86.1 - Numerical Examples on Current Mirror and its Applications (Part-A)
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Understanding Current Mirrors: Basics
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Today we are diving into the concept of current mirrors. A current mirror allows us to output a mirrored current based on a reference current. Can anyone tell me why this might be useful in circuits?
It helps in maintaining consistent current levels for various components, right?
Exactly! It ensures that we have a stable reference current. What are some essential parameters we should consider when designing a current mirror?
Things like threshold voltage and K factor, I think.
Right! The K factor indicates the transconductance of the transistors involved. Remember, we will refer to it as 'current gain'. Let’s do a quick calculation example to reinforce this!
Numerical Example with MOSFETs
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In our first numerical example, we have a current mirror with a reference current of 0.5 mA and two transistors with differing K factors. Using these values, how would we find the output current?
We need to use the K factor equation to calculate the current for the second transistor.
Correct! And we also set our Vgs values accordingly. Do you remember how to calculate Vgs?
Yes! It’s based on the threshold voltage and K value. So, if K is 1 mA/V² for the first transistor, we can find the corresponding Vgs.
Exactly. By calculating Vgs and substituting into the equations, we find the currents for both transistors.
Understanding BJT Current Mirrors
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Now, let's move to current mirrors using BJTs. How do BJTs differ from MOSFETs in this context?
BJTs rely on current gain, beta, rather than transconductance!
Correct! They also have a reverse saturation current. Can anyone explain how this affects the mirroring ratio?
Depending on the reverse saturation current values, the current output can vary! Higher reverse saturation means better performance.
Good observation! We’ll incorporate these characteristics into our numerical calculations for BJTs.
Navigating Non-Ideality Factors
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As we calculate, we must account for non-ideality factors such as Early voltage or losses due to finite beta. Why is this important?
Non-idealities can lead to discrepancies in expected output current!
Exactly! How do we go about quantifying these non-ideal factors?
By using equations that incorporate these factors; for instance, adjusting our output current calculations with these non-ideality terms.
Great! Now let’s practice this through some example problems.
Applications in Amplifiers
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Finally, let’s discuss how current mirrors are used in amplifier circuits. Why do we find them necessary in amplifier designs?
They help to maintain consistent biasing and improve linearity!
Right! Can anyone give an example of an amplifier type that uses current mirrors?
Common emitter and common source amplifiers often utilize them!
Excellent! So, as we wrap up, let’s recap: current mirrors play a crucial role in ensuring stable operations in various amplifiers.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The section explores numerical examples related to current mirrors, illustrating how they operate using both MOSFET and BJT configurations. It outlines their applications in various amplifiers and discusses the theory behind current mirrors and calculations involved.
Detailed
In this section of the course on Analog Electronic Circuits, provided by Prof. Pradip Mandal, an in-depth exploration of current mirrors is conducted through multiple numerical examples. The section starts with a simple current mirror constructed with MOSFETs, where key parameters like reference currents, threshold voltages, and K factors are reviewed. Subsequent calculations determine output currents and minimum voltage requirements for saturation. The discussion extends to BJTs for constructing current mirrors, highlighting differences in parameters like reverse saturation current and mirroring ratios. Moreover, the significance of non-ideality factors due to finite beta of transistors and Early voltage effect are examined in both transistor types. The concepts are supplemented with practical amplifier applications, further emphasizing the importance of current mirrors in real-world circuits.
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Audio Book
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Introduction to Current Mirrors
Chapter 1 of 5
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Chapter Content
Today’s topic of discussion is Numerical Examples on Current Mirror and some Application Circuits, where we are using current mirror. So, primarily we will be talking about numerical examples, to complement whatever the theory you have learnt on current mirror and its application circuit.
Detailed Explanation
This segment introduces the topic of current mirrors, indicating that the discussion will focus on numerical examples. The importance of these examples is to reinforce the theories previously learned about current mirrors and how they are utilized in application circuits. Essentially, current mirrors play a crucial role in analog circuits by providing a stable current source based on a reference current.
Examples & Analogies
Imagine a water fountain system where a single pump supplies water to multiple fountains. The current mirror is like that pump, maintaining consistent water flow (current) to all fountains (circuit components) irrespective of the varying demands from each fountain.
Coverage of Presentation
Chapter 2 of 5
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Chapter Content
The coverage of today’s presentation includes numerical examples of simple current mirror with MOSFETs, current mirror using BJTs, improvised/current precision current mirrors, and amplifiers using current mirrors.
