Transistor as a Switch

Introduction

The Transistor as a Switch is the genesis of many digital circuits. Transistors consist of three layers and have three semiconductor terminal devices used for Amplification and Switching operation. The operating modes of a transistor are active, saturation, and cut-off mode. 

Transistors are used both in analog and digital domains with diverse applications such as amplifier circuits, voltage regulators, digital logic circuits, oscillator circuits, embedded systems, power supply circuits, and control systems. Transistor is one of the finest devices to study and understand. This article explains the operation of the transistor in the circuit and its usages. 

Body

Transistor has two specific families to deal with. The first one is Bipolar Junction Transistors (BJT), and the second is Field Effect Transistors (FETS). The BJT consists of two junction semiconductor devices, three layers and three terminals. It has two PN junctions sandwiched with a middle layer. When it comes to learning about transistors, it is referred to as the Bipolar Junction Transistors.

Transistor

Basically, the Transistor has three terminals, i.e., base, emitter, and collector. The function of the emitter is to emit the electrons to the base as the emitter has a highly doped terminal. 

The function of the base as a lightly doped terminal is to make way for the emitter-injected electrons to the collector, whereas the collector grabs the electrons from the base as the collector has moderately doped properties. Compared to other terminals, the collector is large in size and at times starts generating less heat for the operation’s working.

Types of Transistors

The Bipolar Junction Transistor  has two types i.e.  PNP and NPN transistor. The PNP and NPN have similar functions but vary based on power supply polarity and biasing. Let’s learn a little more about the PNP and NPN transistors. 

When it comes to NPN transistors, the P-type material comes in between the other two N-type materials. And in the case of PNP transistors, the N-type comes in between the other two P-type materials.

 In short, you can understand in this way that the transistor is nothing but a current controlling device where the output current is managed completely by the input current. The Bipolar Junction Transistor carries two charges: the electrons, the negative charge carriers and the positive charge carrier holes. 

To make it a little more specific and vivid, the NPN and PNP transistors can act as a switch in the circuit. A basic transistor can be managed differently by biasing the PNP and NPN transistors simply by an ON/OFF switch through a signal amplifier. One of the most commonly used applications of Bipolar Junction Transistor is to pass a DC signal OFF or ON in switches.

 Some high-power devices like solenoids, lamps, and generators require enormous power to utilize the transistor connection than the logical gate, which uses some milliamperes of voltage (i.e DC) like in LEDs to manage the logical gate output.  

Operating Modes of Transistor as a Switch Regions

The cut-off zone area and the saturation zone are combinedly referred to as the Transistor switch’s working regions. Juggling between the switching absolute off and top-off(saturation), the use of a transistor to initialize the Q-Point and voltage divider circuit for amplification. 

Cut-off Region

In this region, the emitter-base junction and the collector-base junction are put in a reverse way. The PN junction in both terminals is put in reverse biased mode where the passage of the current is near about less or nil except there is any minor leakage of the currents (in some pico amps or nanoamps). The transistor in this region is switched off and acts as an open circuit.

Cut-off Region Characteristics

Some of the characteristics of the cut-off region are mentioned below:

•       The Bipolar Junction Transistor or the transistor act as an ‘open switch’
•       The input and the bases are highlighted as (0V)
•       The emission voltage of the base is  VBE > 0.7 V
•       The emitter in this region is put in reversed order
•       The cut-off area (full-off) bipolar junction transistor or simply transistor (collector flow = 0)  VOUT = VCC = “1′′”
•       Current flow in the collector is zero (IC = 0)

The cut-off region and off mode is in reverse bias with Vb < 0.7 V and IC = 0, when putting a bipolar transistor as a switch.

Saturation Region

In the saturation region, the base bears the highest current. The collector current acquires the average collector-emitter voltage and the leakage is less comparatively where the passage of the maximum current runs in the transistor. This is the reason behind the Fully On transistor being activated.

The saturation field, also known as the On step, can be described as  VW > 0.7 V and IK = complete when using the transistor as a switch.

Let’s take the case of the base-biased transistor in CE pathway and put the Kirchhoff rule of the voltage in the input and the output of the circuit can be mentioned as

VBB = IBRB + VBE … (1)

VCE = VCC – ICRC … (2)

VBB is the input voltage (Vi) and VCE  is the output Voltage DC (Vo). That’s the reason we get the following output;

Vi = IBRB + VBE

Vo = VCC – ICRC

When the shift in Vo as Vi rises from zero the transistor at the cut-off state at Vi is less than 0.6 V. Also, IC= 0. Vo= VCC.

The transistor as a switch is in active mode when Vi goes past 0.6 V. IC >0 and Vo is dropping (as  ICRC is increasing). In the current scenario, the rising Vi, IC increases approximately linearly. Vo also drops approximately linearly till its value decreases below 1 V.  

 Applications Of Transistor as a Switch

The transistor as a switch has the following applications:

1.     In LEDs, such applications are used as transistor connections.
2.   DC motors and light-bulb transistors as switch applications are used in a broadway.
3.   Thermistor, as one of the elements that act as a resistor, is used as a transistor as a switch to monitor and control the connection. With the increase in temperature the resistance drops and in some cases, the resistance enhances the temperature decrease. 

Conclusion 

From the figure, two things are concluded. In case Vi is low, the bipolar junction transistor is not able to put in forward-bias, Vo is high (= VCC).

In case Vi is high, Vo is very low (~ 0) in the transistor.

In case the transistor cannot reflect, it shows as switched Off.  The above description gives the complete idea of the saturation and the cut-off transistors. Suppose we holistically consider the scenario, the small input switches off the transistor whereas the high input turns on the transistor. This is how a Bipolar Junction Transistor or Transistor as a switch works.