Ideally the resistance in the state 'open' is infinite, while it is zero in the state 'closed'.Īt the previous chapter we have learned that the resistance of a bipolar junction transistor can be altered by attaching a voltage to the base pin.

An electromechanical device consists at least of two electrical contacts which can be either in the state open, meaning the contacts are separated from each other, or in the state closed, meaning the contacts are touching each other. Neglecting leakage current, all of the currents are zero, and VCE is equal to VCC.Ĭonditions in Saturation : As you have learned, when the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated.A switch is an electrical or electromechanical component that can interrupt the current running through an electric circuit. Actually, a small voltage drop across the transister of up to a few tenths of a volt normally occurs, which is the saturation voltage, VCE(sat).Ĭonditions in Cutoff : As mentioned before, a transistor is in the cutoff region when the base-emitter junction is not forward-biased. In this condition, there is, ideally, a short between collector and emitter, as indicated by the switch equivalent. In part (b), the transistor is in the saturation region because the base-emitter junction and the base-collector junction are forward-biased and the base current is made large enough to cause the collector current to reach its saturation value. In this condition, there is, ideally, an open between collector and emitter, as indicated by the switch equivalent.

In part (a), the transistor is in the cutoff region because the base-emitter junction is not forward-biased.

The below Figure illustrates the basic operation of a BJT as a switching device.

Download transistor as a switch