In this DIY project, we will design and build a simple audio pre-amplifier using the MMBT5087, a small-signal NPN transistor. The MMBT5087 is often used in low-power, general-purpose amplification circuits due to its compact size, low noise characteristics, and ability to handle small signals with precision. This makes it an ideal choice for audio applications, where clarity and minimal distortion are crucial.

Our goal is to create a pre-amplifier circuit that can boost weak audio signals from sources like microphones, electric guitars, or audio devices, making them strong enough to be processed by power amplifiers or further stages of an audio system. This circuit will serve as an introductory project for those interested in audio electronics, particularly for projects involving microphones or other low-level audio sources.

By the end of this project, you will have a fully functional pre-amplifier that can be used for various audio applications such as microphone amplification, guitar signal processing, or even as a line-level booster for connecting audio devices to a power amplifier.

Key Components Used

MMBT5087 (NPN Transistor)

The MMBT5087 is a general-purpose NPN transistor used for low-power amplification. It is known for its high gain and low noise characteristics, making it a good choice for audio amplification applications.

Resistors

Resistors are used to set the gain and biasing of the transistor. The values will be selected to ensure proper signal amplification and stability.

Capacitors

Capacitors are used for coupling the audio signal between stages of the amplifier and decoupling the power supply, helping to filter out unwanted noise and DC offsets from the signal.

Power Supply

The MMBT5087 transistor will require a simple DC power supply, typically between 5V and 12V. A low-voltage supply is adequate for a pre-amplifier circuit, ensuring the circuit is safe and efficient for audio processing.

Potentiometer

A potentiometer will be included in the circuit to provide volume control for adjusting the output signal level.

Input/Output Connectors

The audio input and output will use standard 3.5mm jacks, making it easy to interface with audio sources like smartphones, microphones, and other audio devices.

Project Overview

The project is designed around a common-emitter amplifier configuration, where the MMBT5087 transistor is used to amplify the audio signal. This simple yet effective design allows the pre-amplifier to boost weak audio signals coming from microphones, electric guitars, or other audio sources to a level that can be processed by power amplifiers or further audio circuitry.

The circuit will have the following key features:

1. Input coupling capacitor to block DC offset and only pass the AC audio signal.

2. Gain control using a potentiometer to adjust the amplification level of the signal.

3. Power decoupling capacitors to filter noise from the power supply, ensuring clean and clear audio.

4. Output coupling capacitor to remove any DC offset at the output and ensure that only the amplified audio signal reaches the next stage (e.g., power amplifier or mixer).

Step-by-Step Build

1. Circuit Design

The audio pre-amplifier circuit we will build is based on the common-emitter amplifier configuration, a popular choice for small-signal audio amplification. This configuration uses a single transistor to amplify the input signal, with the input and output signals both coupled through capacitors to block any DC components.

The basic circuit consists of the following stages:

1. Input Stage: The weak audio signal enters the circuit through a coupling capacitor. This capacitor removes any DC offset from the signal source, ensuring that only the AC audio signal is passed into the amplifier stage.

2. Amplification Stage: The input signal is amplified by the MMBT5087 NPN transistor, which is biased with a pair of resistors to set the operating point and gain. The gain of the circuit can be adjusted by varying the feedback resistance.

3. Output Stage: After amplification, the output signal is passed through another coupling capacitor to ensure that any DC components are blocked before the signal is sent to the next stage or output device.

4. Power Supply Decoupling: A filter capacitor will be used to filter out noise and smooth the supply voltage, ensuring stable operation of the amplifier.

2. Choosing Component Values

The component values for resistors and capacitors will be chosen based on the desired gain of the amplifier and the frequency response. Here's how to calculate and select key components:

Biasing Resistors (R1, R2): The resistors R1 and R2 form the voltage divider network that biases the base of the MMBT5087. The goal is to provide a stable DC bias voltage to the transistor’s base while ensuring that the transistor operates in its active region. Typically, these resistors are chosen to provide a voltage at the base that is about 1.2 to 1.5 times the supply voltage.

Collector Resistor (RC): The collector resistor sets the gain of the amplifier. The value of RC will depend on the desired gain and the operating conditions of the MMBT5087. For an audio pre-amplifier, a typical value for RC could range from 4.7kΩ to 10kΩ.

