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BC557 Transistor Introduction

BC557 is a widely used PNP bipolar junction transistor made of TO-92 small package. It is a general-purpose PNP transistor, which can be used as a switch or amplifier in electronic circuits.

What is BC557 transistor

Bc557 is a widely used PNP bipolar junction transistor made of TO-92 small package. It is a general-purpose PNP transistor, which can be used as a switch or amplifier in electronic circuits. Its HFE ratings range from 125 to 800, making it an ideal transistor for use as a small signal amplifier in electronic circuits, such as for audio signal amplification or others. The HFE rating of bc557 can be determined by the last letter after the number. For example, the HFE rating of bc557a is 110-220, the HFE rating of BC557B is 200-450, and the HFE rating of bc557c is 420-800. The collector has a maximum dissipation of 500MW, which is another advantage of using it in the amplification stage. In addition, when it is used as the switch of load below 100mA, its performance is also very good. The collector emitter (VCE) value of the transistor is – 45V. In addition to these uses, bc557 is also a good general-purpose PNP transistor for your amateur electronic projects, so transistors must be available in the electronic laboratory.

BC557 datasheet

BC557 Pinout &Configuration

BC557 has three terminals, as shown in the table.

Pin NumberPin NameDescription
1CollectorCurrent flows in through collector
2BaseControls the biasing of transistor
3EmitterCurrent Drains out through emitter
BC557 Pinout &Configuration

BC557 Features

  1. Package Type: TO-92
  2. Transistor Type: PNP
  3. Max Collector Current(IC): -100mA
  4. Max Collector-Emitter Voltage (VCE): -45V
  5. Max Collector-Base Voltage (VCB): -50V
  6. Max Emitter-Base Voltage (VBE): -5V
  7. Max Collector Dissipation (Pc): 500 Milliwatt
  8. Max Transition Frequency (fT): 100 MHz
  9. Minimum & Maximum DC Current Gain (hFE): 125 to 800
  10. Max Storage & Operating temperature Should be: -65 to +150Centigrade

BC557 equivalent transistor

  • The BC557 equivalent transistors are: BC157, BC558, 2N3906, 2SA1943, BD140, S8550, TIP127, TIP42
  • The BC557 pnp complementary is: 2N3906, 2N4403, BC556, BC327

How does a BC557 transistor work?

As mentioned above, bc557 is a general-purpose transistor, so it can be used in a variety of general-purpose applications, for example, it can be used as an audio amplifier in small radio, electronic buzzer, electronic bell and other sound circuits requiring small amplification. Because of the good gain of the transistor, it can also be used in the audio preamplifier stage. In addition to these applications, the transistor can also be used as a switch for 100 Ma load in electronic applications, as well as any part of the switch electronic circuit, other high-power transistors, LEDs, relays, etc.

BC557 Transistor as switch

When a transistor is used as a switch it is operated in the Saturation and Cut-Off Region as explained above. As discussed a transistor will act as an Open switch during Forward Bias and as a Closed switch during Reverse Bias, this biasing can be achieved by supplying the required amount of current to the base pin. As mentioned the biasing current should maximum of 5mA. Anything more than 5mA will kill the Transistor; hence a resistor is always added in series with base pin. The value of this resistor (RB) can be calculated using below formulae.

RB  = VBE / IB

Where, the value of VBE should be 5V for BC557 and the Base current (IB depends on the Collector current (IC). The value of Ishould not exceed mA.

BC557 Transistor as Amplifier

A Transistors acts as an Amplifier when operating in Active Region. It can amplify power, voltage and current at different configurations.

Some of the configurations used in amplifier circuits are

  1. Common emitter amplifier
  2. Common collector amplifier
  3. Common base amplifier

Of the above types common emitter type is the popular and mostly used configuration. When uses as an Amplifier the DC current gain of the Transistor can be calculated by using the below formulae

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


  • Driver Modules like Relay Driver, LED driver etc..
  • Amplifier modules like Audio amplifiers, signal Amplifier etc..
  •  Darlington pair

That’s all for today. Hope you enjoyed this article. However, if you have doubts or have any questions about this, you can contact me in the comments section below. I’ll be happy to help you as best I can. Let us know your feedback and suggestions at any time, they allow us to provide you with high-quality work that responds to your needs and expectations, and helps you continue to give back to our products. Thank you for reading this article.

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