![]() ![]() Hence the transistor is said to be in off condition but at the same time, power loss across the transistor switch that is Ic into the VCE is negligible because of the very tiny collector current Ic. So, when the transistor is operated with a base current less or equal to zero the current through the collector to the emitter Ic is very tiny. ![]() When the base current is zero the collector current Ic will have a very small constant value for a wide range of collector-emitter voltage VCE. It is only possible when the transistor is only operated in the marginal region of the characteristics there are two marginal regions in the characteristics of a transistor and these are the cut-off region and saturation region. ![]() When we want a transistor to be operated as a switch it must be operated in such a way that the power loss during on and off conditions should be nearly zero or very low. Similarly, when the switch is on, the current through the switch is as high as the short circuit current, but the voltage across the switch is zero. But the current is zero hence there is no power loss. ![]() There will be significant power loss if the transistor is operated in this region but in a practical switch when it is off the voltage across it will be equal to open-circuit voltage. In the linear region for a wide range of collector-emitter voltage Vce, the collector current Ic remains constant as the voltage has a wide range and the collector current is almost constant. In a transistor characteristic, there are three regions they are cut-off region, linear region, and saturation region. A transistor gives quite a large resistance to the circuit but it is not ideally infinite, also gives very small resistance but it is also not ideally zero. The emitter-collector resistance can be made either nearly infinite or nearly zero. It is controlled by the current in the base-emitter junction by controlling the base-emitter current. A transistor on the other hand can be considered as a controlled resistance between emitter and collector. ![]()
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