Contents
How is a BJT transistor used as a switch?
A switch consists of a BJT transistor that is alternately driven between the saturation and cutoff regions. A simple version of the switch is shown in figure 1. When the input equals -Vin , the base-emitter junction is reverse biased or off so no current flows in the collector.
How is a BJT used as an example?
Using a BJT as a Switch: An Example. Remember that the controlled current through a transistor must go between collector and emitter. Since it is the current through the lamp that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch were.
What is the saturation voltage of a BJT switch?
In practice, even in cutoff there is some leakage current through the transistor. Also, in saturation, there is always some voltage dropped across the transistor’s internal resistance. Typically, this will be between 0.2 and 0.4 V in saturation depending on the collector current and size of the device.
How does a closed SPST switch work in a BJT?
As you can see, the collector and emitter terminals act like a closed SPST switch. Ideally, there’s basically a short between the collector and emitter and the voltage drop across it should be zero.
Why is the base of a BJT not connected to the base?
In the above figures, the base of either BJT is not connected to a suitable voltage, and no current is flowing through the base. Consequently, the transistor cannot turn on. Perhaps, the simplest thing to do would be to connect a switch between the base and collector wires of the transistor as in figure (a) below.
If the circuit uses the BJT transistor as a switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at the both sides of the I-V characteristics curves shown below.
How does a Darlington transistor work as a switch?
Darlington Transistors simply contain two individual bipolar NPN or PNP type transistors connected together so that the current gain of the first transistor is multiplied with that of the current gain of the second transistor to produce a device which acts like a single transistor with a very high current gain for a much smaller Base current.
How is a transistor used in a solid state switch?
Solid state switches are one of the main applications for the use of transistor to switch a DC output “ON” or “OFF”. 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.
What’s the difference between a transistor and a NPN switch?
The equations for calculating the Base resistance, Collector current and voltages are exactly the same as for the previous NPN transistor switch. The difference this time is that we are switching power with a PNP transistor (sourcing current) instead of switching ground with an NPN transistor (sinking current).