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| SEC. 6-1 THE UNBIASED
TRANSISTOR A transistor has three doped regions: an emitter, a base, and a collector. A pn junction exists between the base and the emitter; this part of the transistor is called the emitter diode. Another pn junction exists between the base and the collector; this part of the transistor is called the collector diode. SEC. 6-2 THE BIASED TRANSISTOR For normal operation, you forward bias the emitter diode and reverse bias the collector diode. Under these conditions, the emitter sends free electrons into the base. Most of these free electrons pass through the base to the collector. Because of this, the collector current approximately equals the emitter current. The base current is much smaller, typically less than 5 percent of the emitter current. SEC. 6-3 TRANSISTOR CURRENTS The ratio of the collector current to the base current is called the current gain, symbolized as .dc or hFE. For low-power transistors, this is typically 100 to 300. The emitter current is the largest of the three currents, the collector current is almost as large, and the base current is much smaller. SEC. 6-4 THE CE CONNECTION The emitter is grounded or common in a CE circuit. The base-emitter part of a transistor acts approximately like an ordinary diode. The base-collector part acts like a current source that is equal to .dc times the base current. The transistor has an active region, a saturation region, a cutoff region, and a breakdown region. The active region is used in linear amplifiers. Saturation and cutoff are used in digital circuits. SEC. 6-5 THE BASE CURVE The graph of base current versus baseemitter voltage looks like the graph of an ordinary diode. Because of this, we can use any of the three diode approximations to calculate the base current. Most of the time, the ideal and the second approximation are all that is necessary. SEC. 6-6 COLLECTOR CURVES The four distinct operating regions of a transistor are the active region, the saturation region, the cutoff region, and the breakdown region. When it is used as an amplifier, the transistor operates in the active region. When it is used in digital circuits, the transistor usually operates in the saturation and cutoff regions. The breakdown region is avoided because the risk of transistor destruction is too high. SEC. 6-7 TRANSISTOR APPROXIMATIONS Exact answers are a waste of time in most electronics work. Almost everybody uses approximations because the answers are adequate for most applications. The ideal transistor is useful for basic troubleshooting. The third approximation is needed for precise design. The second approximation is a good compromise for both troubleshooting and design. SEC. 6-8 READING DATA SHEETS Transistors have maximum ratings on their voltages, currents, and powers. Small-signal transistors can dissipate 1 W or less. Power transistors can dissipate more than 1 W. Temperature can change the value of the transistor characteristics. Maximum power decreases with a temperature increase. Also, current gain varies greatly with temperature. SEC. 6-9 SURFACE-MOUNT TRANSISTORS Surface-mount transistors (SMTs) are found in a variety of packages. A simple three-terminal gull-wing package is common. Some SMTs are packaged in styles that can dissipate more than 1 W of power. Other surface-mount devices may contain (house) multiple transistors. SEC. 6-10 TROUBLESHOOTING When troubles arise, they usually produce large changes in transistor voltages. This is why ideal analysis is usually adequate for troubleshooters. Furthermore, many troubleshooters spurn the use of calculators because it slows down their thinking. The best troubleshooters learn to mentally estimate the voltages they want to measure. |