What is a BC557 Transistor : Pin Configuration & Its Working

A semiconductor device like a transistor is a three-terminal component that is used to switch or amplify electrical power & electronic signals. In modern electronics, transistors play a key role. So, a BC557 transistor is a very famous PNP type BJT that is available in a tiny package like TO-92. This kind of transistor is used as a switch otherwise an amplifier within electronic circuits.

What is BC557 Transistor?

BC557 is a general-purpose transistor, used like an amplifier or a switch in electronic circuits. Its hFE ratings of this transistor range from 125 to 800 to make the transistor ideal by using like an amplifier within electronic circuits like audio signal amplification. These ratings can be determined through the final letter after its digit. The highest collector dissipation is 500 milliwatt.

BC557 Transistor
BC557 Transistor

When this transistor is used as a switch for below 100mA loads then it works well. The value of VCE for this transistor is good like -45V. This collector to emitter terminal voltage is used within circuits which require 40V to 45V DC.

BC557 Transistor Pin Configuration

The pin configuration of the BC557 transistor is shown below and each pin and its functionality is discussed below. This transistor includes three terminals like a normal transistor namely base, emitter & collector. The BC557 transistor symbol is shown below.

BC557 Pin Configuration & Symbol
BC557 Pin Configuration & Symbol
  • Pin1 (Collector): The current supplies throughout the collector terminal
  • Pin2 (Base): The base terminal of this transistor controls the biasing
  • Pin3 (Emitter): Flow of current drains out through this terminal

Features & Specifications

The features and specifications of the BC557 transistor include the following.

  • The type of package is T092
  • The type of transistor is PNP
  • Collector Current (IC) Max is -100mA
  • Max Operating & Storage Temperature ranges from -65 to +150 degree centigrade
  • Collector-Emitter Voltage or VCE Max is -45V
  • DC current gain Max or Min (hFE) is 125 – 800
  • Collector Base Voltage or VCB Max is-50V
  • Max fT or Transition Frequency is 100 MHz
  • Emitter-Base Voltage or VBE Max is -5V
  • Collector Dissipation or Pc Max is 500 mW
  • Peak Collector Current (Icm) is 200mA
  • Junction Temperature (Tstg) is 150 Centigrade
  • Process technology used in this transistor is advanced
  • The error voltage is less
  • Switching speed is very quick
  • Complete voltage operation
  • Handling capability of current & power is high

Equivalent BC557 PNP Transistors are BC157, 2N3906, BC558, 2SA1943, S8550, BD140, TIP127 & TIP42. The complimentary of BC557 NPN transistor is BC547.


SMD Versions of BC557 transistors are; BC857-SOT-23, BC860-SOT-23, BC857W-SOT-323, & BC860W – SOT-323.

The current gain of this BC557 transistor ranges from 110 & 800. The BC557A transistor gain ranges from 110 to 220, BC557B transistor ranges from 200 – 450, BC557C transistor ranges from 420 – 800.

How to Protect BC557 Transistor?

The following steps must consider while protecting a BC557 transistor. They are

  • In electronic circuits, this transistor should not operate under -45V DC w
  • At a base terminal, always we should use an appropriate transistor
  • Maintain the temperature below -65 to +150 degrees centigrade.
  • Before placing the transistor, we have to check its gain as wrong input can decrease its performance.

Working Principle

We know that the majority of charge carriers in the PNP transistor are holes whereas, in the NPN transistor, it is electrons. Even though the majority of carriers are holes within PNP, the transistor’s base terminal is very essential within the complete function of this transistor.

In the NPN transistor, the minority charge carriers like holes are produced from the emitter terminal in place of electrons & they are collectors throughout the terminal like a collector.

This kind of transistor is a current-controlled component as the small amount of current at the base terminal is mainly used to manage the huge current there at the emitter & collector terminals.

Once the transistor is switched off, the current will be there at the base terminal, while the transistor is switched on, there is no flow of current at the same terminal.

How to use BC557 Transistor?

The transistor-like BC57 transistor can be used like a switch and an amplifier within the circuit. These are the two main applications of this transistor like a switch and an amplifier.

BC557 Transistor Circuit
BC557 Transistor Circuit

BC557 Transistor as a Switch

Once a BC557 transistor functions like a switch then it is operated within two regions like Cut-Off & Saturation. In forward bias, this transistor works like a switch whereas in reverse bias the same transistor works as an amplifier. This biasing can be obtained by providing the necessary quantity of current toward the base terminal.

The max biasing current is 5mA otherwise it will damage the transistor. So a transistor must be connected with the base terminal in series. This resistor (RB) value can be measured by using the following formula.


For this transistor, the emitter-base voltage like VBE must be 5V & the IB (Base current) mainly depends on the IC (Collector current). The base current value should not go above mA.

BC557 Transistor as Amplifier

Whenever the transistor works in an active region then it performs like an amplifier. This transistor uses different configurations to amplify voltage, current & power like common emitter, common base & common collector.

From the above three types of configurations, the common-emitter or CE is very famous. So this configuration is used in most applications.

When the transistor is used as an amplifier, then the DC current gain can be measured through the following formulae.

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

In a BC557 transistor, the two terminals like emitter & collector will be forward biased by connecting the base pin to the ground terminal. This transistor is reverse biased when a signal is given to the base terminal.

The gain value of the BC557 transistor ranges from 110 – 800, so this value decides the transistor’s amplification capacity.

The highest quantity of current that could supply throughout the collector terminal is 100mA, so by using this transistor, we cannot attach loads that use above 100mA.
Transistor biased when we supply current toward the base terminal, this base current must be restricted to 5mA.

Once this transistor is completely biased then it allows maximum current to supply across the two terminals like emitter & collector, which is known as saturation region. The typical voltage allowed across the CE & BE could be 200mV & 900 mV correspondingly.

Once the current provided to the base terminal is detached then the transistor will be switched off which is called as Cut-off Region & the BE voltage could be approximately 660 mV.

Advantages & Disadvantages

The advantages & disadvantages of the BC557 transistor include the following.

The advantage of the BC557 transistor includes the following.

  • Small size component
  • Responsive to current
  • Less cost
  • Output impedance is less
  • Life span is good
  • Noise operation is less
  • Circuit design is simple
  • Uses less power supply
  • Quick action at the circuit

The disadvantage of the BC557 transistor includes the following.

  • Less Switching
  • Sensitive to temperature
  • Bandwidth is limited
  • For high-frequency signals & high power circuits, it is not appropriate
  • At certain frequencies, phase shifting problems occur
  • High leakage current
  • Power dissipation is high

Where to use/Applications?

The applications of the BC557 transistor include the following.

  • It is used in different driver modules such as LED driver, relay driver, etc.
  • It is used in amplifier modules such as signal, audio, etc.
  • Darlington pair
  • This transistor is used for amplification & switching purposes
  • It is used to control the flow of current within motors
  • It is used in robot & instrumentation based projects
  • Used to drive below 0.1mA/100mA of loads
  • BC557 transistor can be used where small amplification is necessary like an audio amplifier within electronic bells, radios, buzzers, etc.
  • It is used in the stages of audio preamplifier because of the good gain

Thus, this is all about an overview of the BC557 transistor datasheet and its working with applications. The BC557 transistor is the PNP BJT transistor used in electronic circuits. This transistor is very useful for circuit designers as well as students who are very curious about electronic components. Here is a question for you, what is the difference between BC557 & BC558 transistors?