BC557 PNP Transistor

BC557 is a PNP transistor hence the collector and emitter will be closed (Forward biased) when the base pin is held at the ground and will be opened (Reverse biased) when a signal is provided to base pin. This is where a PNP transistor differs from an NPN transistor, a Logic state (blue color) 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 the 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.

Applications

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