DC Load Line and Operating point selection

Consisder a CE amplifier along with the output characteristics as shown in figure 3.18 above. A straight line drawn on the output characteristic of a transistor which gives the various zero signal values (ie. When no signal applied) of V_CE and I_C is called DC load line.

Construction of DC load line

Applying KVL to the collector circuit we get,

V_CC –I_CR_C –V_CE =0-------------------1

V_CE  = V_CC –I_CR_C ----------------------2

The above equation is the first degree equation and can be represented by a straight line. This straight line is DC load line.

To draw the load line we require two end points which can be found as follows.

1.      If I_C =0, equn 2 becomes V_CE  = V_CC

2.      if V_CE = 0, equn 2 becomes V_CC = I_CR_C   ie. I_C = V_CC /R_C

3.14 Operating point (Q)

A point on the d.c. load line which represent the zero signal values of V_CE and I_C in a transistor is called as operating point or silent point or quiescent point or Q-point.

The Q-point is selected where the DC load line intersects the curve of output characteristics for particular value of zero signal current.

  

          i.e. Q-point = (V_CE ,I_C)