This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below. The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the ” I-V ” characteristics curves we have seen previously. However, high power devices such as motors, solenoids or lamps, often require more power than that supplied by an ordinary logic gate so transistor switches are used. Some output devices, such as LED’s only require a few milliamps at logic level DC voltages and can therefore be driven directly by the output of a logic gate. Solid state switches are one of the main applications for the use of transistor to switch a DC output “ON” or “OFF”. However, both the NPN & PNP type bipolar transistors can be made to operate as “ON/OFF” type solid state switch by biasing the transistors Base terminal differently operating the transistor as a switch. Due to this, there is a finite time elapse between the transition of the input waveform and the time when the collector current has dropped to 90 percent of I c(sat) and it is referred to as storage time (t s).When used as an AC signal amplifier, the transistors Base biasing voltage is applied in such a way that it always operates within its “active” region, that is the linear part of the output characteristics curves are used. The transistor cannot respond until this saturation excess charge has been removed. When the transistor is in saturation, it has excess minority carries stored in the base. Where Cc is the collector transition capacitance and w T, is the radian frequency at which the current gain is unity. The collector current increases or decreases along an exponential curve whose time constant is \tau _. The rise time and the fall time are because, if a base current step is used to saturate the transistor or return it from saturation to cutoff, the transistor collector current must traverse the active region. The time required for I c to reach 90% of its maximum level from 10% level is called the rise time (t r ) and the time required for I c to go from 90% to 10% of its maximum level is called fall time (t f).Īlso Read:Schottky diode used for switching Finally, some time is required for the collector current to rise to 10 percent of its maximum.Even when the transistor has been brought to the point where minority carriers have begun to cross the emitter junction into the base, a time interval is required before these carriers can cross the base region to the collector junction and be recorded as collector current. When the driving signal is applied to the transistor input, a nonzero time is required to charge up the emitter junction transition capacitance so that the transistor may be brought from the cut-off to the active region.The delay time exists due to the following reasons. It is the time that elapses the application of the input pulse and current to rise to 10 percent of its maximum (saturation) value I c(sat) = Vcc/Rc. The fall time is specified as the time required for lc to go from 90 % to 10% of its maximum level. It goes to zero level after turn-off time, which is the sum of storage time (t s) and fall time (t f) as shown in Fig. Similarly, when the input current I B is switched OFF, Ic does not go to zero level immediately.
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