BC547- AP TransistorBc

The BC547 is an NPN bipolar-junction triode (BJT) which means that the emitter and collector consist of an n-type substrate while the base consists of a p-type substrate. The two p-n junctions formed between these regions give rise to the various input-output characteristics of the transistors.

As the name itself indicates, a transistor basically works by transferring resistance. In other words, small current input at the base terminal leads to larger currents through the collector and emitter terminals.

The Bc547 junction transistor is mainly used for switching and amplification purposes. These two uses are briefly described below:

• 1. Switching electrical circuits- When the emitter-base junction of the transistor is forward biased (i.e. has a forward voltage greater than 0.7 V across it), a conducting path is created between the collector and emitter terminal and the transistor acts as a switch in ON state. For this, both the emitter-base and the collector-base junctions must be forward biased. The transistor is said to be in the saturation region.

  In case the emitter-base voltage is lower than 0.7 V (say 0 V), there is almost no conduction between the collector and emitter terminals and hence, the transistor acts a switch in OFF state. For this, both the emitter-base and the collector-base junctions must be reverse biased.

  Thus, these transistors are extremely important in electronic circuits where switching needs to be done electrically and not mechanically. The transistor is said to be in the cutoff region. This can be used to drive loads like relays and motors which can’t be directly powered by the output of normal logic circuits.

• 2. Amplification purposes- A small current signal given at the base input can result in a larger output current signal at the collector and base terminals. So, the BC547 NPN transistor can be used for general amplification purposes. In order to do so, it is necessary that the emitter-base junction is forward biased while the collector-base terminal is reverse biased. The maximum current gain for the BC547 transistor is about 800.

Important ratings and input-output characteristics of the Bc547 transistor:

• Maximum collector-base voltage (VCBO) - It is the maximum voltage that can be applied across the collector-base terminals without damaging the transistor. Its value is 80 V for the BC547 transistor.
• Maximum collector-emitter voltage (VCEO) - It is the maximum voltage that can be applied across the collector-emitter terminals without damaging the transistor. Its value is 65 V for the BC547 transistor.
• Maximum emitter-base voltage (VEBO) - It is the maximum voltage that can be applied across the emitter-base terminals without damaging the transistor. Its value is 6 V for the BC547 transistor.
• Maximum collector current- It is the maximum current that can be drawn through the collector without permanently damaging it. For the BC547 transistor, its value is 100 mA.