What is BC547 Transistor Working and Its Applications

The semiconductor device like a transistor is one kind of switch which controls electrically. It consists of three terminals like an i/p, o/p & a control line. These are named as the emitter (E), collector(C) and base (B). A transistor works like a switch as well as an amplifier to convert the waves from audio to electronic. Transistors are smaller in size, long life and can operate with low voltage supplies. The first transistor was designed with Ge (germanium). In modern electronics, it is the basic building block and used in various electrical and electronic systems. This article discusses an overview of BC547 transistor working and its applications.

What is a BC547 Transistor?

The BC547 transistor is an NPN transistor. A transistor is nothing but the transfer of resistance which is used for amplifying the current. A small current of the base terminal of this transistor will control the large current of emitter and base terminals. The main function of this transistor is to amplify as well as switching purposes. The maximum gain current of this transistor is 800A.


The similar transistors are like BC548 & BC549. This transistor works in a fixed DC voltage in the preferred region of its characteristics which is called the biasing. Further, the series of this transistor can be divided into three groups based on the current gain like BC547A, BC547B & BC547C.

BC547 Transistor Pin Configuration

The BC547 transistor includes three pins which include the following.

  • Pin1 (Collector): This pin is denoted with symbol ‘C’ and the flow of current will be through the collector terminal.
  • Pin2 (Base): This pin controls the transistor biasing.
  • Pin3 (Emitter): The current supplies out through emitter terminal

A Transistor works as an amplifier while functions in the active region to amplify voltage, current, and power at various configurations. The amplifier circuit uses three configurations which include the following.

  • Common emitter (CE) amplifier
  • Common collector (CC) amplifier
  • Common base (CB) amplifier

From the above three configurations, CE is the most widely used configuration.


Working States of Transistor

The working states of BC547 transistor include the following.

  • Forward Bias.
  • Reverse Bias.

In a forward bias mode, the two terminals like emitter & collector are connected to allow the flow of current through it. Whereas in a reverse bias mode, it doesn’t allow the flow of current through it because it works as an open switch.


The features of the BC547 transistor include the following.

  • The gain of DC current (hFE) = 800 A
  • Continuous Ic (collector current) = 100mA
  • VBE (emitter-base voltage) = 6V
  • IB (base current) = 5mA
  • The polarity of the transistor is NPN
  • The transition frequency is 300MHz
  • It is obtainable in semiconductor package like-92
  • Power dissipation is 625mW

BC547 Transistor Circuit Diagram

The ON/OFF touch switch using transistor BC547 is shown below. The circuit is activated once the power supply is given to the circuit. Once the supply is given to the circuit, the relay gets off mode. Thus, the base terminal of the Q3 transistor is high throughout the R7 resistor to maintain in cut-off condition.


When the S2 switch is ON, the Q4 transistor will start conducting & relay ‘L3’ can be latched. The base terminal of the Q3 transistor will be pulled downward, and then L2 LED will blink to indicate power is ON. The Q4 transistor is ON because of the voltage at the collector terminal of the transistor Q3 using R8 resistor

When the switch S1 is pressed for a moment the base terminal of transistor Q3 will be pulled up then the L2 will turn off because of the Q4 transistor’s pull-down base throughout the R8 resistor so the relay L3 will be turned off.

Precautions of this Transistor

The precautions of this transistor include the following.

  • To run the transistor for a long time in a circuit, it is very important that it doesn’t increase the load more than 100mA.
  • The voltage should not exceed to 45V DC across the transistor.
  • The base resistor should be used for providing the necessary current intended for saturation.
  • Maintain the temperature from the above +150oC to -65 oC.
  • Always verify the three terminals of the transistor while connecting in-circuit otherwise the performance can be reduced and the circuit can be damaged.


Applications of BC547 transistor include the following.

  • This BC547 transistor is used general-purpose, widely used and it is used as an alternative as well as a substitute to different kinds of transistors. Thus, it can use in different electronic circuits
  • The utmost transition frequency of BC547 is 300MHz so that it will perform well within RF circuits.
  • Amplification of current
  • Audio Amplifiers
  • Switching Loads < 100mA
  • Transistor Darlington Pairs
  • Drivers like an LED driver, Relay Driver, etc.
  • Amplifiers like Audio, signal, etc..
  • Darlington pair
  • Quick switching
  • PWM (Pulse Width Modulation)

These transistors are used to build various electrical and electronic circuits which include the following.

  • Alarm circuits
  • LED flasher circuit
  • Water level indicator
  • Sensor-based circuits
  • Audio Preamp circuits
  • RF Circuits
  • Touch-sensitive switch circuit
  • Heat sensor circuit
  • Moisture sensitive alarm
  • Latch circuit
  • Street light circuit
  • Relay driver based on one channel
  • Indication of volume level

Thus, this is all about BC547 transistor and it is an NPN BJT. A transistor is usually used for amplifying current. A small amount of current at the base terminal of the transistor will control the high current at collector and emitter terminals of the transistor. These transistors are specially used for switching as well as amplification purposes. The highest gain of current is 800A. Here is a question for you, what are the advantages of BC547?