Inversion
Use an operational amplifier to invert the phase of the input signal going to one of the LM380 ICs. This phase inversion is critical for the bridge configuration, as it ensures that the two LM380s produce opposite polarity signals at their outputs.
Step 3: Connect the Load
Attach the speaker across the outputs of the two LM380 ICs. One amplifier output will push the current while the other pulls, creating a high voltage differential across the load.
Step 4: Test the Circuit
Once the connections are made, apply the input signal (typically an audio signal) and observe the output. The bridge amplifier should significantly boost the speaker’s volume compared to a single LM380 setup.

Output Power Calculation

In a bridge amplifier, the output power can be calculated using the following formula:

P_{out} = \frac{V_{rms}^2}{R_{load}

Where:

$V_{rms}$ is the root mean square (RMS) value of the voltage across the load.
$R_{load}$ is the impedance of the speaker (usually 8 ohms).

In a bridged configuration, the output voltage is approximately doubled compared to a single amplifier. Therefore, the output power is roughly four times greater. For example, if a single LM380 can provide 2 watts of power to an 8-ohm speaker, a bridged LM380 setup can potentially deliver up to 8 watts of power.

Advantages of Using LM380 in Bridge Configuration

Higher Power Output: By using a bridge amplifier, you can achieve higher power output without increasing the supply voltage or changing the load impedance.
Efficient Use of Power: The bridge configuration allows more efficient use of the available supply voltage, resulting in better performance without the need for expensive power supplies.
Simpler Design: With the LM380’s built-in gain and thermal protection, designing a robust bridge amplifier is relatively simple compared to using discrete transistors.

Applications of Bridge Amplifiers with LM380

Home Audio Systems: The LM380 in a bridge amplifier configuration is ideal for driving small speakers in home audio amplifier systems, delivering high-quality sound without the need for large or expensive components.
Portable Speakers: For battery-powered applications, such as portable Bluetooth speakers, the LM380 can provide sufficient power while minimizing battery drain.
Public Address Systems: Bridge amplifiers are often used in public address systems where higher power is needed to drive large speakers or multiple speakers simultaneously.

Conclusion

Building a bridge power amplifier using the LM380 is a straightforward way to increase output power for audio applications. By driving the load with two amplifiers in a push-pull configuration, you can significantly boost the signal strength without the need for higher supply voltages. The LM380’s simplicity and reliability make it an excellent choice for a wide range of audio amplification projects.

With a proper understanding of bridge amplifier design, you can apply this configuration to achieve higher output power, making your audio circuits more efficient and effective.