Transistor as a switch theory with block diagram & Characteristics curve

This tutorial is about transistor as a switch with block diagram and circuit diagram. After reading this tutorial you ll get to know how Transistor acts as a switch if it is NPN or PNP.

Transistor as a switch

Transistor is a three terminal semiconductor  device that amplify the current and used as a open close switch . Transistors plays a very important role in replacing vacuum tubes. Because transistors are ideal component for using in the digital circuits as switch.

           A transistor can be used as a switch because its collector current is directly controlled by the base current . If the base current is greater than 0.7 volts than the transistor acts as a closed switch. To understand this let’s take a look of current characteristics of a transistor .

 The characteristic curve above shows the three regions.

• Saturation region
• Cut off region
• Active region

When we are using the transistor as switch we deals with only saturation region and cut off region.

Transistor in Cut off region :

In the cut off region the base voltage is less than 0.7 V so the base current is also negligible. In the transistor the collector current is directly proportional to the base current. So the collector current Ic is also negligible. But the collector to emitter voltage V_CE is maximum. Which creates a large depletion region and no current flows through the transistor. So this region is called as cut off region.

The conditions for the transistor in the saturation region are Vb < 0.7 V, Collector Current ( Ic) = minimum, Collector to Emitter Voltage (  V_CE ) = maximum. 

Transistor in Saturation region :

In this mode the maximum base current is applied that results with high collector current and lower collector to emitter voltage. And this results a lower depletion region layer. By this the large amount of current can flow through the transistor.

The conditions for the transistor in the saturation region are V_b > 0.7 V, Collector Current ( Ic) = maximum, Collector to Emitter Voltage ( V_CE ) = minimum.  

When the base bias voltage is lower than 0.7 V. The transistor operated in the cut off region. But when the base bias voltage increases from 0.7 V the base saturated the collector and the current starts flow. In this state the transistor acts as a closed switch.

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So now you can understand how a transistor acts a switch. Its totally depends upon the Base bias voltage V_b . If  V_b is greater than 0.7V it acts as a closed switch and when V_b is lower than 0.7 V it acts a open switch.

In the figure below a comparison is made with the mechanical switch and the transistor as a switch . In this figure a buzzer is operated with a mechanical switch. When switch is in closed condition the circuit completed and current flows across the buzzer. But when the switch is open the its breaks the circuit and buzzer doesn’t blow.

Same condition is in the case of transistor. When the transistor operated in the cut off region means its acting as a open switch and the buzzer doesn’t blow in this case. But when transistor goes into the saturation region the transistor acts as a closed switch and current completes the path. By this buzzer start blowing.

You can use a LED instead of buzzer with a 330 ohm resistor to perform this experiment in lab or at your home.

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