![]() If more than one defective transistor is present in the equipment where the trouble has been localized, this testing method becomes cumbersome, as several transistors may have to be replaced before the trouble is corrected. This method of testing is highly accurate and sometimes the quickest, but it should be used only after you make certain that there are no circuit defects that might damage the replacement transistor. In many cases, you can substitute a transistor known to be good for one that is questionable and thus determine the condition of a suspected transistor. To determine if a transistor is good or bad, you can check it with an ohmmeter or a transistor tester. If C bp is large enough, rapid signal variations will not change its charge materially and no degeneration of the signal will occur. However, to provide long-term or dc thermal stability, and at the same time, allow minimal ac signal degeneration, the bypass capacitor (C bp) is placed across R3. This reaction to an increase in I E by R3 is another form of degeneration, which results in less output from the amplifier. Should I E increase, the voltage drop across R3 would also increase, reducing V C. Resistor R3, which is connected in series with the emitter, provides the emitter with self-bias. The dc current flowing through the voltage-divider network biases the base positive with respect to the emitter. Fixed bias is provided in this circuit by the voltage-divider network consisting of R1, R2, and the collector supply voltage (V CC). One of the most widely used combination-bias systems is the voltage-divider type shown in figure 2-14. However, this is enough voltage on the base, along with ground on the emitter and the large positive voltage on the collector, to properly bias the transistor.Ī combination of fixed and self-bias can be used to improve stability and at the same time overcome some of the disadvantages of the other two biasing methods. Since the current in the base circuit is very small (a few hundred microamperes) and the forward resistance of the transistor is low, only a few tenths of a volt of positive bias will be felt on the base of the transistor. ![]() ![]() Current flows in the emitter-base bias circuit from ground to the emitter, out the base lead, and through R B to V CC. Resistor R B provides the necessary forward bias for the emitter-base junction. Notice in figure 2-12 that the emitter-base battery has been eliminated and the bias resistor R B has been inserted between the collector and the base. In addition to eliminating the battery, some of these biasing methods compensate for slight variations in transistor characteristics and changes in transistor conduction resulting from temperature irregularities. By inserting one or more resistors in a circuit, different methods of biasing may be achieved and the emitter-base battery eliminated. ![]()
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