Building amplifier is not complicated task. You can easily build amplifier using a single BD139 transistor. Hear i am sharing my project of simple audio amplifier circuit i built with a BD139 transistor. This circuit can drive a small speaker from a low level audio input signal to high output. In this article you will get the details about the circuit and how to build your own amplifier. You can use this small circuit as Portable audio amplifier.
Components Required
| Value/Part Number | Quantity |
|---|---|
| R1 – 2.2kฮฉ | 1 |
| R4 – 100ฮฉ | 1 |
| C1 – 47ยตF | 1 |
| C2 – 10ยตF | 1 |
| C3 – 100ยตF | 1 |
| C4 – 100ยตF | 1 |
| C5 – 0.47ยตF | 1 |
| Q1 – BD139 | 1 |
| Q2 – 2N3904 | 1 |
| Speaker | 1 |
| 12V DC | 1 |
Circuit Diagram
Construction and Working Principle
This audio amplifier circuit is build with two NPN transistors of 2N3904 and BD139. The 2N3904 is using as a preamplifier and BD139 as a power output transistor. The input audio signal is first passed through capacitor C1, which blocks any DC voltage and it only allows the AC audio signal entering into the base of 2N3904 transistor. This transistor amplifies the weak input signal and the amplified output is coupled through capacitor C2 to the base of BD139 transistor.
The BD139 transistor acts as the main power amplifier transistor, it will boosting the current of the audio signal to drive the connected speaker. Capacitor C3 passes the amplified AC signal to the speaker and it will blocking DC voltage reaching to speaker. Capacitor C4 will smooths the 12V DC power supply and capacitor C5 will improves the signal stability. The resistors R1 and R4 set the biasing and limit current.
Applications of BD139 Transistor Audio Amplifier Circuit
- Mini Audio Amplifiers
- DIY Electronics Projects
- Pre-Amplifier Stage
- Home Audio Systems
