An Introduction to Transistors
A Transistor is a type of semiconductor device; that has many functions, such as switching, amplifying, detecting, signal modulation, and more. In almost all modern electronics, Transistors are the most important active components. As a result, many people consider the Transistor to be one of the most important innovations of the twentieth century. In this article, you will understand the Transistor, its Input, Output, and Current transfer characteristics. So, with no further ado, let us start by understanding a Transistor.
What is a Transistor?
A Transistor is an Electric device that regulates the flow of Electric Current and Voltage. It acts as a switch or gate of Electric signals. A Transistor is usually composed of three layers of semiconductor components that carry Current. Most of the Transistors are composed of pure silicon, with some made of germanium however, sometimes other semiconductor materials are used.
Transistors can be used for a wide range of digital and analogue functions, including amplifiers, switches, Voltage stabilizers, signal modulation, and oscillators, because of their high response and accuracy. Transistors can be packaged individually or in a tiny space, allowing for integrating up to 100 million Transistor integrated circuits.
Parts of Transistor
A Transistor is made up of three layers of semiconductor materials, or terminals that help to link the Transistor to an external circuit and carry Current. The Current that is applied through one pair of terminals of a Transistor is controlled by the Current applied to any other pair of terminals of the Transistor. For a Transistor, there are three terminals. They are as follows:
Base: The base is used to activate the Transistor.
Collector: The Transistor's positive lead is known as the collector.
Emitter: The Transistor's emitter is the negative lead.
Characteristics of Transistor
Transistor Characteristics is the basis that represents the relationship between the Electric Current and Electric Voltage of a circuit. There are three types of Transistor characteristic curves based on the configuration of the circuit.
Input Characteristic - The Input characteristics describe any changes that occur in the Input Current because of the variation of the Input Voltage by keeping the Output Voltage constant.
Output Characteristic - This is a graph of Output Current on one axis and Output Voltage on another, at a constant Input Current.
Current Transfer Characteristic - This is a characteristic curve that points to the fluctuation of the Output Current to that of the Input Current. Here, the Output Voltage is kept constant.
Transistor Configuration
Any type of Transistor circuit can be designed by using the above mentioned three Transistor characteristics. The configuration of the Transistors is based on the Transistor terminals. There are three types of Transistor circuit configuration, these are:
Common Emitter Transistor
Common Base Transistor
Common Collector Transistor (emitter follower).
Each circuit configuration has a different characteristic curve. Based on the requirement of the circuit, the Transistor configuration is chosen accordingly.
Few things are considered while using the correct Transistor for the circuit. These are the maximum Voltage rating between the emitter and the collector (UCEmax), maximum power to build a circuit, and maximum collector Current (ICEmax). An Electric circuit must not exceed these maximum values to function properly. Permanent damage to the circuit may occur if it exceeds the value. It is also important to maintain proper Current amplification and frequency.
Common Emitter Configuration
In this kind of configuration, an Emitter is used as a common terminal for both Input and Output. It works as an Inverting Amplifier Circuit. Here, the Input is applied in the region of the base Emitter and the Output is obtained from between the Terminals of the Collector and Emitter.
In this case,
VBE is the Input Voltage,
IB is the Input Current,
VCE is the Output Voltage, and
IC is the Output Current.
The Common-Emitter Configuration is usually based on Transistor-Based Amplifiers. Under this condition, the Emitter Current is equivalent to the sum of base Current and collector Current.
Hence,
IE = IC + IB
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This equation is the Transistor equation for the CE configuration. The ratio of the collector Current to that of emitter Current gives Current gain alpha in the Common Base Configuration. Similarly, the ratio of the Collector Current to that of the base Current gives Current gain beta in the Common-Emitter Configuration.
The Relationship Between the Two Current Gains is:
Current gain (α) = IC/IE
Current gain (β) = IC/IB
Collector Current IC =αIE = βIB
This configuration uses one of the three circuit configurations. It has average Input and Output values of impedance. It also has an average Current and Voltage gains. The Output signal of this configuration has a phase shift of 180⁰, so the Input and the Output are inversely proportional to each other.
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Input and Output Characteristics of Common-Emitter Configuration
Input Characteristics of Transistor
The Input characteristic of a Transistor is obtained between the Input's Current IB and the Input Voltage VB by having a constant Output Voltage VCE. By keeping the Output Voltage VCE constant and changing the Input Voltage VBE of different points, we can examine the values of the Input Current of each of the points. Now, using the values obtained from different points, a graph is drawn by plotting the values of IB and VBE at constant VCE.
Rin = VBE/IB (at a constant VCE)
This is the required equation to calculate the Input resistance Rin.
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Output Characteristics
The Output characteristic of a common emitter is obtained between the Output Voltage VCE and Output Current IC at a constant Input Current IB. By keeping the base Current IB constant and changing the value of Output Voltage VCE at different points, we can calculate the value of collector IC for each point. Now, if we plot a graph between IC and VCE, we get the Output characteristics of a common-emitter configuration.
Rout = VCE/IC (at a constant IB)
This is the equation to calculate Output resistance.
