We will discuss here the topic, transistor is a switching device
Before going through actual topic, let us know, what does a switch do
It offers open circuit when it is, off, and offers short circuit when it is, on
In other words, it can be said that, a switch provides infinite resistance during its off condition, and provides zero resistance during its on condition
Hence, a switch can be imagined as an, an off controlled resistor, which provides either zero, or infinite resistance to the circuit without any intermediate value
A transistor, on the other hand, can be considered as a controlled resistor, as the resistance between, emitter, and, collector, is controlled by current in the base emitter junction
By controlling base emitter current, the emitter collector resistance can be made either nearly infinite, or nearly zero
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
In a transistor characteristic, there are three regions. They are, cut off region, linear region, and saturation region
In linear region, for wide range of collector emitter voltage, V C E, collector current, I C, remains constant
As the voltage has a wide range and collector current is almost constant, 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, but the current is zero, hence, there is no power loss
Similarly, when the switch is on, the current through the switch is as high as short circuit current, but the voltage across the switch is zero, hence, there is no power loss again
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, condition should be nearly zero, or very low
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, cut off region and saturation region
In a figure, when base current is zero, the collector current, I C, will have very small constant value for a wide range of collector emitter voltage, V C E
So, when a transistor is operated with base current less or equal to zero, the current through the collector to emitter, I C, is very tiny, hence the transistor is said to be in off condition, but at the same time, power loss across the transistor switch that is I C into V C E is negligible, because of very tiny collector current I C
Hence, the transistor is said to be operated as, off switch. Now, suppose the transistor is connected in series with a load of resistance, R L. In normal condition, the voltage across the load is V L. Hence current through the load is, I L equals V L by R L
If the transistor is operated with a base current I B 1, for which collector current is I C 1. If the load current V L by R L that is I L is less than I C 1, then the transistor is operated in saturation region
Here, for any current less than I C 1 through the transistor's collector to emitter, there will be very tiny collector emitter voltage V C E. Hence, at this situation the current through the transistor is as high as load current, but the voltage across the transistor V C E quite low, hence power loss in the transistor again is negligible
The transistor behaves as approximate short circuited switch, or on switch. So, for using the transistor as a switch we should make sure, that the applied base emitter current must be sufficiently high to keep the transistor in saturation region, for given load current
So, from the previous explanation, we can conclude that the transistor behaves as a switch only when it is operated in, cut, off, and saturation region of its characteristic. In switching application, the linear region of characteristics is avoided. As we already told, the power loss in transistor switch is very low but not zero. So, it is not an ideal switch but accepted for specific applications
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