BC547 NPN Transistor

The BC547 NPN transistor is a versatile semiconductor component used for amplification and switching in electronic circuits. It operates efficiently as a switch and amplifier, making it ideal for driver modules, audio amplifiers, and signal processing applications. With a DC current gain (hFE) of up to 800 and a maximum collector current of 100mA, it is reliable for low-power electronics projects.

How BC547 Transistor Works?

Being an NPN transistor, the collector and emitter are open (reverse-biased) when the base pin is at ground, and closed (forward-biased) when a signal is applied to the base. The transistor's gain value, ranging from 110 to 800, determines its amplification capability. Maximum current through the collector pin is 100mA, so loads above this value are not recommended. The base current (IB) should be limited to 5mA to prevent damage.

Saturation and Cut-Off Operation

When fully biased, the BC547 allows up to 100mA to flow across the collector and emitter, known as the Saturation Region. Typical voltages are 200mV across Collector-Emitter (VCE) and 900mV across Base-Emitter (VBE). When base current is removed, the transistor enters the Cut-Off Region with Base-Emitter voltage around 660mV.

Pin Configuration

• Pin 1 (Collector): Current enters through the collector
• Pin 2 (Base): Controls transistor biasing
• Pin 3 (Emitter): Current exits through the emitter

Features

• Bi-Polar NPN Transistor
• DC Current Gain (hFE) up to 800
• Continuous Collector current (IC) 100mA
• Emitter-Base Voltage (VBE) 6V
• Maximum Base Current (IB) 5mA
• Available in TO-92 Package

Equivalent Transistors

• BC549, BC636, BC639
• 2N2222 (TO-92 / TO-18), 2N2369
• 2N3055, 2N3904, 2N3906, 2SC5200

Applications

• Driver modules such as relay drivers and LED drivers
• Amplifier modules like audio amplifiers and signal amplifiers
• Darlington pair circuits

Using BC547 as a Switch

When operated as a switch, the BC547 works in Saturation and Cut-Off Regions. Forward bias turns it on, and reverse bias turns it off. Base current should not exceed 5mA; hence a series resistor (RB) is required. The resistor value can be calculated using:

RB = VBE / IB

Where VBE is typically 5V and IB depends on the desired Collector current (IC).

Using BC547 as an Amplifier

In the Active Region, the BC547 functions as an amplifier, boosting voltage, current, or power. Common amplifier configurations include:

• Common Emitter Amplifier (most widely used)
• Common Collector Amplifier
• Common Base Amplifier

The DC current gain can be calculated using:

DC Current Gain = Collector Current (IC) / Base Current (IB)