83.2.1 - Usage of Current Mirror (Part-A)
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Introduction to Current Mirrors
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Today, we'll begin by revisiting current mirrors. Can anyone remind me what a current mirror does?
It duplicates a reference current into another branch of a circuit, right?
Exactly! This mirroring allows us to control currents effectively in amplifiers. Remember the acronym 'MIRROR'—Mimic, Input, Resistance, Regulation, Output, Reassure. It helps us recall the key features of current mirrors. Now, can you think of a situation where this would be useful?
In amplifiers, so we can maintain consistent operating points?
Correct! Current mirrors stabilize amplifier circuits. Let's dive deeper into their applications.
Small Signal Model of Current Mirrors
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Now, let's focus on the small signal model for current mirrors. Why do you think we need this model?
To analyze how the circuit behaves with small variations in input, like in real scenarios!
Exactly! Under DC conditions, the small signal model is straightforward. When can we consider this model?
When no signal is present, right?
Yes, perfect! And under signal conditions, we need to account for AC signals. Can anyone explain why?
Because the mirror’s behavior changes when it has to mirror signal current?
Correct! Let’s move on to see the implications of input signals on the mirror's output.
Application in Common Emitter Amplifiers
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Now let’s examine how current mirrors are applied in common emitter amplifiers. Who can explain the role of the active load here?
I think it helps maximize output voltage without increasing complexity.
Exactly! By using a current mirror as the active load, we achieve better performance. Remember, having matched transistors ensures similar currents. What’s our goal when designing these circuits?
To ensure they operate in their active region and maintain stability!
Right! The more consistent our currents, the better our amplifier functions. Now, can anyone summarize these benefits?
They save space, reduce component count, and increase efficiency!
Differential Amplifier Usage
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Finally, let’s touch on the significance of current mirrors in differential amplifiers. Who knows why they are crucial here?
They help to bias the differential pair and ensure balanced currents?
Exactly. They provide stable reference currents essential for proper operation. Can anyone name a factor that affects these currents in real circuits?
Base current loss might cause discrepancies, right?
Spot on! Ensuring accurate matching and compensation for these losses results in a robust circuit design. Let’s summarize countermeasures for these issues!
Introduction & Overview
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Quick Overview
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The section explores the use of current mirrors in amplifier circuits such as common emitter, common source, and differential amplifiers, detailing their small signal models under both DC and signal conditions. It emphasizes the importance of understanding these concepts for practical electronics design.
Detailed
Detailed Summary
In this section, we delve into the practical applications of current mirrors in various amplifier structures, particularly common emitter, common source, and differential amplifiers. The content begins with a recap of current mirrors and their operating principles established in previous lectures. The focus is on the significance of small signal modeling for these circuits, where we analyze the behavior of current mirrors under different conditions:
- DC Condition: When the current mirror operates under DC conditions, we establish its small signal equivalent model to predict how it behaves in amplifying circuits. For this, both BJT and MOSFET implementations are considered to illustrate the corresponding small signal models.
- Signal Input Condition: The section progresses to evaluate the scenario when the current mirror carries signal currents. Here, the small signal model must account for these AC components. The teacher guides students in deriving the algebraic relationships governing the current flow and output voltage across the mirror, which is crucial for analyzing amplifier performance.
- Applications in Amplifiers: The practical applications of current mirrors as active loads in amplifiers are illustrated through examples of common emitter amplifiers with active loads and common source amplifiers. The discussion highlights how current mirrors help maintain matched currents and improve overall amplifier performance while accounting for non-ideal factors that may affect the amplification process, such as base current losses and early voltage effects.
- Differential Amplifiers: Finally, the section briefly touches upon the utilization of current mirrors within differential amplifiers, emphasizing their dual role as biasing and active current components. The understanding of how current mirrors affect the operation of differential amplifiers forms a vital part of analog electronic circuit design.
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Introduction to Current Mirror Applications
Chapter 1 of 7
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Chapter Content
Dear students welcome back to our online certification course on Analog Electronic Circuits, myself, Pradip Mandal from E and EC department of IIT Kharagpur. Today’s topic of discussion is Usage of Current Mirror. So, we have started the current mirror circuit and today we will be talking about its application.
Detailed Explanation
This chunk introduces the topic of current mirrors, setting the context for its applications in electronic circuits. It emphasizes that this segment will focus on discussing how current mirrors are used, specifically in amplifiers and other circuit configurations.
Examples & Analogies
Think of a current mirror like a water fountain. Just as a water pump ensures a steady flow of water to different areas of the fountain, a current mirror guarantees a consistent flow of electrical current to various parts of a circuit.
Overview of Current Mirror Applications
Chapter 2 of 7
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Chapter Content
We have different applications for this current mirror. One is in common emitter and common source amplifier based on whether it is BJT or MOSFET based implementation. Likewise, in common collector and common drain and common base and common gate and also in differential amplifier.
Detailed Explanation
This chunk lists various applications of current mirrors in electronic circuits. It mentions specific amplifier types where current mirrors are pivotal, including common emitter, common source, common collector, and differential amplifiers. Each amplifier type can use either BJT (Bipolar Junction Transistor) or MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) configurations.
Examples & Analogies
Imagine a toolbox filled with different tools. Depending on what needs repairing, you would choose different tools (e.g., a hammer for nails and a screwdriver for screws). Similarly, different amplifier types require specific configurations of current mirrors to function effectively.
