Lecture - 15: Stability of Operating Point

Let us consider three operating points of transistor operating in common emitter amplifier.

  1. Near cut off
  2. Near saturation
  3. In the middle of active region

If the operating point is selected near the cutoff region, the output is clipped in negative half cycle as shown in fig. 1.

Fig. 1

If the operating point is selected near saturation region, then the output is clipped in positive cycle as shown in fig. 2.

Fig. 2
Fig. 3

If the operating point is selected in the middle of active region, then there is no clipping and the output follows input faithfully as shown in fig. 3. If input is large then clipping at both sides will take place. The first circuit for biasing the transistor is CE configuration is fixed bias.

In biasing circuit shown in fig. 4(a), two different power supplies are required. To avoid the use of two supplies the base resistance RB is connected to VCC as shown in fig. 4(b).

Fig. 4(a) Fig. 4(b)

Now VCC is still forward biasing emitter diode. In this circuit Q point is very unstable. The base resistance RB is selected by noting the required base current IB for operating point Q.

IB = (VCC VBE ) / RB

Voltage across base emitter junction is approximately 0.7 V. Since VCC is usually very high

i.e. IB = VCC/ RB

Since IB is constant therefore it is called fixed bias circuit.

GOTO >> 1 || 2 || Home