Emitter Resistor (RE): The emitter resistor stabilizes the operating point of the transistor and helps set the gain of the circuit. It also provides local negative feedback, which helps linearize the amplification. A typical value for RE is 1kΩ to 2.2kΩ.

Coupling Capacitors (C1, C2): The input and output capacitors are used to block any DC offset from the signal. The values for these capacitors are chosen based on the desired frequency response of the amplifier. Typically, 10µF to 100µF electrolytic capacitors are used for audio coupling.

Decoupling Capacitor (C3): A decoupling capacitor is used to smooth out any ripple or noise in the power supply. A value of 100nF to 1µF is typical.

3. Building the Circuit

With the component values chosen, we can now begin assembling the circuit on a breadboard or PCB.

Power Supply:

1．Start by connecting a 5V to 12V DC power supply to the circuit. Ensure that the power supply is stable and capable of supplying sufficient current for the pre-amplifier.

Input Coupling Capacitor (C1):

1．Connect a coupling capacitor (C1) in series with the input audio signal. The input can come from any standard audio source, such as a microphone or an MP3 player. The capacitor blocks any DC offset that may be present at the input.

Biasing Network:

1．Connect resistors R1 and R2 in a voltage divider configuration to bias the base of the MMBT5087 transistor. The base should be biased at a stable voltage to ensure proper operation of the transistor.

Amplification Stage (Transistor):

1．Connect the collector of the MMBT5087 transistor to the power supply through a collector resistor (RC).

2．Connect the emitter of the MMBT5087 to ground through an emitter resistor (RE). This will help set the transistor’s operating point.

3．The base of the transistor will be connected to the voltage divider network (R1 and R2), which biases the base at the correct operating point.

Output Coupling Capacitor (C2):

1．After the amplification stage, connect the output capacitor (C2) to the collector of the transistor. This capacitor ensures that only the AC audio signal is passed to the output.

Volume Control (Potentiometer):

1．Add a potentiometer between the output and ground to provide volume control. This allows you to adjust the signal level that is sent to the next stage or output.

Power Supply Decoupling:

1．Place a decoupling capacitor (C3) close to the power pins of the MMBT5087 to filter out noise and provide stable voltage to the transistor.

4. Testing the Circuit

Once the circuit is assembled, it's time to test it.

Initial Power-Up:

1．Apply the power supply (e.g., 9V or 12V) to the circuit.

2．Use a multimeter to check the voltage at key points, especially the base and emitter of the transistor, to ensure the transistor is correctly biased.

Signal Input:

1．Connect an audio source (such as a smartphone, microphone, or audio signal generator) to the input of the amplifier.

2．Monitor the output with a speaker or headphones. The audio signal should be amplified, and the volume should be adjustable via the potentiometer.

Adjusting Gain:

1．Use the potentiometer to adjust the output signal level.

2．Listen for any distortion or clipping, and ensure the amplifier operates cleanly across the entire volume range.

Testing with Different Sources:

1．Test the circuit with various audio sources, such as a microphone, guitar, or audio player, to ensure it works well in all scenarios.

5. Troubleshooting

If the amplifier is not working as expected, here are some common troubleshooting steps:

1. Check the Biasing: Incorrect biasing can result in distortion or low output. Ensure that the base of the MMBT5087 has the correct bias voltage.

2. Verify Component Values: Double-check the values of the resistors and capacitors. Incorrect values can lead to improper gain or frequency response.

3. Test for Shorts or Open Circuits: Check for any shorts in the circuit, especially at the transistor terminals. A multimeter in continuity mode can help identify any issues.

6. Conclusion

In this DIY project, you’ve successfully built a simple audio pre-amplifier using the MMBT5087 NPN transistor. This pre-amplifier circuit will boost weak audio signals from sources like microphones or guitars, making them suitable for processing by power amplifiers or other audio equipment.

This project introduces key concepts of audio amplification, including signal coupling, biasing, gain control, and power decoupling. It’s an excellent starting point for anyone interested in audio electronics and can serve as the foundation for more complex audio projects in the future. Whether you’re looking to build a microphone preamp, guitar amplifier, or simply learn about audio circuits, this project provides valuable hands-on experience and insight into the world of audio electronics.