The zero signal values of IC and VCE are known as theoperating point.
It is call operating point because the variations of IC and VCE 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 IC− VCE 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 IC and VCE conditions in the circuit must be represend by some point on IB = 5 μA characteristic. But IC and VCE 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 IB = 5 μA,
The zero signal values are :
It follows, therefore, that the zero signal values of IC and VCE(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 VCE is give by ;
This locates the point B of the load line on the collector-emitter voltage axis.
When VCE = 0, then,
= 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 VCC = 12V and RC
= 6 kΩ, draw
the d.c. load line. What will be the Q point if zero signal base current is 20µA and β = 50 ?
The collector-emitter voltage VCE is given by :
VCE = VCC – IC RC
= 0, VCE = VCC = 12 V. This locates the point B of the load line. When VCE = 0,
= VCC /RC
= 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, IB
= 20 µA = 0.02 mA
Current amplification factor, β = 50
∴ Zero signal collector current, IC
= β IB
= 50 × 0.02 = 1 mA
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