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The Output characteristics can be divided into three regions:
The active region of the Transistor
Saturation region of the Transistor
Cut-off region of the Transistor
The Active Region of the Transistor
The active region of the Transistor is the area on the Output curve where the Output Current is almost constant and independent of the Output Voltage. The Transistor operates in the Active region if the base resistance is greater than the maximum value allowed. A Transistor can only be used as an amplifier if it is in the active region. In addition, the emitter junction should be in forwarding bias, and the collector junction should be in reverse bias for operation in the active region.
Saturation Region of the Transistor
The saturation region of the Transistor is the area where the collector Current rapidly increases with a little increase in Output Voltage. The base resistance should be less than the maximum allowed value to run the Transistor in the saturation region. Both emitter and collector junctions should be in forwarding bias for operation in the saturation region. The Transistor works like the ON stage of a switch in the saturation zone.
Cut-off Region of the Transistor
The base Current is effectively zero in the Cut-Off region. As a result, even at higher Output Voltage, collector Current becomes zero. To operate a Transistor in the cut-off region, both the emitter and collector junctions must be in reverse-biased condition. A Transistor operates like the OFF stage of a switch in the cut-off region.
FAQs on Characteristics of a Transistor
1. What are the V-I (Voltage-Current) characteristics of a transistor?
The V-I characteristics of a transistor are graphical representations that show the relationship between the currents and voltages of its terminals. These graphs are essential for understanding how a transistor behaves in a circuit. They are typically divided into three main types: input characteristics, output characteristics, and current transfer characteristics. These curves help in designing and analysing electronic circuits like amplifiers and switches.
2. What do the input characteristics of a transistor in a common-emitter (CE) configuration show?
The input characteristics in a common-emitter configuration describe the relationship between the input current (base current, I_B) and the input voltage (base-emitter voltage, V_BE) while keeping the output voltage (collector-emitter voltage, V_CE) constant. This curve is similar to that of a forward-biased diode and helps determine the transistor's input dynamic resistance (R_in).
3. What are the three operating regions shown in the output characteristics of a transistor?
The output characteristics curve of a transistor shows three distinct regions of operation:
- Active Region: Here, the collector current is almost constant and is controlled by the base current. The transistor functions as an amplifier in this region.
- Saturation Region: In this region, the collector current increases rapidly and is at its maximum value. The transistor acts like a closed (ON) switch.
- Cut-off Region: Here, the base current is nearly zero, causing the collector current to be negligible. The transistor acts like an open (OFF) switch.
4. What is the significance of the current transfer characteristic curve of a transistor?
The current transfer characteristic curve illustrates how the output current (collector current, I_C) changes in response to the input current (base current, I_B), while the output voltage (V_CE) is held constant. Its primary significance is that its slope gives the DC current gain (β) of the transistor, which is a crucial parameter for amplifier design.
5. How do the fundamental characteristics differ between an NPN and a PNP transistor?
The primary differences in characteristics between NPN and PNP transistors lie in their biasing and current flow direction.
- Biasing: An NPN transistor is turned ON when a positive voltage is applied to its base relative to the emitter. A PNP transistor turns ON when its base is made negative relative to the emitter.
- Current Flow: In an NPN transistor, conventional current flows from the collector to the emitter. In a PNP transistor, current flows from the emitter to the collector.
- Majority Carriers: In NPN transistors, electrons are the majority charge carriers, while in PNP transistors, holes are the majority charge carriers.
6. How do a transistor's characteristics allow it to function as both a switch and an amplifier?
A transistor's versatility comes from its different operating regions. For amplification, it operates in the active region, where a small change in the input base current produces a large, proportional change in the output collector current. To function as a switch, it utilizes the other two regions: the cut-off region acts as an 'OFF' or open switch with no current flow, and the saturation region acts as an 'ON' or closed switch where maximum current flows.
7. Why is the common-emitter (CE) configuration the most widely used for building amplifiers?
The common-emitter (CE) configuration is preferred for amplifiers because it is the only configuration that provides both significant current gain (β) and voltage gain. The common-base configuration has voltage gain but no current gain, while the common-collector configuration has current gain but no voltage gain. The CE configuration's ability to amplify both current and voltage (and thus, power) makes it the most practical and efficient choice for most amplifier circuits.
8. What happens to a transistor's behaviour if it is biased to operate outside the active region?
If a transistor operates outside the active region, its amplifying properties are lost.
- In the saturation region, the collector current reaches its maximum and is no longer controlled by the base current. The transistor behaves like a closed switch with a small voltage drop across it, not an amplifier.
- In the cut-off region, the base current is zero, which stops the flow of collector current entirely. The transistor acts like an open circuit or an open switch.
9. What are the real-world implications of a transistor's operational limitations, such as thermal sensitivity?
A transistor's operational limitations have significant real-world consequences. For example, thermal sensitivity means that as a transistor's temperature increases, its current gain can also increase, potentially leading to a condition called 'thermal runaway' where the device heats up uncontrollably and gets damaged. This requires designers to include heat sinks or special biasing circuits to ensure stability. Another limitation, such as its finite switching speed, determines the maximum frequency at which the transistor can be used in digital logic or communication circuits.
