BJT Working Principle, Current Flow & Configurations Explained
The Beer-Lambert Law is a foundational concept in the study of how light interacts with matter. It explains how the intensity of light passing through a substance decreases depending on the material’s properties, concentration, and how far the light travels through the sample. This law is essential in Physics and Chemistry, especially in fields involving absorption and spectroscopy.
- The Beer-Lambert Law, also known as Beer’s Law, directly connects the absorbance of light by a solution to both its concentration and the path length that the light follows within it.
- This provides a simple, reliable method to determine the concentration of a solute in a solution just by measuring how much light the solution absorbs.
- To visualize this, imagine monochromatic (single color) light shining through a solution held in a cuvette (a small transparent container).
- The light enters with intensity I0 and emerges from the other side with intensity I. By comparing these values, we can learn about how much light the solution absorbs and thus deduce information about its contents.
The Relationship: Transmittance and Absorbance
Two key quantities are defined when light passes through a medium: transmittance (T) and absorbance (A).
- Transmittance (T): The ratio of transmitted light intensity (I) to incident light intensity (I0). It is typically expressed as either a decimal (0 to 1) or percentage (0% to 100%).
- Absorbance (A): A logarithmic measure of how much light is absorbed by the sample. Absorbance is unitless and given by:
A solution with high absorbance transmits very little light, while one with low absorbance transmits most of the incident light. Absorbance and transmittance are inversely and logarithmically related.
Key Formula: The Beer-Lambert Law
The Beer-Lambert Law is mathematically given by:
Here,
- A: Absorbance (no units)
- ε: Molar absorption coefficient (L mol-1 cm-1), specific to the substance and wavelength
- c: Concentration of the solution (mol L-1)
- l: Optical path length (cm), usually the width of the cuvette
This formula shows that absorbance is directly proportional to both the concentration of the absorbing species and the path length the light travels. If you double the concentration or the path length, the absorbance doubles as well.
Step-by-Step: Solving Beer-Lambert Law Problems
Follow these steps to solve problems involving the Beer-Lambert Law:
- Identify the given data: absorbance (A), molar absorption coefficient (ε), concentration (c), or path length (l).
- If only transmittance or light intensities are given, calculate absorbance using:
A = -log10(I/I0) - Use the Beer-Lambert Law (A = εcl) to find the unknown variable.
- Be sure to use consistent units—typically, l in cm and c in mol/L.
Illustrative Example
Suppose a solution in a 1 cm cuvette has a measured absorbance (A) of 1.0 at a particular wavelength. If the molar absorption coefficient (ε) is 100 L mol-1 cm-1, what is the concentration?
From A = ε c l,
1.0 = 100 × c × 1
c = 1.0 / 100 = 0.01 mol/L
So, the solution’s concentration is 0.01 mol/L.
Table: Absorbance and Transmittance Values
| Absorbance (A) | Transmittance (%) |
|---|---|
| 0 | 100% |
| 1 | 10% |
| 2 | 1% |
| 3 | 0.1% |
| 4 | 0.01% |
| 5 | 0.001% |
Practical Applications
The Beer-Lambert Law is widely used in chemical analysis. It enables measurement of an unknown substance's concentration by simply measuring its absorbance with a spectrophotometer.
Calibration curves are often constructed by measuring the absorbance of solutions with known concentrations, plotting absorbance against concentration, and using the resulting linear relationship to determine concentrations of unknown samples.
Key Points to Remember
- Absorbance is a dimensionless, unitless quantity.
- Molar absorption coefficient (ε) depends on the substance and wavelength.
- The relationship between absorbance and transmittance is logarithmic, not linear.
Vedantu Resources & Next Steps
- Reinforce your understanding of wave and light concepts with Bipolar Junction Transistor pages.
- Explore real-life applications of Physics topics at Uses of Transistor.
- Deepen your knowledge about electronic configurations at Common Emitter Configuration.
