Transistors

The transistor's finction is to amplify an electric current.
Many different kinds of transistors are used in analog circuits, for different reasons. This is not the case for digital circuits. In a digital circuit, only two values matter; on or off. The amplification abilitiy of a transistor is not relevant in a digital circuit. In many cases, a circuit is built with integrated circuits(ICs).
Transistors are often used in digital circuits as buffers to protect ICs. For example, when powering an electromagnetic switch (called a 'relay'), or when controlling a light emitting diode. (In my case.)

Two different symbols are used for the transistor.

PNP type and NPN type

The name (standard part number) of the transistor, as well as the type and the way it is used is shown below. 2SAXXXX PNP type high frequency 2SBXXXX PNP type low frequency 2SCXXXX NPN type high frequency 2SDXXXX NPN type low frequency The direction of the current flow differs between the PNP and NPN type. When the power supply is the side of the positive (plus), the NPN type is easy to use.