Transistor as a Switch

Blog Number:-010

Hello Everybody,

I hope you all will be fine.

In our last discussion, we discussed about the Configuration of Bipolar Junction Transistor. Today in this session, we will discuss about the behavior of a transistor as a switch.So, let's start.

Transistor as a 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. 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.

          If the circuit uses the Bipolar Transistor as a Switch, then transistor is arranged to operate the transistor at both sides of the “ I-V ” characteristics curves.

The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. We 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.

     1. Cut-off Region

     Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

     Cut-off Characteristics

• The input and Base are grounded ( 0v ) • Base-Emitter voltage VBE < 0.7v • Base-Emitter junction is reverse biased • Base-Collector junction is reverse biased • Transistor is “fully-OFF” ( Cut-off region ) • No Collector current flows ( IC = 0 ) • VOUT = VCE = VCC = ”1″ • Transistor operates as an “open switch”

    2. Saturation Region

     Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results is maximum current flowing through the transistor. Therefore the transistor is switched “Fully-ON”.

    Saturation Characteristics

• The input and Base are connected to VCC • Base-Emitter voltage VBE > 0.7v • Base-Emitter junction is forward biased • Base-Collector junction is forward biased • Transistor is “fully-ON” ( saturation region ) • Max Collector current flows ( IC = Vcc/RL ) • VCE = 0 ( ideal saturation ) • VOUT = VCE = ”0″ • Transistor operates as a “closed

        Then the transistor operates as a “single-pole single-throw” (SPST) solid state switch. With a zero signal applied to the Base of the transistor it turns “OFF” acting like an open switch and zero collector current flows. With a positive signal applied to the Base of the transistor it turns “ON” acting like a closed switch and maximum circuit current flows through the device.

So, that's all for this session. If you have any doubt regarding the topic. please comment.

Thank You,

Have a good day.