Lecture - 14: Biasing Techniques for CE Amplifiers

Biasing Circuit Techniques or Locating the Q - Point:

Fixed Bias or Base Bias:

In order for a transistor to amplify, it has to be properly biased. This means forward biasing the base emitter junction and reverse biasing collector base junction. For linear amplification, the transistor should operate in active region ( If IE increases, IC increases, VCE decreases proportionally).

The source VBB, through a current limit resistor RB forward biases the emitter diode and VCC through resistor RC (load resistance) reverse biases the collector junction as shown in fig. 1.

Fig. 1

The dc base current through RB is given by

IB = (VBB - VBE) / RB      

or          VBE = VBB - IB RB

Normally VBE is taken 0.7V or 0.3V. If exact voltage is required, then the input characteristic ( IB vs VBE) of the transistor should be used to solve the above equation. The load line for the input circuit is drawn on input characteristic. The two points of the load line can be obtained as given below

For   IB = 0,          VBE = VBB.

and      For    VBE = 0,         IB = VBB/ RB.

The intersection of this line with input characteristic gives the operating point Q as shown in fig. 2. If an ac signal is connected to the base of the transistor, then variation in VBE is about Q - point. This gives variation in IB and hence IC.

Fig. 2

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