Transistor as a Switch or Bipolar Junction Transistor or BJT as SwitchPublished on 06/10/2016 & updated on 22/11/2018
In a transistor characteristic, there are three regions. They are
- Cutoff Region
- Active Region
- Saturation Region
It is only possible when the transistor is only operated in the marginal region of the characteristics. The cutoff region and saturation region are two marginal regions in the transistor characteristics.
In the figure, when base current is zero, the collector current (IC) has very small constant value for a wide range of collector-emitter voltage (VCE). So when the transistor is operated with base current ≤ 0, the collector current (IC ≈ 0) is very tiny, hence the transistor is said to be in OFF condition but at the same time, power loss across the transistor switch i.e. IC × VCE is negligible because of very tiny IC.
The transistor is connected in series with an output resistance RC. Hence, current through the output resistance is If the transistor is operated with a base current IB3 for which collector current is IC1. IC is less than IC1, then the transistor is operated in the saturation region. Here, for any collector current less than IC1, there will be very tiny collector-emitter voltage (VCE < VCE1). Hence in this situation, the current through the transistor is as high as load current, but the voltage across the transistor (VCE < VCE1) quite low, hence power loss in the transistor is negligible. The transistor behaves as an ON switch. So for using the transistor as a switch we should make sure that the applied base current must be sufficiently high to keep the transistor in the saturation region, for a collector current.So, from the above explanation, we can conclude that bipolar junction transistor behaves as a switch only when it is operated in the cutoff and saturation region of its characteristic. In switching application, the active region or active 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 as a switch for specific applications.
Although this finite time is very small, maybe less than few microseconds but still, it is not zero. During switch ON period, the current (IC) will be building up while the collector-emitter voltage VCE will be falling towards zero. As the current increases from zero (ideal) to its maximum value and voltage falls from the maximum value to zero (ideal) there will be an instant where both of them will be maximum. At that point, peak power loss happens. In the same way, maximum power loss happens in a transistor when it comes at OFF state from ON state. Hence maximum power loss happens in a transistor during the transition period of changing state but still, the amount of energy dissipated is quite moderate as the period of transition is quite small. For low-frequency operation, the generated heat may be moderated. But if the frequency of operation is quite high, there will be significant power loss and corresponding generation of heat.
This is to be noted that, heat generation does not occur only during transient condition also during steady ON or OFF condition of the transistor but the amount of heat during steady condition is quite small and negligible.
Please Rate this Article