3.0 MAIN CONTENT 3.1 Operating Point
Fig 3.1: Operating Point Of A Transistor
Fig 3.1 shows a general output device characteristic with four operating points indicated. The biasing circuit can be designed to set the device operation at any of these points or others within the active region. The maximum ratings are indicated on the characteristics of fig 3.1 by the horizontal line for the maximum collector current and the vertical line at the maximum collector-to-emitter voltage . The maximum power constraint is defined by the curve in the same figure. At the lower end of the scales are the cutoff region, defined by , and the saturation region, defined by .
The BJT device could be biased to operate outside its maximum limits, but the result of such operation would be either a considerable shortening of the lifetime of the device or destruction of the device.
Confining ourselves to the active region, one can select many different operating areas or points. The chosen Q-point often depends on the intended use of the circuit. Still, we can consider some difference among the various points shown in fig 3.1 to present some basic ideas about the operating point and thereby, the bias circuit.
If no bias were used, the device would initially be completely off, resulting in a Q-point at A – namely, zero current through the device (and zero voltage across it). Since it is necessary to bias a device so that it can respond to the entire range of an input signal, point A would not be suitable. For point B, if a signal is applied to the circuit, the device
device to react to (and possibly amplify) both the positive and negative excursions of the input signal. If the input signal is properly chosen, the voltage and current of the device will vary but not enough to drive into cutoff or saturation. Point C will allow some positive and negative variation of the output signal, but the peak-to-peak would be limited by the proximity of . Operating at point C also raise
some concern about the non-linearities introduced by the fact that the space between IB curves is rapidly changing in this region. In general, it is preferable to operate where the gain of the device is fairly constant (or linear) to ensure that the amplification of the entire swing of input is the same. Point B is a region of more linear spacing and therefore more linear operation, as shown in fig 3.1. Point D sets the device operating near the maximum voltage and power level. The output voltage swing in the positive direction is thus limited if the maximum voltage is not exceeded. Point B therefore seems the best operating point in terms of linear gain and largest possible voltage and current swing. This is usually the desired condition for small-signal amplifier but not the case necessary for power amplifiers.
One other very important biasing factor must be considered. Having selected and biased the BJT at a desired operating point, the effect of temperature must also be taken into account. Temperature causes the device parameters such as transistor current gain (βac) and the transistor leakage current ( ) to change. Higher temperatures result in increased leakage current in the device, thereby changing the operating condition set by the biasing network. The result is that the network design must also provide a degree of temperature stability so that temperature changes result in minimum changes in the operating point. This maintenance of the operating point can be specified by a stability factor, S, which indicates the degree of change in the operating point due to temperature variation. A highly stable circuit is desirable, and stability of a few basic bias circuits will be compared.
• For BJT to be biased in its linear or active operating region the following must be true
• The base-emitter junction must be forward-biased (p-region voltage more positive), with a resulting forward-bias voltage of about 0.6 to 0.7 V.
The base-collector junction must be reversed-biased (n-region more positive), with the reverse-bias voltage being any value within the maximum limit of the device
A BJT has two junctions i.e. base-emitter and base-collector junctions either of which could be forward-biased or reverse-biased. With two junctions, there are four possible combinations of bias condition.
(i) both junctions reverse-biased (ii) both junctions forward-biased
(iii) BE junction forward-biased, BC junction reverse-biased.
(iv) BE junction reverse-biased, BC junction forward-biased.
Since condition (iv) is generally not used, we will consider the remaining three conditions below.
(a) Cut-off – Open Circuit Condition
This condition corresponds to reverse-bias for both base- emitter and base collector junctions. In fact, both diodes act like open circuits under these conditions as shown in Fig.
3.2, which is true for an ideal transistor. The reverse leakage current has been neglected. As seen, the three transistor terminals are uncoupled from each other. In cut-off,
(b) Saturation - Short Circuit Condition
This condition corresponds to forward-bias for both base- emitter and base-collector junctions. The transistor becomes saturated i.e. there is perfect short-circuit for both base-emitter and base-collector diodes. The ideal case is shown in Fig 3.3, where the three transistor terminals have been connected together thereby acquiring equal potentials. In this case, VCE = 0.
(c) Active Region
This condition corresponds to forward-bias for base-emitter junction and reverse bias for base-collector junction. In this, Vce >
0
Operation in the cutoff, saturation, and linear regions of the BJT characteristic are summarized as follows:
Linear-region operation:
Base-emitter junction forward biased Base-collector junction reversed biased Cutoff-region operation:
Base-emitter junction reverse biased Saturation-region operation:
Base-emitter junction forward biased Base-collector junction forward biased
4.0 CONCLUSION
In this unit, you have been introduced to the different operation points (Q-point) and the characteristics of these Q-points with the bias conditions.
5.0 SUMMARY
The operating point defines the characteristics of the region that will be employed for amplification of transistor amplifier’s applied signal. Since the operating point is a fixed point on the on the characteristics, it is also called the quiescent point (Q-point).
6.0 TUTOR-MARKED ASSIGNMENT
i. State the conditions for a Bipolar Junction Transistor operating in each of the following regions
a. Cutoff Region b. Active Region c. Saturation Region
7.0 REFERENCES/FURTHER READING
Robert, L. B. (1999). Electronic Devices and Circuit Theory. 7th Edition Prentice-Hall Inc. New Jersey.
Theraja, B. L. and Theraja, A. K. (2010). Electrical Technology. S. C.
Chand, New Delhi, India.