Table of Contents
- Introduction to the LM386
- Key Features and Specifications
- Pin Configuration and Functions
- Operating Principles
- Application Circuits
5.1 Basic Audio Amplifier
5.2 Adjustable Gain Amplifier
5.3 Low-Voltage Operation
5.4 Audio Mixer
5.5 Intercom System - PCB Layout Considerations
- Troubleshooting and Common Issues
- Frequently Asked Questions (FAQ)
- Alternatives to the LM386
- Conclusion
1. Introduction to the LM386
The LM386 is an audio power amplifier IC designed for use in low-voltage consumer applications. It is part of the LM3xx series of monolithic integrated circuits developed by National Semiconductor (now part of Texas Instruments). The chip is capable of delivering up to 1 watt of output power into an 8-ohm load, making it suitable for driving small speakers or headphones.
One of the key advantages of the LM386 is its simplicity. It requires minimal external components to function, which makes it an attractive option for beginners and those looking to create compact audio projects. Additionally, the chip’s wide supply voltage range (4V to 12V) and low quiescent current make it suitable for battery-powered applications.
2. Key Features and Specifications
Here are some of the most important features and specifications of the LM386:
- Wide supply voltage range: 4V to 12V
- Low quiescent current: 4mA
- Voltage gain: 20 to 200 (26dB to 46dB)
- Output power: 325mW (8Ω load, 6V supply)
- Low distortion: 0.2% (at 1kHz, 6V supply, 125mW output)
- Input impedance: 50kΩ
- Inbuilt short circuit and thermal protection
- Available in 8-pin DIP, SOIC, and MSOP packages
| Parameter | Value |
|---|---|
| Supply Voltage Range | 4V – 12V |
| Quiescent Current | 4mA |
| Voltage Gain | 20 – 200 |
| Output Power (8Ω, 6V) | 325mW |
| Distortion (1kHz, 125mW) | 0.2% |
| Input Impedance | 50kΩ |
3. Pin Configuration and Functions
The LM386 is available in three package types: 8-pin DIP, SOIC, and MSOP. The pin configuration is the same for all packages. Here’s a breakdown of the pin functions:
| Pin | Name | Function |
|---|---|---|
| 1 | GAIN 1 | Gain control pin 1. Connect to pin 8 for 20x gain. |
| 2 | -INPUT | Inverting input |
| 3 | +INPUT | Non-inverting input |
| 4 | GND | Ground |
| 5 | VS | Positive supply voltage |
| 6 | BYPASS | Bypass capacitor connection for internal biasing |
| 7 | VOUT | Output voltage |
| 8 | GAIN 2 | Gain control pin 2. Connect to pin 1 for 20x gain. |
4. Operating Principles
The LM386 is designed to operate as a basic audio amplifier, taking a small input signal and amplifying it to drive a speaker or headphones. The chip internally consists of a differential amplifier input stage, followed by a voltage amplification stage and a class AB output stage.
The gain of the amplifier can be set between 20 (26dB) and 200 (46dB) by connecting a capacitor between pins 1 and 8. If left unconnected, the default gain is 20. To increase the gain, a capacitor of 10µF or larger should be connected between these pins.
The LM386 also features a BYPASS pin (pin 6) that allows for additional control over the frequency response and gain characteristics of the amplifier. By connecting a capacitor between this pin and ground, you can create a low-pass filter that helps to reduce high-frequency noise and improve stability.
5. Application Circuits
In this section, we will explore several practical application circuits using the LM386 IC.
5.1 Basic Audio Amplifier
The most basic application of the LM386 is as a simple audio amplifier. Here’s a schematic of a basic audio amplifier circuit:
+5V
|
+-+
| |
| | 10μF
| |
+-+
|
10kΩ +-+
IN>----/\/\/\/------|+|--- Pin 3 (Non-inverting input)
+-+
|
+-+
| | 0.1μF
| |
+-+
|
+------ Pin 2 (Inverting input)
|
+------ Pin 4 (GND)
|
+++
| | 10μF
| |
+-+
|
+------ Pin 6 (BYPASS)
|
|
+------ Pin 7 (VOUT) ----> To Speaker
|
+++
| | 220μF
| |
+-+
|
+------ Pin 5 (VS)
Note: Connect Pin 1 (GAIN 1) and Pin 8 (GAIN 2) for 20x gain.
In this circuit, the input signal is coupled to the non-inverting input (pin 3) through a 10kΩ resistor and a 10µF capacitor. The 0.1µF capacitor between the inverting input (pin 2) and ground helps to reduce noise and improve stability. The 10µF capacitor connected to the BYPASS pin (pin 6) sets the low-pass filter cutoff frequency, while the 220µF capacitor at the output helps to filter the power supply and minimize output distortion.
5.2 Adjustable Gain Amplifier
To create an amplifier with adjustable gain, you can replace the direct connection between pins 1 and 8 with a potentiometer. Here’s the modified schematic:
+5V
|
+-+
| |
| | 10μF
| |
+-+
|
10kΩ +-+
IN>----/\/\/\/------|+|--- Pin 3 (Non-inverting input)
+-+
|
+-+
| | 0.1μF
| |
+-+
|
+------ Pin 2 (Inverting input)
|
+------ Pin 4 (GND)
|
+++
| | 10μF
| |
+-+
|
+------ Pin 6 (BYPASS)
|
|
+------ Pin 7 (VOUT) ----> To Speaker
|
+++
| | 220μF
| |
+-+
|
+------ Pin 5 (VS)
|
|
Pin 1 (GAIN 1) +---/\/\/\/---+ Pin 8 (GAIN 2)
10kΩ
Potentiometer
By adjusting the 10kΩ potentiometer, you can vary the gain of the amplifier between 20 and 200.
