Another workhorse of electronic equipment is the transistor. There are two types of transistors commonly used: PNP and NPN. As shown in Fig. 1, the PNP transistor consists of a thin layer of N-type material between two layers of P-type material. The NPN transistor has the opposite: a thin layer of P-type material between two layers of N-type material. (See April 1995 Back To Basics for information on P-type and N-type materials.) Also shown in Fig. 1 are the symbols used for these two types.
Transistors have three terminals: an emitter (E), a base (B), and a collector (C). These terminals have identical locations for both types of transistors. Note that the PNP transistor symbol has an arrow called the transmitter arrow pointing toward the base while an NPN transistor has the transmitter arrow pointing away from the base. In both types, however, the transmitter arrow points away from the P-type material and towards to the N-type material.
Transistors are available with two or three leads extending from their case. If you need a specific shaped transistor, you specify a transistor outline (TO) number. Fig. 2. shows typical TO number configurations. Note that the TO-3 configuration has only two leads or terminals. Many times, the transistor's metal case is used as the C lead.
You can identify specific leads by their spacing. For example, as shown in Fig. 3, the E and B leads are usually close together, while the C lead is farther away. Also, the B lead is always in the middle.
Sometimes, transistors come with index pins, as shown in Fig. 4. To identify terminals on such transistors, start with the lead closest to the index pin and work clockwise. The first lead is always the E lead; the B lead (always closest to the E lead) is next, and the C lead is last.
Transistor junction biasing
The base-emitter junction in a transistor circuit must always be foward biased while the base-collector junction must always be reverse biased. (See April 1995 Back To Basics for discussion of forward and reverse biasing.) Fig. 5 shows the base-emitter junction of an NPN transistor. Note that the bias voltage is connected so that the positive terminal connects to the base (P-type material) and the negative terminal connects to the emitter (N-type material). Thus, we have a forward biased base-emitter junction, with current flowing in the external circuit as shown.
As shown in Fig. 6, the base-collector junction has the bias voltage connected so that the negative terminal connects to the base (P-type material) and the positive terminal connects to the collector (N-type material). These connections reverse-bias the base-collector junction. This results in a very small amount of current (basically leakage current) to flow in the external circuit. This is identical to that of a reverse-biased semiconductor diode.
Voltage in a transistor
There are certain voltages in the base circuit of a transistor. Looking at Fig. 7, we see a bias voltage ([V.sub.BB]) applied to a transistor's base-emitter circuit, a voltage drop ([V.sub.BE]) across the base-emitter junction, and another voltage drop ([V.sub.R1]) across a series-limiting resistor ([R.sub.1]). Most of [V.sub.BB] will be across [R.sub.1] because the resistance of the base-emitter junction is less than that of [R.sub.1].
There are other voltages associated with transistors, namely those in the collector circuit. As shown in Fig. 8, we have a bias voltage ([V.sub.CC]) applied across the emitter-collector circuit. This voltage will be divided between the emitter-base and collector-base junctions, based on their individual resistances, because these junctions are in series with each another. The voltage across the collector and emitter is [V.sub.CE] and the voltage across the collector and base is [V.sup.CB]. [V.sup.CB] is greater than [V.sub.CE] because the collector-base junction has a much greater resistance than the base-emitter junction. However, base voltage [V.sub.CB] is greater than base-emitter voltage [V.sub.BE].