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| SEC. 7-1 VARIATIONS IN
CURRENT GAIN The current gain of a transistor is an unpredictable quantity. Because of manufacturing tolerances, the current gain of a transistor may vary over as much as a 3.1 range when you change from one transistor to another of the same type. Changes in the temperature and the collector current produce additional variations in the dc gain. SEC. 7-2 THE LOAD LINE The dc load line contains all the possible dc operating points of a transistor circuit. The upper end of the load line is called saturation, and the lower end is called cutoff. The key step in finding the saturation curent is to visualize a short between the collector and the emitter. The key step to finding the cutoff voltage is to visualize an open between the collector and emitter. SEC. 7-3 THE OPERATING POINT The operating point of the transistor is on the dc load line. The exact location of this point is determined by the collector current and the collector-emitter voltage. With base bias, the Q point moves whenever any of the circuit values change. SEC. 7-4 RECOGNIZING SATURATION The idea is to assume that the npn transistor is operating in the active region. If this leads to a contradiction (such as negative collector-emitter voltage or collector current greater than saturation current), then you know that the transistor is operating in the saturation region. Another way to recognize saturation is by comparing the base resistance to the collector resistance. If the ratio is in the vicinity of 10 .1, the transistor is probably saturated. SEC. 7-5 THE TRANSISTOR SWITCH Base bias tends to use the transistor as a switch. The switching action is between cutoff and saturation. This type of operation is useful in digital circits. Another name for switching circuits is two-state circuits. SEC. 7-6 EMITTER BIAS Emitter bias is virtually immune to changes in current gain. The process for analyzing emitter bias is to find the emitter voltage, emitter current, collector voltage, and collector-emitter voltage. All you need for this process is Ohm’s law. SEC. 7-7 LED DRIVERS A base-biased LED driver uses a saturated or cutoff transistor to control the current through an LED. An emitter-biased LED driver uses the active region and cutoff to control the current through the LED. SEC. 7-8 THE EFFECT OF SMALL CHANGES Useful to both troubleshooters and designers is the ability to predict the direction of change for a dependent voltage or current when one of the circuit values changes. When you can do this, you can better understand what happens for different troubles and can more easily analyze circuits. SEC. 7-9 TROUBLESHOOTING You can use a DMM or ohmmeter to test a transistor. This is best done with the transistor disconnected from the circuit. When the transistor is in the circuit with the power on, you can measure its voltages, which are clues to possible troubles. SEC. 7-10 MORE OPTOELECTRONIC DEVICES Because of its .dc, the phototransistor is more sensitive to light than a photodiode. Combined with an LED, the phototransistor gives us a more sensitive optocoupler. The disadvantage with the phototransistor is that it responds more slowly to changes in light intensity than a photodiode. |