5.3 Low-Voltage Operation
The LM386 can operate with supply voltages as low as 4V, making it suitable for battery-powered applications. When using lower supply voltages, it’s essential to ensure that the input signal is biased correctly to avoid clipping and distortion. Here’s an example circuit for low-voltage operation:
+3.3V
|
+-+
| |
| | 10μF
| |
+-+
|
10kΩ +-+
IN>----/\/\/\/------|+|--- Pin 3 (Non-inverting input)
+-+
|
+-+
| | 0.1μF
| |
+-+
|
+------ Pin 2 (Inverting input)
|
+------ Pin 4 (GND)
|
+++
| | 10μF
| |
+-+
|
+------ Pin 6 (BYPASS)
|
|
+------ Pin 7 (VOUT) ----> To Speaker
|
+++
| | 220μF
| |
+-+
|
+------ Pin 5 (VS)
Note: Connect Pin 1 (GAIN 1) and Pin 8 (GAIN 2) for 20x gain.
```
In this circuit, the supply voltage is reduced to 3.3V, which is still within the operating range of the LM386. The input coupling capacitor and the BYPASS capacitor values remain the same as in the previous examples.
### 5.4 Audio Mixer
The LM386 can also be used to create a simple audio mixer, allowing you to combine multiple audio sources into a single output. Here's a schematic for a basic two-channel audio mixer:
+5V
|
+-+
| |
| | 10μF
| |
+-+
|
10kΩ +-+
IN1>—-/\/\/\/——|+|
+-+
|
10kΩ +-+
IN2>—-/\/\/\/——|+|— Pin 3 (Non-inverting input)
+-+
|
+-+
| | 0.1μF
| |
+-+
|
+—— Pin 2 (Inverting input)
|
+—— Pin 4 (GND)
|
+++
| | 10μF
| |
+-+
|
+—— Pin 6 (BYPASS)
|
|
+—— Pin 7 (VOUT) —-> To Speaker
|
+++
| | 220μF
| |
+-+
|
+—— Pin 5 (VS)
Note: Connect Pin 1 (GAIN 1) and Pin 8 (GAIN 2) for 20x gain.
In this circuit, two input signals (IN1 and IN2) are combined using 10kΩ resistors before being fed into the non-inverting input (pin 3) of the LM386. The rest of the circuit remains the same as the basic audio amplifier example.
### 5.5 Intercom System
The LM386 can be used to create a simple intercom system, allowing two-way communication between two points. Here's a schematic for a basic intercom system:
+5V
|
+-+
| |
| | 10μF
| |
+-+
|
10kΩ +-+
MIC1>—-/\/\/\/—–|+|— Pin 3 (Non-inverting input)
+-+
|
+-+
| | 0.1μF
| |
+-+
|
+—— Pin 2 (Inverting input)
|
+—— Pin 4 (GND)
|
+++
| | 10μF
| |
+-+
|
+—— Pin 6 (BYPASS)
|
|
+———+—— Pin 7 (VOUT) —-> SPK1
| |
+++ +++
| | 220μF | | 10μF
| | | |
+-+ +-+
| |
+———+—— Pin 5 (VS)
|
| 10kΩ
+++–/\/\/\/——> To MIC2 (on the other LM386)
| |
| | 10μF
| |
+-+
|
+—————- From SPK2 (on the other LM386)
Note: Connect Pin 1 (GAIN 1) and Pin 8 (GAIN 2) for 20x gain on both LM386 ICs.
“`
In this circuit, two LM386 ICs are used, one for each intercom station. The microphone input (MIC1) of the first LM386 is connected to the speaker output (SPK2) of the second LM386 through a 10kΩ resistor and a 10µF capacitor. Similarly, the microphone input (MIC2) of the second LM386 is connected to the speaker output (SPK1) of the first LM386. This configuration allows for two-way communication between the intercom stations.
6. PCB Layout Considerations
When designing a PCB for the LM386, there are a few key considerations to keep in mind:
-
Power supply decoupling: Place the decoupling capacitors (220µF and 10µF) as close to the LM386 power supply pins as possible to minimize noise and ensure stable operation.
-
Input and output signal routing: Keep the input and output signal traces away from the power supply traces to avoid noise coupling. Use ground planes to provide a low-impedance return path for the signals.
-
Grounding: Provide a solid ground connection to the LM386 and use ground planes to minimize ground loops and improve overall circuit performance.
-
Component placement: Place the input and output coupling capacitors close to the LM386 pins to minimize stray capacitance and ensure proper frequency response.
-
Heat dissipation: If the LM386 is expected to operate at high output levels for extended periods, consider using a heatsink or a larger PCB copper area to help dissipate heat and prevent thermal damage to the IC.
7. Troubleshooting and Common Issues
If you encounter problems while using the LM386, here are some common issues and troubleshooting tips:
-
No output: Check the power supply connections, ensure that the input signal is present, and verify that the speaker or load is connected correctly. Also, make sure that the GAIN pins are configured correctly for your desired gain setting.
-
Distorted output: Ensure that the input signal is not clipping, check the power supply voltage, and verify that the output load is within the LM386’s capabilities. If the distortion persists, try reducing the gain or adding a small capacitor (0.1µF to 1µF) between the BYPASS pin and ground to improve stability.
-
Noisy output: Check the power supply for noise and ripple, ensure that the input signal is clean, and verify that the PCB layout follows best practices for noise reduction. Additionally,
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