BIPOLAR JUNCTION TRANSISTOR AS SWITCH BASIC AND TUTORIALS

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?

Fig. 9.1 A transistor switch


The input to the circuit in Fig. 9.1 is connected to the base terminal via the resistor R b whilst 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 BJT switch, and other BJT applications to follow, we just need to know the following.

1. 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, I b, by I~ =hf~Ib, where hfe is called the current gain and is typically 100. In this condition the transistor is in the on state, called saturation, and the voltage across the collector to emitter is approximately 0.2 V and is called V~.

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.

TYPES OF SWITCH BASICS AND TUTORIALS

WHAT ARE THE TYPES OF SWITCH?

(a) Push-button.

These are found on calculators, telephones, electronic games and most equipment with a digital display.

(b) Rotary.

These are switches controlled by a knob, and are turned instead of moved up and down. Figure 2 shows the rear of such a switch and its circuit symbol.

The commoner types of rotary switch are: 1-pole, 12-way; 2-pole, 6-way; 3-pole, 4-way; 4-pole, 3-way; 6-pole, 2-way. All these switches have 12 click positions, as you may have guessed, but each one comes with an adjustable end-stop so that you can set the correct number of ways according to the contacts on the switch.

(c) Slide.

This switch is common on the cheaper types of radio, mainly as an ON/OFF or band-changing switch. They are not very rugged, but are small and cheap to produce. Very small types are manufactured for use on PCBs.

(d) Toggle.

Available as two-way types or three-way, with a centre-off position. Can be manufactured to handle very high currents.

(e) Micro-switch.

These are devices which are usually operated indirectly, such as when the cover is removed from a high-voltage power supply, or when the door of a fridge is opened.

The ‘micro’ part of the name doesn’t refer to the size of the switch, but to the small movement that is required to activate it.