introduction:
a bipolar junction transistor, (bjt) is very versatile. it can be used in many ways, as an amplifier, a switch or an oscillator and many other uses too. before an input signal is applied its operating conditions need to be set. this is achieved with a suitable bias circuit,
some of which i will describe. a bias circuit allows the operating conditions of a transistor to be defined, so that it will operate over a pre-determined range. this is normally achieved by applying a small fixed dc voltage to the input terminals of a transistor.
bias design can take a mathematical approach or can be simplified using transistor characteristic curves. the characteristic curves predict the performance of a bjt. there are three curves, an input characteristic curve, a transfer characteristic curve and an output characteristic curve.
of these curves, the most useful for amplifier design is the output characteristics curve. the output characteristic curves for a bjt are a graph displaying the output voltages and currents for different input currents. the linear (straight) part of the curve needs is utilized for an amplifier or oscillator.
for use as a switch, a transistor is biased at the extremities of the graph, these conditions are known as "cut-off" and "saturation".
there are three stander types of bjts dc biasing as follows:
1-the first type is called fixed bias dc analysis is depend on connecting base resistor rb between supply voltage and base terminal of bjt transistor. in this type of dc biasing the value of ib is constant.
2- the second type is called voltage divider-biasing. this type depend on connecting two transistors for base terminal which are divide the value of vcc according to the value of r1 and r2 to the base terminal.
3-the third type is called feedback dc biasing is depend on connecting the resistor between the collector terminal and base terminal.
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 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 basic relationships for a transistor:
in fact, once the analysis of the first few networks is clearly understood, the path toward the 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 through 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 dropingping or adding a term or two.
the primary of this chapter is to a level of familiarity with the bjt transistor that would permit a dc analysis of any system that might employ the bjt