TRANSISTOR

Transistors are frequently used as amplifiers. Some transistor circuits are current amplifiers with a small load resistance; other circuits are designed for voltage amplification and have a high load resistance; others amplify power.

Transistors, like rectifiers, are made in two polarity designs. Figure 4-5 shows the polarity inherent in an NPN transistor. In the NPN transistor, the current flows into the transistor through the emitter and emerges through the collector. There should be no confusion in finding the correct type of transistor to use when making a replacement, as each transistor must be replaced

Figure 4-5.\NPN transistor.

with the exact type number (for instance, 2N1475), which automatically guarantees the correct polarity.

In a transistor of the opposite polarity, called a PNP transistor, the current enters the collector and emerges from the emitter.

With a direct current of the correct polarity for the NPN transistor, negative on the emitter and positive connected to the collector, current flows through the transistor if a current signal (known as base current) is applied between the base and the emitter. This transistor conducts current from the emitter to the collector. The stronger the base current, the greater the flow of current through the emitter-collector circuit. This feature of its operation gives the transistor its amplification characteristics.

Relatively small base currents (microamperes) and voltage (1 volt is typical) cause relatively large currents (several milliamperes) to flow in the emitter circuit.

UNIJUNCTION TRANSISTOR

Another type of transistor is the unijunction. The unijunction transistor (UJT) operates in an entirely different manner from other transistors. It is used in voltage regulators to generate the current pulses that fire the SCRs.

The UJT (fig. 4-6) is a three-terminal semiconductor device that has electrical characteristics quite different from the conventional two-junction transistor. Physically, this transistor consists of two ohmic contacts, called base 1 and base 2, on opposite ends of a bar of silicon. A single rectifying junction is located on base 2.

In normal operation, a positive voltage is applied to base 2, and base 1 is grounded with no emitter current flowing. The device acts like a simple high-resistance voltage divider. When sufficient positive voltage is applied to the emitter, with respect to base 2, emitter current will flow. The net result is a decrease in the resistance between the emitter and base 1, so that as the emitter current increases, the emitter voltage decreases, and a negative resistance characteristic is obtained. The output can be used as a temperature-stable firing circuit for an SCR control circuit.

INTEGRATED CIRCUIT

An integrated circuit (IC) is a device that integrates (combines) both active components (such as transistors and diodes) and passive components (such as resistors and capacitors) of a complete electronic circuit in a single chip (a tiny slice or wafer of semiconductor crystal or insulator).

ICs have almost eliminated the use of individual components as the building blocks of electronic prints. Instead, tiny chips have been developed, the functions of which are not that of a single component, but of dozens of transistors,

Figure 4-6.\Unijunction transistor.

Figure 4-7.\Integrated circuit packages.

resistors, capacitors, and other electronic elements, all interconnected to perform the task of a complex circuit.

These chips are packaged to protect them and help dissipate the heat generated in the device. One of these packages may contain one or several stages and often has several hundred components. Some of the most common package styles are shown in figure 4-7.

ICs are composed of parts so closely associated with one another that repair becomes almost impossible. In case of failure, the entire IC package is replaced as a single component.