Transistor as A Switch

By Mohit Uniyal|Updated : October 3rd, 2022

There are a lot of applications with a Transistor as a switch. We have various transistors like Bipolar Junction Transistor (BJT), Junction Field Effect Transistor (JFET), and Metal Oxide Semiconductor Field Effect Transistor (MOSFET). Each transistor has multiple applications. Among all, the transistor switching action is the basic application. 

Based on the requirement and application, we will use the respective transistor. Once you study and understand the working of BJT, then it is easy to understand the working of other transistors like JFET and MOSFET. This article will discuss the Transistor as a Switch, its characteristics, operation, etc.

What is Transistor as a Switch?

A diode is a two-terminal electronic switch. Whereas a Transistor switch is a three-terminal electronic switch. Among these, one terminal is used as a controller by which we will connect or disconnect the other two terminals of the transistor. Among all the Transistors, Bipolar Junction Transistor (BJT) is the basic one.

Based on the requirement, we can connect the BJT in one of the three configurations. These are Common Base (CB), Common Emitter (CE), and Common Collector (CC) configurations. In this article, let’s consider the CE configuration to easily understand the transistor as a switching action and its operation.

Transistor as a Switch Circuit Diagram

The circuit diagram of the Bipolar Junction Transistor in Common Emitter (CE) mode is shown in the diagram below.

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Transistor as a Switch Characteristics

As the name implies, an Emitter is common to both input and output in the Common Emitter (CE) configuration. Here, we will consider the Base and Collector terminals of the BJT as the input and output terminals. From the transistor as a switching characteristic, we will understand the regions in which the transistor will be turned ON and similarly turned OFF.

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The graph between base current and base to emitter voltage is known as the input characteristics of the BJT in CE configuration. The graph between collector current and collector-to-emitter voltage is known as the output characteristics of the BJT in CE configuration. The input and output characteristics of CE configuration are shown below. From these characteristics, we will get to know how the transistor switch from one state to another state.

Transistor as a Switch in Different Regions

Broadly, we can classify the operating regions of the BJT into three regions. These are the Cut-off region, Active region, and Saturation region. The active region is also called a linear region since there is a linear relation between IC and VCE. In this operating region, BJT will work as an amplifier.

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In Which Region does a Transistor Act as an Open Switch?

The transistor acts as an open switch in the cutoff region. In Cut-off region, IB = 0A & VCE = VCC. Hence, BJT will work as an OFF switch in this operating region. In the Saturation region, BJT will work as an ON switch. Here, VCE=VCE(sat). In general, we use switches for closing or opening the circuit terminals.

Similarly, we can use a transistor as a switch by operating it in cut-off and saturation regions.

In Which Region does a Transistor Act as an Closed Switch?

The transistor acts as a closed switch when it is operated in a saturation region.

Hence, the transistor as a switching characteristic will be in the two operating regions of BJT. We can use a transistor as a switch to drive Light Emitting Diodes (LEDs) and relays.

Implementation of Universal Logic Gates Using Transistor as a Switches

NAND & NOR gates are called universal gates because we can implement any Boolean function by using NAND gates alone and similarly by using NOR gates alone. We can represent any digital circuit or logical expression using basic operations like logical AND, logical OR & NOT.

The circuit diagram we have considered earlier is an example of a NOT gate or an Inverter. We will get a NAND gate if we connect the transistors in series in the basic inverter circuit. Similarly, we will get a NOR gate if we connect the transistors in parallel in the basic inverter circuit. The circuit diagrams of two input NAND & NOR gates using a transistor as a switch are shown below.

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FAQs on Transistor Switch

  • We can classify the transistors mainly into three types. Those are BJTJFET & MOSFET. Further, we can classify these three transistors into two types each. n-p-n and p-n-p transistors are examples of BJT, which is widely used as the Transistor Switch. n-channel and p-channel JFETs are examples of JFET. Similarly, n-channel and p-channel MOSFETS are examples of MOSFET. In short, we will call them NMOS & PMOS.

  • The transistors BJT and MOSFET are the examples of current controlled and voltage controlled devices respectively. By controlling base current, we can control the output of BJT. By controlling Gate voltage, we can control the output of MOSFET. These are the basic differences between BJT and MOSFET. The other differences are the input impedance and switching frequency are low for BJT and high for MOSFET.

  • Transistors are used in many applications. Among which, the transistor switching action (ON / OFF) and amplifier are the prominent applications. For designing any digital circuit, we require logic gates. By using transistors (along with other electrical / electronic components) we can implement various logic gates. 

  • The transistor is an electronic component that has three terminals. The transistor as a switching action is the basic application. It is used to control the flow of electrons. The three terminals are Emitter, Base, and Collector for Bipolar Junction Transistor. The JFET or MOSFET has three terminals: Gate, Drain, and Source.  

  • The region in which we are operating the BJT is known as the operating region of BJT which is often uses as the Transistor as a Switch. We can operate the BJT in three regions mainly. Those are Saturation, Linear and Cut-off regions. Whenever we want to utilize BJT as a switch, then we will operate the BJT in cut-off and saturation regions. Whenever we want to utilize BJT as an amplifier, then we will operate the BJT in Active region or Linear region.

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