BC557 PNP Transistor

BC557 Pin Configuration

BC557 Equivalent PNP Transistors

BC157, BC558, 2N3906, 2SA1943, BD140, S8550, TIP127, TIP42

Brief Description on BC557 Transistor

BC557 is a PNP transistor hence the collector and emitter will be closed (Forward biased) when the base pin is held at ground and will be opened (Reverse biased) when a signal is provided 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 biased then it can allow a maximum of 100mA to flow across the collector and emitter. This stage is called 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 removed 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 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 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 (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