Use of Transistor as a Switch | switching circuit

A switch or key is useful to ON or OFF the circuit connection to activate or deactivate a device. For example, if we want to ON or OFF a fan or a TV or a Tube light we need to press on switches. Sometimes, in physics laboratory we use tapping key to connect the circuit to get a current flow. These all keys are to be operated manually. In many cases people need an automatic switch or a voltage dependent switch. In this purpose, we are going to discuss how a transistor can be used as a switch in electronic circuits.

Contents in this article:

  • Benefits of using transistor as a switch
  • Transistor switching circuit
  • How to use a bipolar junction transistor as a switch?
  • Experimental verification of transistor switching circuit

Benefits of using transistor as a switch

As I mentioned above that many of the circuits need an automatic or a voltage dependent switch to operate. Now, an electronic circuit uses currents i.e. voltage to be active. So, if we can construct a switch that depend on the current flow or the voltage applied in the circuit then it will be like a diamond. Because, in this case we don’t need to work manually, rather the current or voltage of the circuit itself decides whether the circuit will be ON or OFF. Using a Transistor in an electronic circuit one can create such types of switching operation in that circuit. Therefore, the application of transistor as a switch is more useful than a manual switch. In recent days, people are using transistor as a switch in the electronic circuits.

Transistor switching circuit

This is the circuit diagram to use the transistor as a switch. This is a common emitter (CE) configuration circuit of an NPN transistor which can be used as the transistor amplifier circuit as well. Just the regions of operation of the transistor are different. One can also use a PNP based circuit for the same purpose.

Transistor Amplifier Circuit
Transistor switching Circuit

Here, VBE gives the input voltage, VCE is the output voltage, IB is input current and IC gives the output current. We have to notice the output voltage VCE corresponding to the input voltage VBE.

Operation of a Transistor as a switch

One can realize the transistor switching operation with the help of above circuit diagram. To operate the transistor like a switch, we need to operate in the saturation region and the cut off region. In saturation region, it acts like closed switch (ON stage) and in cut off region it acts like opened switch (OFF stage). Now, let us realize this with some electronic equations.

Using Kirchhoff’s law in the input terminal (Base to Emitter) we get, VBB = IBRB + VBE …………(1)

And, using Kirchhoff’s law for the output terminal (Collector to Emitter) we get, VCE = VCC – ICRC …………(2)

Here, the effective input voltage is Vin=VBB which is given by equation-(1) and the effective output voltage is Vo=VCE which is given by equation-(2).

Now, for a Silicon made transistor, VBE=0.7 volt. So, when VBB is very low (less than 0.8 volt), the input current or the base current IB remains zero. At this condition, the transistor is not ON yet i.e. the transistor is at cut off region. Then collector current IC will also be zero. Hence, from equation-(2) we get the output voltage, VCE = VCC (very high). Again, when VBB is very high, the base current IB will become higher. Then, the collector current IC will also become very high. This leads the transistor in saturation region of operation. Hence, the output voltage VCE becomes very low (nearly zero).

Input voltage
VBB
Output voltage
VCE
Region of operation
Low (<0.8 V)High (nearly VCC)Cut off
HighLow (nearly 0.2 V)Saturation

Observation and discussion

At the ON state of a switch, the voltage across it remains very low (nearly zero) as both ends of the key are directly connected or shorted. In transistor operation we also see that when the transistor operates in saturation region the output voltage becomes very low. This situation is like the ON state of the switch. Again, at the OFF state of a switch the voltage across it becomes very large as it remains opened. In the cut off region of transistor one can achieve the same situation. So, a transistor behaves like the OFF state of a switch when it acts in cut off region. So, a BJT acts as ON state of a switch in saturation region and OFF state of a switch at Cut off region of operation. Clearly, the ON or OFF state of transistor depends on the input voltage across it.

How to verify the Transistor switching operation practically?

I strongly recommend you to try this circuit practically and verify the transistor switching operation for a BJT. You need a Transistor (use BJT), a power supply (a 10 volt source is enough), a variable input voltage supply, two resistors (RB and RC), a Multimeter to take the data for inputs and outputs. You can place the transistor on a bread board and construct the circuit. Note the data of output voltage for low input and high input respectively and check whether it works or not. You can measure the output voltage directly by a multimeter. However, you can calculate output voltage using equation-(2) after taking the data for the output current IC. In this way, one can verify the working principle of a transistor as a switch.

This is all from this article. Hope you have gathered a great knowledge from this article. If you have any doubt on this topic you can ask me in the comment section.

Thank you!

Related Posts:

  1. Bipolar Junction Transistor (BJT)
  2. Input and Output curves of a Transistor
  3. Difference between NPN and PNP Transistor
  4. Classification of BJT Amplifiers

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