General output device characteristic with four operating points indicated. Biasing circuit can be developed to set device operation at any of these points or others in active region. Maximum ratings are indicated on characteristics of figure by horizontal line for maximum collector current ICmax and vertical line at maximum collector-to-emitter voltage VCEmax. Maximum power constraint is stated by PCmax curve in same figure. At lower end of scales are cutoff region, stated by IB < 0μA, and saturation region, atated by VCE ≤ VCEsat
BJT device could be biased to operate outside maximum limits, but result of such operation would be either considerable reduction of lifetime of device or destruction of device. Confining ourselves to active region, one can choose several different operating areas or points. Chosen Q-point often depends on intended use of circuit.
If no bias were used, device would originally be entirely off, resulting in Q-point at A - namely, zero current through device (and zero voltage across it). As it is essential to bias device so that it can respond to complete range of input signal, point A would not be appropriate. For point B, if the signal is applied to circuit, device will differ in current and voltage from operating point, letting device to react to (and possibly amplify) both positive and negative excursions of input signal. If input signal is correctly chosen, voltage and current of device will differ but not enough to drive into cutoff or saturation. Point C will let some positive and negative variation of output signal, but peak-to-peak would be restricted by proximity of VCE = 0 V/IC 0mA. Operating at point C concern about non-linearities introduced by fact that space between IB curves is rapidly changing in this region. It is preferable to operate where gain of device is quite constant (or linear) to make sure that amplification of complete swing of input is same. Point B is a region of more linear spacing and thus more linear operation. Point D sets device operating near maximum voltage and power level. Output voltage swing in positive direction is therefore limited if maximum voltage is not exceeded. Point B thus appears best operating point in terms of linear gain and largest possible voltage and current swing. This is generally desired condition for small-signal amplifier but not case essential for power amplifiers.
One other very significant biasing factor should be considered. After selected and biased BJT at preferred operating point, effect of temperature should also be taken into account. Temperature causes device parameters like transistor current gain (βac) and transistor leakage current (ICEO) to change. Higher temperatures result in increased leakage current in device, thereby changing operating condition set by biasing network. Result is that network design should also give degree of temperature stability so that temperature changes result in minimum changes in operating point. This maintenance of operating point can be specified by the stability factor, S, that indicates degree of change in operating point due to temperature variation. Highly stable circuit is desirable, and stability of the few basic bias circuits will be compared.
For BJT to be biased in linear or active operating region the following should be true:
The base-emitter junction should be forward-biased (p-region voltage more positive), with the resulting forward-bias voltage of about 0.6 to 0.7 V.
Base-collector junction should be reversed-biased (n-region more positive), with reverse-bias voltage being any value within maximum limit of device
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.
(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 conditions, which is true for ideal transistor. Reverse leakage current has been neglected. As seen, three transistor terminals are uncoupled from each other. In cut-off, VCE = VCC
(b) Saturation - Short Circuit Condition: This condition corresponds to forward-bias for both base- emitter and base-collector junctions. Transistor becomes saturated that is there is perfect short-circuit for both base-emitter and base-collector diodes. Ideal case is, where three transistor terminals have been joined together thereby obtaining 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.
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