Transistor as a Switch
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 –
- Cutoff Region
- Linear Region
- Saturation Region
In linear region, for wide range of collector – emitter voltage (VCE) collector current (IC) 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, 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 there are cut-off region and a saturation region. In the figure when base-emitter current or simply base current is zero, the collector current (IC) will have very small constant value for a wide range of collector-emitter voltage (VCE). So when the transistor is operated with base current ≤ 0, the current through the collector to emitter (IC) 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. Hence the transistor is said to be operated as open circuited switch or OFF switch.
Now, suppose the transistor is connected in series with a load of resistance RL. In normal condition, the voltage across the load is VL. Hence current through the load is If the transistor is operated with a base current Ib1 for which collector current is IC1. IL is less than IC1, then the transistor is operated in the saturation region. Here, for any current less than IC1 through the transistor’s collector to emitter, there will be very tiny collector-emitter voltage VCE. Hence at this situation the current through the transistor is as high as load current, but the voltage across the transistor (VCE) 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 the saturation region, for given load current. So, from the above 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.
Now, for regulating output DC power in load, we must operate a transistor switch in such a way that it periodically switch ON and OFF the circuit to provide desired output power. For that, we need a specific waveform of base current due to which transistor goes to its cut-off and saturation region periodically for a given load current. The typical periodic base current waveform is generally obtained from a microprocessor based pulse generator.