The BC108 is one of the most iconic NPN small-signal transistors used in early and modern electronics. This article will discuss the BC108 transistor’s features, electrical characteristics, pinout, equivalents, applications, and more.

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BC108 Transistor Overview

The BC108 is a classic NPN small-signal transistor originally popular in early audio and electronics designs. Manufactured by Central Semiconductor Corp in a durable TO-18 metal-can package. It offers low noise, stable gain, and reliable performance for amplification and switching tasks. With typical ratings of 25 V, 200 mA, and around 600 mW dissipation, it handles low-power applications efficiently while maintaining good high-frequency behavior thanks to its ~150 MHz transition frequency. Its metal-can construction also provides excellent thermal stability and shielding compared to plastic-encased transistors.

As part of the BC108/BC109 transistor family, it continues to serve as a reliable general-purpose transistor for modern and legacy designs. If you are interested in purchasing the BC108, feel free to contact us for pricing and availability.

BC108 Gain-Group Variants

BC108 Transistor CAD Models

BC108 Transistor Pinout Configuration

BC108 Transistor Equivalents

BC107

BC109

BC548

BC547

BC549

BC546

BC108 Transistor Specifications (BC108B Version)

BC108 Electrical Characteristics

BC108 Transistor Application Circuit

BC108 Microphone Preamplifier Circuit

In this microphone preamplifier, the BC108 transistor operates as a common-emitter low-noise amplifier. The dynamic microphone (M1) produces a very small AC audio signal, which is coupled through capacitor C1 into the base of the BC108. Resistors R1 and R2 form a bias network that sets the correct base voltage for linear amplification. The emitter resistor R3, bypassed by C2, provides gain stabilization while allowing AC amplification. The amplified audio signal appears at the collector and is coupled out through capacitor C5, while C3 and C4 help filter unwanted noise and stabilize frequency response. In this design, the BC108’s low noise and good gain characteristics make it ideal for boosting weak microphone signals into a usable audio level.

100-Watt Audio Power Amplifier (BC108 as Input Transistors)

In the high-power audio amplifier circuit, the two BC108 transistors (Q6 and Q7) serve as the input differential pair. Their job is to amplify the small audio input signal and convert it into a balanced signal suitable for driving the following gain stages. Because BC108 transistors have good linearity and low noise, they ensure clean and stable amplification at the first stage, which is critical for high-fidelity audio. These BC108 input transistors feed driver transistors such as 2N6107 and 2N5294, which then drive the high-power output devices 2N3773 to deliver up to 100 watts to the speaker. The BC108 plays a key role in determining the amplifier’s sensitivity, distortion level, and overall clarity by providing a clean and stable initial amplification stage.

Relay Driver / Trigger Control Using BC108

In the relay control circuit, the BC108 transistor acts as a signal amplifier and switch. When transistor Q1 (BC141) senses an appropriate input condition and switches on, it sends current into the base of Q2 (BC108). This causes Q2 to conduct, allowing current to flow through R1, R2, and the relay coil. The BC108 effectively amplifies the small control signal into a higher current needed to energize the relay. Once activated, the relay switches its contacts, providing an isolated and higher-power output. In this configuration, the BC108 works as a sensitive low-power current amplifier, making it suitable for triggering electromechanical devices such as relays.