BC558 Transistor : Pin Configuration & Its Applications

For the conductivity of any type of transistor, charge carriers play a key role. For PNP transistors, the majority of charge carriers are holes whereas, for NPN transistors, electrons are the majority charge carriers. There are different types of PNP and NPN transistors available. From them, the BC558 transistor belongs to the PNP family.


This transistor is one kind of current-controlled device where the small amount of current at the base pin generates a huge current across the remaining two terminals. BC558 transistor is designed with Si material and it is available in the TO-92 package.

What is BC558 Transistor?

BC558 is a PNP family-based bipolar junction transistor, used in switching & amplification-based applications. It includes three terminals that are collector, base & emitter which are dissimilar in terms of doping concentrations, functions, and size.

BC558 Transistor
BC558 Transistor

This transistor is composed of 3-layers P, N & P where the ‘N’ layer is arranged in between two ‘P’ layers. The amplification factors of this transistor range from 110 – 800 which calculates the amplification capacity of the transistor.

Pin Configuration

The pin configuration of the BC558 Transistor is shown below. This transistor includes three pins which are discussed below.

BC558 Transistor Pin Configuration
BC558 Transistor Pin Configuration
  • Pin1 (Collector): This terminal allows the flow of current
  • Pin2 (Base): The transistor biasing can be controlled through this pin
  • Pin3 (Emitter): This pin drains out the flow of current

Features & Specifications

The features and specifications of the BC558 transistor include the following.

  • PNP type transistor
  • The available package is TO-92
  • Voltage from collector to the emitter (Vce) is 30 V
  • Storage temperature (Tstg) is 150 C
  • Voltage from collector to base (Vcb) is 30 V
  • Dissipation of power (Ptot) is 500 mW
  • Voltage from emitter to base (Veb) is 5V
  • Collector peak current (Icm) is 200mA
  • Collector current (Ic) is 100mA
  • Max DC gain or hFE is 800
  • Transition frequency or fT is 100 MHz
  • Collector current max is -100mA
  • Peak base current or IB is 200mA
  • Peak collector current or IC is 200mA
  • Nonstop collector current or IC is 100mA
  • Operating & storage temperature ranges from -65 to +150 C

Equivalent BC558 transistors are BC556 & BC557. Alternative PNP transistors are BC157, 2N3906, BC556, 2SA1943S8550, , BD140, TIP42 & TIP127.

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The working of BC558 transistor is; whenever the transistor is connected in forward bias then there is no flow of current at the base terminal and the remaining two terminals like emitter & collector will be closed. So the transistor will be switched ON.

Similarly, when the transistor is connected in reverse bias, then there is current flow throughout the base terminal, and the remaining two terminals like emitter & collector will be opened. So the transistor will be switched OFF.

The base terminal in the transistor mainly controls its conductivity whereas the emitter terminal in the transistor holds the entire current. As compared to base and collector terminals, the emitter terminal is doped extremely. So both the emitter & collector terminals are positive when the base terminal is negative.

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How to use BC558 Transistor?

The application circuit of the BC558 transistor is a rain detector circuit. This rain detector circuit can be built with two transistors which are used to give an alert before rainfall. This circuit is used in many fields like agriculture areas, homes, etc.

The operation of this circuit is very simple to understand. The required components to build this rain detector circuit mainly include two transistors like BC548 NPN and BC558 PNP, resistors -330K, 10K, 0.01 uF capacitor, and speaker with 8 ohms/16 ohms.


Rain Detector Circuit using Transistors
Rain Detector Circuit using Transistors

The current used by this simple circuit is extremely low like two 1.5V batteries in series to get 3V. The working of the rain detector circuit is; the two transistors within the circuit like Q1 & Q2, two resistors like R1 & R2, and the capacitor C1 are used to form an oscillator. Here this oscillator works when the flow of current reaches the Q1 transistor’s base terminal.

Once this oscillator gets active, then it regenerates a sound throughout the speaker used in the circuit. When the two terminals of the sensor are not short-circuited then the Q1 transistor will be in the cutoff region, thus the Q2 transistor also moves into the cutoff region & the oscillator will stop working.

When the rainfall shortens the sensor terminals, then the flow of current at the base terminal will allow the Q1 transistor to enable the Q2 transistor. This Q2 transistor starts conducting and the flow of current throughout the speaker will make to generate sound. So this procedure can be continuously repeated when the terminals of the sensor are shorted. This portable circuit is very simple to design with basic electronics components on a small PCB with two 1.5 Volts batteries.

Where to Use/Applications

This transistor is made to use in different electronic circuits applications. For instance, if it is used as a switch then it controls the load blow 100mA such as ICs, relays, LEDs, transistors, and many more. Alternatively, it can also be used as an audio amplifier, a preamplifier for amplification of the signal. The general applications of the BC558 transistor include the following.

  • Signal amplification
  • Darlington pair
  • Audio amplifier
  • Any type of signal amplification
  • Preamplifier
  • Use to drive different loads < 100mA
  • Design audio circuits
  • Used as an amplifier
  • Used in different driver modules such as LED driver, relay driver, etc
  • Used in amplifier modules such as signal amplifiers, audio amplifiers, etc
  • Used for switching & amplification purposes
  • Used for controlling the motor.
  • Used for buffering the impedance
  • Used in different projects based on instrumentation and robotics
  • Used in different circuits like a current mirror, oscillator, comparator & H- Bridge.
  • Used to design multivibrators like astable and bistable

Thus, this is all about an overview of the BC558 Transistor datasheet which includes its pin configuration, specifications, features, circuit working with applications. This is one kind of semiconductor device mainly used to switch or amplify electronic signals as well as electrical power. To use this transistor for a long time in any application, the load must be operated >100mA and should not drive the load >30V. Always use a base resistor and verify pinout before connecting within the circuit. Here is a question for you, what are the disadvantages of the BC558 transistor?

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