Transistor (BC557, PNP)

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
• To-92 Package

BC557 Transistor Uses

As you know a transistor can be used in various applications. The two major use cases for the BC 557 transistor are as a switch or as an amplifier just like any other BJT transistor.

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 (R_B) can be calculated 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 Transistor 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 (I_C) / Base Current (I_B)

Difference between BC557A , BC557B and BC557C

The major difference between these variants is that they offer different gain(hfe). While the BC557A variant offers the lowest gain of 110 to 200, the BC557B offers a gain of 200-450 and the BC557C gives the highest gain of 450-800.

Applications

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