Electrical Engineering ⇒ Topic : Biasing of a Transistor

BIASING OF A TRANSISTOR

When transistor is used as an amplifier, it is essential that the emitter-base junction should be forward biased and collector-base junction should be reverse biased. In other words, for the normal operation of a transistor amplifier circuit, the active region should be selected. The amount of bias required is also important to establish the Q-point that is dictated by the mode of operation desired. Improper biasing leads to the following drawbacks:

• The transistor will work inefficiently.
• The transistor will produce distortion in the output signal.

Once the Q-point is selected, it is desirable that the Q-point should remain stable. It should not change its position due to temperature rise. It is difficult to maintain stable Q-point in practice. Therefore, special efforts should be made for the purpose

Graphical analysis

Figure (1) shows a common emitter configuration.

                       Figure (1) Common emitter circuit.

In Figure (1) transistor is biased with V_cc and V_BB to obtain certain values of I_B, I_C, I_E and V_CE.

Applying KVL to the collector circuit, we have

                                                    .............. (1)

If we compare the Eq. (1) with y = mx + C, we can write

       m = (1/R_C), i.e., the slope and C = (V_CC/R_C) is the intercept on Y-axis. Thus the plot if l_C (taken on Y-axis) vs V_CE (taken on X-axis) gives a straight line having slope m = (-1/R_C) and intercept on Y-axis is (V_CC/R_C). This is the output characteristics of CE mode of the transistor.

From Eq. (1), we have

When V_CE = V_CC, I_C = 0 and

When V_CE = O , I_C = V_CC/R_C.

Figure (2) shows the locations of these two points

            Figure (2) Common emitter output characteristics with dc load line and Q-point.

From Figure (2), A [0, (V_CC/R_CC] and B (V_CC, 0). The line joining the two points A and B is the equation of the straight represented by Eq. (1). This is called DC load line.

Applying KVL to the base circuit, we get

                       V_BB - I_BR_B - V_BE = 0

                         V_BB = I_BR_B + V_{BE                       ............. (2)}

where V_BE is 0.7 V for Si transistor and 0.3 V for Ge transistor.

From Eq. (2), we get

                                 .......... (3)

The intersection of the characteristic curve for this value of I_B and the DC load line is the operating point. Since this point is fixed on the characteristics, so it is known as quiescent point or Q-point. Transistors are mainly used for the purpose of amplification of the electrical signal.The Q-point must be fixed in such a way that there should not be any kind of distortion of the amplified signal. The emitter-base junction must be forward biased and collector-base junction must be reverse biased for proper amplification. During designing of the biasing circuit, certain ratings of the transistor like I_C(max), V_CE(max) and P_D(max) must be considered, which are specified by the manufacturers. The variation in collector current and collector voltage, corresponding to a given variation of base current, must be observed by fixing the Q-point at different positions.Different cases have been shown in Figure (3), Figure (4) and Figure (5) respectively when Q-point (P) is very near to the saturation region, very near to cut-off region and at the centre of active region, respectively. When the point P is very near to the saturation region

Figure (3) Operating point near the saturation region gives clipping at the positive peaks.

Figure (4) Operating point near the cut-off region gives clipping at the negative peaks.

Figure (5) Operating point near the centre of active region is most suitable.

the collector current is clipped at the positive half cycle. When the point P is very near to the cutoff region, the collector current is clipped at the negative half cycle. When the point P is at the centre of active region, the collector current is distortionless. This Q-point is the best operating point.