2-20 desired to prevent amplitude distortion (an output signal that fails to follow the input exactly) and self-bias may be used for this purpose. Combination BiasA 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. One of the most widely used combination-bias systems is the voltage-divider type shown in figure 2-14. Fixed bias is provided in this circuit by the voltage-divider network consisting of R1, R2, and the collector supply voltage (VCC). The dc current flowing through the voltage-divider network biases the base positive with respect to the emitter. Resistor R3, which is connected in series with the emitter, provides the emitter with self-bias. Should IE increase, the voltage drop across R3 would also increase, reducing VC. This reaction to an increase in IE by R3 is another form of degeneration, which results in less output from the amplifier. However, to provide long-term or dc thermal stability, and at the same time, allow minimal ac signal degeneration, the bypass capacitor (Cbp) is placed across R3. If Cbp is large enough, rapid signal variations will not change its charge materially and no degeneration of the signal will occur. Figure 2-14.—A basic transistor amplifier with combination bias. In summary, the fixed-bias resistors, R1 and R2, tend to keep the base bias constant while the emitter bias changes with emitter conduction. This action greatly improves thermal stability and at the same time maintains the correct operating point for the transistor. Q18. Which biasing method is the most unstable? Q19. What type of bias is used where only moderate changes in ambient temperature are expected? Q20. When is degeneration tolerable in an amplifier? Q21. What is the most widely used combination-bias system?
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