1. The BJT as a switch
The bipolar junction transistor or BJT as it is more commonly known can be considered in digital terms as a simple single-pole switch. It physically consists of three layers of semiconductor (which can be either N-type or P-type) of which two transistor types exist - NPN or PNP. We shall consider the operation of the NPN device since this device is used mainly in bipolar digital switching circuits.
The symbol for the NPN transistor is shown in Fig. 9.1 and is connected as a simple switch. The transistor consists of three terminals: base (b); emitter (e); and collector (c). Notice that the arrow on this type of transistor is pointing out from the emitter which indicates the direction of current flow. For the PNP the arrow points in. A simple rule for remembering the direction of the arrow is that with an NPN transistor the arrow is Not Pointing iN!
The input to the circuit in Fig. 9.1 is connected to the base terminal via the resistor Rh whiist the output is taken from the collector. Several text books are available that discuss the operation of a bipolar transistor in detail. However, for this simple 8JT switch, and other BJT applications to follow, we just need to know the following.
To turn the transistor on a voltage at the base with respect to the emitter of greater than 0.7 V is needed. Under this condition a large collector current, I flows through the transistor. The amount of current that flows is related to the base current, Ib, by IC=HFEIB where is called the current gain and is typically 100. In this condition the transistor is in the on state, called saturation, and ^ voltage across the collector to emitter is approximately 0.2 V and is called Vcesat.
2. To turn the transistor off the voltage at the base with respect to the emitter has to be less than 0.7 V. The collector current that flows is now zero (or more accurately a very small current called the leakage current). The transistor in this off state is called cut-off and the voltage across it is the supply voltage, Vcc which is usually 5 V.