At the base a DC source is used and we are going to see the output at the collector by varying input voltage at two states i.e. ImplementationĪssume beta value of transistor to be 100 base resistance Rb= 1k ohm and collector resistance Rc=300 ohm, even you can select any value but always select resistor value such that Rb > Rc as we need only a small base current to perform the operation. Similarly, when Vin=0v or no input voltage is applied at the input of the transistor then transistor operates in cuttoff region and hence transistor acts as an open circuit and hence collector to emitter voltage is equal to the bias voltage of the transistor. Therefore act as a short circuit and collector to emitter voltage is approximately equal to zero. Vin > 0.7 V) is applied between Base and Emitter then transistor will be in ON condition hence the collector current Vcc/Rc flows through the transistor. When some voltage (above threshold voltage i.e. The transistor operation can be performed by applying some voltage at base terminal. In this section we will see how to use both of them in both active high and active low cases. Transistor as a Switchīoth NPN and PNP transistors can be used as switch. This type of switching application can be used in digital electronics and also in domestic applications. Hence, from above discussion, it is clear that transistor can be made to work as ON / OFF switch by operating transistor in cuttoff and saturation regions. In this mode, both emitter-base and collector-base junctions are reverse biased.in this mode transistor is completely switched off as a result current flowing through the transistor is zero. We will use this mode of the transistor to use it as a switch. In this mode, the transistor acts almost as a short circuit. Here, the transistor acts as a constant current source and transistor is completely switched ON. In Saturation mode both emitter-base and collector-base junctions are forward biased as shown below. In active mode the current flows between emitter and collector and amount of current flow is proportional to the base current. In active mode of a transistor emitter-base junction should be forward biased and collector-base junction should be reverse biased as shown below. ![]() When transistor is in active mode then it can be used as a current amplifier. Operating Modes of a Transistorīased on the biasing conditions transistor mainly operates in three regions namely cutoff, active and saturation regions. Before moving forward we have to understand the operating modes of a transistor.
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