D.C. Bias and its Stabilization

We have seen that for operation of a bipolar transistor as an amplifier, the emitter-base junction is forward biased and the collector-base junction is reverse biased. For amplification of an input signal and its faithful transfer to the load, (Lc. currents and voltages corresponding to a proper point in the linear region of the output characteristics are applied to the transistor. This point is defined 'cly particular values of In and VCE, or Ig and Ic, or Ic and VeE, and it is called the operating point, or quiescent point or Q-point.
When the operating point is established in the central region where the output characteristics are most linear and uniformly spaced, variations in input signal (base current) will produce proportional changes in output (collector current). In a good circuit design it is necessary that once the operating point is established, its position should not appreciably vary. If it shifts to a position near the saturation line or to a position near the cut-off region of the output characteristics, the signal, after amplification, by the transistor, will be distorted. In this chapter we consider various biasing methods for common-emitter amplifier circuits.
4.2. Factors which cause shift of the Operating Point.
The factors which cause shift in the position of the operating point are as follows :
(1) Parameter variations from unit to unit. There is usually a considerable spread in characteristics among transistors of the same type. The value of a may differ by a few percent from unit to unit of the same type. But this introduces a much wider spread into the factor (1—a) and hence into R. For example, suppose the value of a for a transistor is 0.98. The value of f3 for this transistor is



0.98
1 —0.98 —49.



If cc is 0.99 for a substitute transistor, then 13 for this transistor is 99. Thus a change in a by about 1% introduces a large change in p. There is also production spread in the collector leakage currents Icso and /cEo, and in the base-emitter voltage. Hence replacing a transistor in a given circuit may cause a large shift in the position of the operating point.
(2) Temperature variations.

SCE E
Fig. 4-1.
Variation in temperature is a very important cause for shift in the position of the operating point. We have seen that the reverse leakage current IcB0 of the reverse-biased collector-base junction changes greatly with temperature, it approximately doubles for every 10°C rise in temperature. Therefore, an increase in temperature increases the value of Ics(). Since /c is related to /cso by the equation (See Sec. 3.7),
jc ----13/B+(13+1) !coo
an increase of /cao increases lc. Consequently all the output characterist'es are raised upward as shown by dotted curves in Fig. 4.1. Hence the operating point Q shifts upward towards the saturation line and the collector-current wave form will be distorted.
Another effect of rise of temperature of the transistor is called thermal runaway. An increase of temperature increases the leakage current IcgO. This increases the collector current Ic. The increase of Ic increases the power dissipated at the collector junction . and thereby increases the junction temperature.