BC547 NPN Transistor

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

How BC547 Transistor Works?

Being an NPN transistor, the collector and emitter remain open (reverse-biased) when the base pin is at ground, and they close (forward-biased) when a signal is applied to the base. The gain value of the BC547 transistor, ranging from 110 to 800, determines its amplification strength. Since the maximum collector current is 100mA, high-power loads should not be used. Also, the base current (IB) must be limited to 5mA for safe operation.

Saturation and Cut-Off Operation

When fully biased, the BC547 transistor enters the Saturation Region, allowing up to 100mA to flow between collector and emitter. Typical voltages include 200mV across Collector–Emitter (VCE) and 900mV across Base–Emitter (VBE). When base current is removed, the transistor enters 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 — relay drivers, LED drivers
• Amplifier modules — audio amplifiers, signal boosters
• Darlington pair circuits

Using BC547 as a Switch

The BC547 transistor functions as an efficient switch by operating in Saturation and Cut-Off Regions. Forward bias turns it ON, while reverse bias turns it OFF. The base current should not exceed 5mA, requiring a series resistor (RB). The resistor value can be calculated using:

RB = VBE / IB

Here, VBE is typically around 5V, while IB depends on the required collector current (IC).

Using BC547 as an Amplifier

In the Active Region, the BC547 transistor works as an amplifier, boosting voltage, current, or power levels. 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)

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