Brief Description on BC557 Transistor

BC557 is a PNP transistor hence the collector and emitter will be close (Forward biase) when the base pin is hold at ground and will be open (Reverse biase) when a signal is provide to base pin. This is where a PNP transistor differs from a NPN transistor, a Logic state (blue colour) is used to toggle between Ground and Signal Voltage (Emitter-Base Voltage V_BE) as shown below:

BC557 has a gain value of 110 to 800, this value determines the amplification capacity of the transistor. The maximum amount of current that could flow through the Collector pin is 100mA, hence we cannot connect loads that consume more than 100mA using this transistor. To bias a transistor we have to supply current to base pin, this current (I_B) should be limited to 5mA.

When this transistor is fully biase then it can allow a maximum of 100mA to flow across the collector and emitter. This stage is call Saturation Region and the typical voltage allowed across the Collector-Emitter (V­_CE) or Base-Emitter (V_BE) could be 200 and 900 mV respectively. When base current is remove the transistor becomes fully off, this stage is called as the Cut-off Region and the Base Emitter voltage could be around 660 mV.

BC557 Pin Configuration

Features

• Bi-Polar PNP Transistor
• DC Current Gain (h_FE) is 300 maximum
• Continuous Collector current (I_C) is 100mA
• Emitter Base Voltage (V_BE) is 6V
• Base Current(I_B) is 5mA maximum
• Available in To-92 Package

BC557 Transistor as switch

When a transistor is use as a switch it is operate in the Saturation and Cut-Off Region as explain in above paragraph. As discussed a transistor will act as an Open switch during Forward Bias and as a Close switch during Reverse Bias, this biasing can be achieved by supplying the require 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 (R_B) can be calculate using below formulae.

R_B  = V_BE / I_B

Where, the value of V_BE should be 5V for BC557 and the Base current (I_B depends on the Collector current (I_C). The value of I_B should 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 use in amplifier circuits are

1. emitter amplifier
2. Common collector amplifier
3. 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 (I_C) / Base Current (I_B)

Applications

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

Datasheet

https://components101.com/sites/default/files/component_datasheet/BC557%20Datasheet.pdf

2D model of the component

If you are designing a PCD or Perf board with this component then the following picture from the BC557 Datasheet will be useful to know its package type and dimensions.