Understanding the Small Signal Model
Chapter 3 of 7
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Chapter Content
To understand or to appreciate the effect of common current mirror in amplifier standard amplifier where, we normally talk about the linearize circuit whether it is common emitter or common source or common collector or common drain or for that matter even for differential amplifier.
Detailed Explanation
This chunk introduces the importance of the small signal model for current mirrors in amplifiers. It explains that understanding how current mirrors affect amplifier behavior is crucial, particularly when discussing linear circuits. The small signal model helps simplify complex circuits into manageable ones for analysis.
Examples & Analogies
Think of the small signal model like creating a simplified map for a complex city. Instead of showing every single detail, you focus only on the key streets and landmarks needed for navigation. Similarly, the small signal model focuses on the necessary elements of a circuit to understand its behavior without getting lost in the details.
Analyzing the Small Signal Model of MOSFET Current Mirror
Chapter 4 of 7
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Chapter Content
Let me talk about the small signal model of current mirror implemented by MOSFET. Here we do have the current mirror circuit, we do have transistor-1 it is diode connected and the we do have reference current DC current that is getting mirrored into this branch through this transistor-2.
Detailed Explanation
This chunk focuses on the implementation of the small signal model for a MOSFET-based current mirror. It describes how the current flows from a reference current through transistor-1 and into transistor-2, emphasizing the function of the differential circuit design to manage and mirror currents.
Examples & Analogies
Consider a factory assembly line where one worker (transistor-1) directs the flow of materials (current) to another worker (transistor-2) who processes them further. Each worker's role ensures a smooth operation, just like how currents are managed in a current mirror circuit.
Small Signal Model Analysis for DC and AC Conditions
Chapter 5 of 7
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Chapter Content
For small signal model this is DC current so; obviously, we have to make the current here it is since it is 0. So, we can say it is this circuit is open.
Detailed Explanation
In this chunk, the process of determining the small signal model in a DC condition is discussed. Since there’s no AC component (i.e., the current is 0 for small signal analysis), the circuit can be simplified to an open circuit for analysis, indicating that no current flows during this specific moment.
Examples & Analogies
Imagine a dam holding back water. When the valve is closed (representing DC condition), no water flows down the stream (analogous to current). The dam's water pressure is like the DC current that remains static until the valve opens.
Revisiting the Small Signal Model with AC Signals
Chapter 6 of 7
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Now, if you want to have said small signal model of this entire circuit along with the application circuit, we need to know what will be the small signal model of the current mirror.
Detailed Explanation
This chunk transitions to how the small signal model changes when AC signals are introduced. It notes that if the current mirror carries a signal current, the analysis must consider these signals to represent the circuit appropriately, leading to different behaviors compared to the DC condition.
Examples & Analogies
Think of a speaker system. When it's silent (like the DC condition), no sound travels through the speakers (no current). Once music plays (representing an AC signal), sound waves flow through the system, creating an entirely different scenario compared to the silent state.
Understanding the Gain in Current Mirrors
Chapter 7 of 7
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Chapter Content
Whenever we do have a current mirror having getting implemented by a BJT instead of MOS, then also we will be getting similar kind of circuit.
Detailed Explanation
In this chunk, it is clarified that similar small signal models apply when using BJTs rather than MOSFETs. The principles remain consistent regardless of the type of transistor used, emphasizing the importance of the gain and the current factor in determining the overall circuit performance.
Examples & Analogies
Consider two chefs (BJT and MOSFET) preparing the same dish. While they may use different tools and methods, the end goal is the same: to create a delicious meal (accurate current mirroring). The principles of cooking (circuit performance) apply to both, thus achieving a consistent flavor (circuit function).
Key Concepts
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Current Mirror: A circuit that duplicates a reference current.
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Small Signal Model: A method for analyzing circuits under small signal variations.
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DC vs AC Conditions: Conditions that determine circuit behavior based on static (DC) or dynamic (AC) signals.
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Common Emitter Amplifier: A typical amplifier circuit configuration utilizing a current mirror as an active load.
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Differential Amplifier: An amplifier type that outputs the difference between two input signals.
Examples & Applications
An application of current mirror in a common emitter amplifier allows for higher gains and efficient coupling without additional passive components.
Differential amplifiers utilize current mirrors to ensure that both branches maintain matched currents, improving common-mode rejection.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Current mirrors in a circuit flow, duplicating currents in a steady show!
Stories
Imagine a conductor in a band, matching rhythms perfectly, hand in hand! That's how current mirrors copy currents; they ensure harmony in circuits!
Memory Tools
AC/DC: Active Currents / Direct Copy - remember that current mirrors manage both types!
Acronyms
MIRROR
Mimic Input
Resistance Regulation
Output
Reassure - the functions of a current mirror.
Flash Cards
Glossary
- Current Mirror
A circuit configuration that duplicates a reference current into another branch.
- Small Signal Model
A simplified representation of a circuit used for analyzing small variations in signals.
- Amplifier
A device that increases the power, voltage, or current of a signal.
- DC Condition
A state where the circuit operates without time-varying signals, i.e., steady-state.
- AC Condition
A state where the circuit includes alternating current components, often indicating signal activity.
- Differential Amplifier
An amplifier that amplifies the difference between two input signals.
- Active Load
A component in an amplifier design that substitutes passive loads, improving performance and efficiency.
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