Biasing Circuit Techniques/ Locating the Q - Point:

Fixed Bias or Base Bias:

For a transistor to amplify, the transistor has to be properly biased. This means that the forward biasing base emitter junction and reverse biasing collector base junction. For the linear amplification, transistor should operate in the active region ( If I_E increases, I_C increases, V_CE decreases).

The source V_BB, through the current limit resistor R_B  forward biases emitter diode and V_CC through resistor R_C reverse biases collector junction as shown in the figure 1.

Fig. 1

The DC base current through RB can be given by

I_B = (V_BB - V_BE) / R_B

or          V_BE = V_BB - I_B R_B

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

For   I_B = 0,          V_BE = V_BB.

and      For    V_BE = 0,         I_B = V_BB/ R_B.

The intersection of this line with the input characteristic gives operating point Q as shown in figure 2. If the AC signal is connected to base of transistor, then variation in the V_BE is about Q - point. This provides variation in the I_B and hence I_C.

Fig. 2

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