- For detailed characteristics of transistors, visit Characteristics of a Transistor.
Summary
The Beer-Lambert Law provides a clear linear link between absorbance, concentration, and path length. This concept is crucial for students aiming to understand optical properties of solutions and for practical applications in analytical laboratories.
By applying its formulae and interpreting data tables, students can solve problems efficiently and gain confidence for Physics examinations.
FAQs on Bipolar Junction Transistor (BJT): Definition, Working & Exam Guide
1. What is a Bipolar Junction Transistor (BJT) and how does it work?
A Bipolar Junction Transistor (BJT) is a three-layer, three-terminal semiconductor device used for amplification and switching. It works by allowing a small base current to control a larger collector-emitter current. The emitter-base junction is forward biased, and the collector-base junction is reverse biased, enabling the movement of majority and minority charge carriers that result in current amplification.
2. What are the differences between NPN and PNP transistors?
NPN and PNP transistors are two types of BJTs with different charge carrier flows:
• NPN: Majority carriers are electrons; current flows from collector to emitter.
• PNP: Majority carriers are holes; current flows from emitter to collector.
• Symbol direction: The arrow in the emitter leg indicates the direction of current flow (outward for NPN, inward for PNP).
3. Where are BJTs used in circuits?
BJTs are used in various electronic circuits, primarily for:
• Amplification of signals in audio, radio, and instrumentation circuits
• Switching operations in logic devices, relays, and digital circuits
• Oscillator and modulator circuits
• Voltage regulation and current control applications
4. What is the difference between BJT and MOSFET?
BJT is a current-controlled device, while MOSFET is voltage-controlled. Key differences:
• Input impedance: BJT: Low; MOSFET: Very high
• Switching speed: BJT: Moderate; MOSFET: Very high
• Applications: BJT in analog amplifiers, MOSFET in digital and power electronics
• Control type: BJT needs base current; MOSFET needs gate voltage
5. What are the three regions of operation for a BJT, and what do they mean?
The three regions of BJT operation are:
• Active: Emitter-base forward biased, collector-base reverse biased; used for amplification.
• Saturation: Both junctions forward biased; transistor is fully ON (switch closed).
• Cut-off: Both junctions reverse biased; transistor is OFF (switch open).
6. What is the current gain (β) of a BJT?
Current gain (β) is the ratio of collector current (IC) to base current (IB) in a BJT:
β = IC / IB
It indicates how much the input current is amplified at the output. Typical values range from 20 to 200 in BJTs.
7. How do you calculate collector current in a BJT?
You can calculate collector current (IC) using the base current (IB) and current gain (β):
IC = β × IB
This formula is essential for solving BJT circuit problems in exams.
8. What are the main configurations of a BJT and their applications?
Main BJT configurations:
• Common Emitter (CE): High voltage and current gain, used for amplification
• Common Base (CB): High voltage gain, low input impedance, used in high-frequency circuits
• Common Collector (CC): High current gain, used as a buffer or voltage follower
9. What is the relation between α and β in a BJT?
The relationship between α (common-base current gain) and β (common-emitter current gain) is:
β = α / (1 - α)
This formula helps convert parameters between BJT configurations.
10. Are BJTs still used in modern electronics?
Yes, BJTs are still widely used in analog circuits, audio amplifiers, and situations where precise current control and linear amplification are required. Although MOSFETs dominate digital and high-speed circuits, BJTs remain essential for specific applications.
11. How is the BJT symbol different for NPN and PNP transistors?
The BJT symbol shows the direction of current flow:
• NPN transistor: Arrow points outward from the emitter
• PNP transistor: Arrow points inward towards the emitter
This helps quickly identify transistor type in circuit diagrams.
12. What are the advantages and disadvantages of BJT?
Advantages of BJT:
• Excellent linear amplification
• High transconductance
Disadvantages:
• Lower input impedance compared to MOSFETs
• Higher power consumption
• Slower switching speed than MOSFETs