Detailed Explanation
This section outlines what will be covered in the lesson. It starts with numerical examples of simple current mirrors constructed with both MOSFETs and BJTs. The session will also delve into more advanced current mirrors designed for precision, showing how the functionality can be improved. Moreover, the applicability of current mirrors in amplifiers will be touched upon, particularly in the context of single-ended and differential amplifiers.
Examples & Analogies
Think of a high school science project where you first learn how to create a simple circuit with batteries. Later, as you gain more knowledge, you add sensors and controls to make a more advanced project. Similarly, this presentation starts with basic current mirrors and progresses to more complex applications.
Example of a Simple Current Mirror with MOSFET
Chapter 3 of 5
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Chapter Content
A current mirror constructed by MOSFET has two transistors forming the current mirror. The reference current is given along with the details of transistor parameters; then, calculations for gate-source voltage and output current are performed.
Detailed Explanation
Here, an example circuit of a simple current mirror using MOSFET transistors is provided. It connects two transistors, known as M1 and M2, which mirror a reference current of 0.5 mA. The goal is to find the gate-source voltages and output current for both transistors based on their parameters. The calculation involves aspects such as ignoring the channel length modulation effect and determining the voltages and currents using known equations.
Examples & Analogies
Imagine that the first MOSFET (M1) is like a teacher showing a student how much they should study (the reference current); the second MOSFET (M2) then mimics the study habits of the student, trying to match the efforts of the teacher.
Effect of Lambda (λ) on Current Variation
Chapter 4 of 5
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Chapter Content
The calculation continues with consideration of the finite values for λ, showing how the output current of transistor-2 varies with changes in the drain-source voltage.
Detailed Explanation
This part elaborates on how the finite value of the channel-length modulation parameter (λ) affects the output current from the current mirror. As the drain-source voltage increases, adjustments must be made to compute the output current accurately, reflecting the non-ideal behavior in real-world circuits. The analysis leads to two different currents obtained at different drain-source voltages, demonstrating the practical implications of λ on circuit performance.
Examples & Analogies
Consider that λ is like a small adjustment needed in a music amplifier. If the amplifier isn't recognizing subtle sound variations, you need to tweak some settings (in this case, λ) to ensure the output matches the desired audio quality.
Small Signal Output Resistance
Chapter 5 of 5
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Chapter Content
To find the small-signal output resistance at the output of the current mirror, we analyze the slope of the current versus voltage change.
Detailed Explanation
This section deals with deriving the small-signal output resistance of the current mirror. By observing how the output current changes with changes in the output voltage, the slope can be computed, which in turn helps in calculating the small-signal resistance. This provides insight into how effectively the current mirror can maintain a constant output in response to varying voltage conditions.
Examples & Analogies
Think of a bungee cord attached to a bridge. When the pressure (voltage) applied to it changes, the tension of the cord (output current) also adjusts slightly. The bungee’s tendency to return to its original position represents the output resistance of the current mirror.
Key Concepts
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Current Mirror: A device for duplicating reference current.
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MOSFET Configuration: Uses voltage-controlled devices to mirror current.
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BJT Configuration: Utilizes current-controlled devices for increased accuracy in mirroring.
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Non-Ideality Factors: Adjustments that must be made for accurate current duplication.
Examples & Applications
Calculating output currents using simple MOSFET current mirror configurations.
Analyzing how different K factors affect the output current in BJTs.
Memory Aids
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Rhymes
In circuits where currents flow, a mirror is for show- duplexing the flow, never too slow.
Stories
Imagine two friends, one who always mirrors the other. The first friend holds a constant light, while the other adjusts to keep up, highlighting the role of current mirrors in circuits.
Memory Tools
Remember 'MIRROR' for M - Maintain, I - Input, R - Reference, R - Resistor effects, O - Output current, R - Reproduce current.
Acronyms
MOSFET = Magnify Output via Significant Field Effect Transistor.
Flash Cards
Glossary
- Current Mirror
A circuit that copies (mirrors) the current flowing in one active device by controlling the current in another device, maintaining a constant output current.
- Threshold Voltage
The minimum gate-to-source voltage that is needed to create a conducting path between the source and the drain of a MOSFET.
- K Factor
The transconductance parameter, relating the drain current to the gate voltage in a MOSFET.
- Reverse Saturation Current
The small current that flows through a diode when it is reverse-biased, often denoted as I0 in BJTs.
- NonIdeality Factor
Factors affecting the ideal behavior of a transistor, which can include the Early effect and base current losses.
Reference links
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