Transistor as a Switch Different ways of connecting Loads Load can be connected to transistor in different ways, some of them are shown below. Note : In the case of a PNP transistor just replace GND with Vcc , Vcc with GND and the transistor will be ON when the input voltage Vin is LOW. To ensure that the transistor is in saturation we will take Ib’ = 10Ib The minimum base current can be find out from the equation Ib = Ic/ß To keep transistor in saturation enough base current Ib must flow. Ic = Vcc/Rc, since Vce is less than 0.3V. Ic = (Vcc – Vce)/Rc, which is the maximum Ic that can flow through the circuit. So collector current Ic can be written as, For a silicon transistor it usually 0.3v. When the transistor is ON ( saturation region ), the collector emitter voltage Vce is approximately equal to zero.
Leackage current, Iceo can be neglected, so Where ß is the gain in common emitter configuration and lceo is the leckage current. During this state transistor acts as an open circuit and thus the entire voltage Vcc will be available at collector. Transistor will be in OFF ( cutoff ) when the input Vin equal to zero. Transistor as a Switch – OFF Transistor as a Switch OFF Thus collector current Ic = Vcc/Rc will flows. For a silicon transistor it is equal to 0.3v. During this condition the Collector Emitter voltage Vce will be approximately equal to zero, ie the transistor acts as a short circuit. Transistor will become ON ( saturation ) when a sufficient voltage V is given to input. Transistor as a Switch – ON Transistor as a Switch ON In most cases output is taken from collector but in some cases load is connected in the place of Rc. Circuit Diagram – Transistor as a Switch Transistor as a Switch Circuit Diagramįrom the above circuit we can see that the control input Vin is given to base through a current limiting resistor Rb and Rc is the collector resistor which limits the current through the transistor. It is also a very useful circuit for an electronics hobbyist as it can be used as a driver, inverter etc. In most digital IC’s transistors will work as a switch to make power consumption very low. But in saturation region both junctions are forward biased. Switch is a very useful and important application of transistors. In the Cutoff State both Emitter Base Junction and Collector Base junctions are reverse biased. When a transistor works as a Switch it works in Cutoff and Saturation Regions. A transistor works in active region when worked as an Amplifier. We all know that a transistor has 4 regions of operation, in which Active, Cutoff and Saturation are commonly used.
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