Chapter 4: DC Biasing–BJTs
 

Chapter 4: DC Biasing–BJTs

The analysis or design of a transistor amplifier requires a knowledge of both the dc and the ac response of the system. Too often it is assumed that the transistor is a magical device that can raise the level of the applied ac input without the assistance of an external energy source. In actuality,
the improved output ac power level is the result of a transfer of energy from the applied dc supplies.
The analysis or design of any electronic amplifier therefore has two components: the dc portion and the ac portion. Fortunately, the superposition theorem is applicable and the investigation of the dc conditions can be totally separated from the ac response. However, one must keep in mind that during the design or synthesis stage the choice of parameters for the required dc levels will affect the ac response, and vice versa.
The dc level of operation of a transistor is controlled by a number of factors, including the range of possible operating points on the device characteristics. In Section 4.2 we specify the range for the bipolar junction transistor (BJT) amplifier. Once the desired dc current and voltage levels have been defined, a network must be constructed that will establish the desired operating point. A number of these networks are analyzed in this chapter. Each design will also determine the stability of the system, that is, how sensitive the system is to temperature variations, another topic to be investigated in a later section of this chapter.
Although a number of networks are analyzed in this chapter, there is an underlying similarity in the analysis of each configuration due to the recurring use of the following important basic relationships for a transistor:

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In fact, once the analysis of the first few networks is clearly understood, the path toward the solution of the networks to follow will begin to become quite apparent. In most instances the base current IB is the first quantity to be determined. Once IB is known, the relationships of Eqs. (4.1) through (4.3) can be applied to find the remaining quantities of interest. The similarities in analysis will be immediately obvious as we progress through the chapter. The equations for IB are so similar for a number of configurations that one equation can be derived from another simply by dropping or adding a term or two. The primary function of this chapter is to develop a level of familiarity with the BJT transistor that would permit a dc analysis of any system that might employ the BJT amplifier.