Fig 4.14 (a) Biasing Circuit

Fig 4.14(b) CE o/p Characteristics

And Load Line

The supply voltage and resistances in the circuit must be suitably chosen in order to produce distortion -free output in amplifier circuits. These voltages and resistances establish a set of values to operate the transistor in the active region. These values are called quiescent values and they determine the operating point or Q - point of the transistor. The process of giving proper supply voltages and resistances for obtaining the desired Q - point is called biasing and the circuits used for the purpose are called biasing circuits.

D.C. load line

Referring to Fig. 4.14 (a), we see that values of V_CC and R_C are fixed and I_C and V_CE are dependent on R_B.

Applying Kirchoff's voltage law to the collector circuit we get,

The straight line represented by AB in Fig. 4.14 (b) is the d.c. load line. The co-ordinates at the end point 'A' are obtained by substituting V_CE = 0 in the above equation. Then I_c= V_cc. Therefore, the co-ordinates of A are V_CE =0 and I_c= V_cc/R_c

The co-ordinates of B are obtained by substituting I_c = 0 in the equation.

Then V_CE =V_cc. Therefore, the co-ordinates of B are V_CE =V_cc and I_c= 0.

As shown in the Fig. 4.14(b), the optimum Q-point is located at the midpoint of the d.c. load line AB between saturation and cut-off regions. In order to get faithful amplification the Q-point must be well within the active region.

However, in practice, the Q-point tends to shift its position due to the following reasons :-