Operating Point 

The zero signal values of I_C and V_CE are known as theoperating point.

It is call operating point because the variations of I_C and V_CE take place about this point when signal is apply. It is also call quiescent (silent) point or Q-point because it is the point on I_C− V_CE characteristic when the transistor is silent i.e.in the absence of the signal.

Suppose in the absence of signal, the base current is 5μA. Then I_C and V_CE conditions in the circuit must be represend by some point on I_B = 5 μA characteristic. But I_C and V_CE conditions in the circuit should also be represent by some point on the d.c. load line AB. The point Q where the load line and the characteristic intersect is the only point which satisfies both these conditions. Therefore, the point Q de-scribes the actual state of affairs in the circuit in the zero signal conditions and is call the operating point. Referring to Fig.1, for I_B = 5 μA,

The zero signal values are :

V_CE=OC volts

I_C=OD mA

It follows, therefore, that the zero signal values of I_C and V_CE(i.e. operating point) are deter-mine by the point where d.c. load line intersects the proper base current curve.

Example 1.For the circuit shown in Fig. 2 (i), draw the d.c. load line.

Solution. The collector-emitter voltage V_CE is give by ;

This locates the point B of the load line on the collector-emitter voltage axis.

When V_CE = 0, then,
IC
= V_CC/R_C
= 12.5 V/2.5 kΩ = 5 mA

This locates the point A of the load line on the collector current axis. By joining these two points,
we get the d.c. load line AB as shown in Fig. 2 (ii).
Example 2. In the circuit diagram shown in Fig. 3 (i), if V_CC = 12V and R_C
= 6 kΩ, draw
the d.c. load line. What will be the Q point if zero signal base current is 20µA and β = 50 ?
Solution.

The collector-emitter voltage V_CE is given by :

V_CE = V_CC – I_C R_C
When I_C
= 0, V_CE = V_CC = 12 V. This locates the point B of the load line. When V_CE = 0,
IC
= V_CC /R_C
= 12 V/6 kΩ = 2 mA. This locates the point A of the load line. By joining these two
points, load line AB is constructed as shown in Fig. 3(ii).
Zero signal base current, I_B
= 20 µA = 0.02 mA
Current amplification factor, β = 50
∴ Zero signal collector current, I_C
= β I_B
= 50 × 0.02 = 1 mA

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