transistor biasing and stabilization of operating point pdf

This results in the variation of collector current I C for a given I B. If the transistor is replaced by another one of the same type, one cannot ensure that the new transistor will have identical parameters as that of the first one. If base current I B is kept constant since I B is approximately equal to Vcc/R B. Transistor current gain, h FE or β which increases with temperature.Base emitter Voltage ,VBE, which decreases by 2.5 mV per oC.

Reverse saturation current, Ico, which doubles for every 10 oC raise in temperature.In practice, the Q-point tends to shift its position due to any or all of the following three main factors. If the Q point shifts nearer to either A or B, the output voltage and current get clipped, thereby distorting the o/p signal. In order to get faithful amplification, the Q point must be well within the active region of the transistor.Įven though the Q point is fixed properly, it is very important to ensure that the operating point remains stable where it is originally fixed. Thus the dc load line AB can be drawn if the values of R C and V CC are known.Īs shown in the fig4.2b, the optimum POINT IS LOCATED AT THE MID POINT OF THE MIDWAY BETWEEN A AND B. Therefore the coordinates of B are V CE =Vcc and I C=0. The coordinates of B are obtained by substituting I C=0 in the above equation. I CC = V CC/R C Therefore The coordinates of A are V CE =0 and I CC = V CC/R C. The coordinates of the end point A are obtained by substituting V CE =0 in the above equation. The straight line represented by AB in fig4.2b is called the dc load line.

Referring to the biasing circuit of fig 4.2a, the values of V CC and R C are fixed and I C and V CE are dependent on R B.Īpplying Kirchhoff’s voltage law to the collector circuit in fig. Hence operating point for a transistor amplifier is selected to be in the middle of active region. It also provides linear gain and larger possible o/p voltages and currents. It will allow both positive and negative half cycles in the o/p signal. Point B is located in the middle of active region.

Therefore the o/p voltage swing in the positive direction is limited. Point C is too close to P D(max) curve of the transistor. It does not satisfy the above stated conditions necessary for faithful amplification. If operating point is selected at A, A represents a condition when no bias is applied to the transistor i.e, I C=0, V CE =0. rating of the transistor I c(max), V CE (max) and P D(max) should not be exceeded at any value of i/p signal.Ĭonsider fig1. The value of the signal I c when no signal is applied should be at least equal to the max.For V CE less than V CE (sat) the collector base junction is not probably reverse biased.

V ce voltage should not fall below V CE (sat) (0.3V for Si, 0.1V for Ge) for any part of the i/p signal.

Emitter base junction must be forward biased (V BE=0.7Vfor Si, 0.2V for Ge) and collector base junction must be reverse biased for all levels of i/p signal.

There are four conditions to be met by a transistor so that it acts as a faithful amplifier: This process of selecting proper supply voltages and resistance for obtaining desired operating point or Q point is called as biasing and the circuit used for transistor biasing is called as biasing circuit. To establish an operating point (proper values of collector current I C and collector to emitter voltage V CE) appropriate supply voltages and resistances must be suitably chosen in the circuit. That means an operating point has to be established in this region. If the o/p signal must be a faithful reproduction of the i/p signal, then transistor must be operated in active region.

Transistor biasing and stabilization of operating point pdf