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Fundamentals of Current Mirrors
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Today, we will begin with the fundamentals of current mirrors. Can anyone explain what a current mirror is?
It's a circuit that replicates a current from one branch to another.
Exactly! The primary purpose of a current mirror is to maintain a constant current, regardless of voltage variations. This stability is critical in amplifier circuits.
What are the main types of current mirrors we use?
Great question! We typically work with MOSFET and BJT current mirrors. Now, letβs take a look at how these work through numerical examples.
Remember, C.A.M. - Constant Current from Active Mirror. This acronym can help you recall the main function of current mirrors.
I find that helpful!
Let's summarize: Current mirrors are essential for maintaining constant currents in various applications, including amplifiers. Ready to explore some numerical examples?
Numerical Example: MOSFET Current Mirror
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Let's look at a specific numerical example using a MOSFET current mirror. What parameters do we have?
We have transistors M1 and M2, with K factors of 1 mA/VΒ² and 4 mA/VΒ² respectively, a reference current of 0.5 mA, and supply voltage of 12 V.
Correct! Now, what is the first step in calculating the output voltage for M1?
We use the equation for Vgs. Ignoring any lambda effect initially, we can calculate Vgs1 as Vth + Id/K.
Right! What do we find if we substitute our values?
We get Vgs1 as 2.5 V, leading us then to assess the output current for M2.
Correct! Thus, the output current here is crucial because it ties back to maintaining the system's performance. Remember, Iout = Iref multiplied by the ratio of K factors. Always keep track of K and I.
I can see how these values change based on transistor characteristics.
Exactly! Itβs fundamental to understand how varying these parameters affects your current mirror. Letβs summarize: Key calculations involve Vgs, Iout, and relationships between K factors.
BJT Current Mirror Concepts
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Now, let's move from MOSFET to BJT current mirrors. How do the parameters change?
BJTs consider base currents and saturation currents.
Correct! Can anyone tell me how the mirroring ratio is affected?
It can be 1:3 in some configurations, depending on the saturation current values.
Right! In our example, the reverse saturation currents are critical. How do we compute the reference current with a resistor at the input?
We can use Ohm's Law. If we know the supply voltage and resistance, we can derive the reference current.
Perfect! Always remember Ohm's Law - V = IR. Now, how would we account for a Beta-helper in this setup?
The Beta-helper reduces the effect of base current loss, keeping the current ratio closer to ideal.
Well said! Remember the concept of Beta-helper can significantly enhance current stability in your designs. Let's recap: BJTs add complexity through the base current and require adjustments for optimal performance.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The section provides detailed numerical examples demonstrating the use of current mirrors, focusing on both MOSFET and BJT configurations. It highlights calculations related to simple and improvised current mirrors and explores their applications in various amplifier circuits.
Detailed
Detailed Summary
In this section, we delve into numerical examples surrounding current mirrors, illustrating their applications in amplifier circuits. The lesson begins with a review of the theoretical base, followed by practical scenarios involving both MOSFET and BJT current mirrors.
Key Points:
- Current Mirrors: Understanding the fundamental concept of current mirrors, their configurations, and their operational principles is essential.
- Numerical Examples: The section elaborates on calculations for different types of current mirrors β starting with simple ones made from MOSFETs and transitioning to more precise configurations utilizing BJTs.
- Amplifier Applications: We look at how these current mirrors are implemented in amplifiers, namely single-ended amplifiers (common emitter and common source amplifiers) and differential amplifiers.
- Key Calculations: It covers calculations for output currents, minimum voltage requirements for operation, and factors affecting output resistance (like the Early effect in BJTs).
- Practical Insights: The insights aim to bridge theory with practical applications in electronics design, emphasizing the importance of current mirrors in modern analog circuits.
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Overview of Numerical Examples
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Todayβs topic of discussion it 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 section introduces the focus of the lecture, which is on numerical examples related to current mirrors and their applications in circuits. The idea is to bridge the gap between theoretical knowledge and practical application through calculations and examples.
Examples & Analogies
Think of this as learning to bake. You may have read through a recipe, understanding all the ingredients and methods. However, actually following the recipe and baking a cake gives you practical skills you can see and taste. Similarly, these numerical examples provide hands-on experience with current mirrors.
Types of Current Mirrors
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We shall start with numerical examples of simple current mirror. We may start with current mirror having MOSFET transistor and then we can go for current mirror using BJT and then we will be moving to numerical examples on improvised current mirror or more precision current mirror.
Detailed Explanation
The section outlines the structure of the lecture, indicating that it will cover different types of current mirrors, starting with simple ones made from MOSFETs and BJTs, then advancing to more precise designs. This progression allows students to build upon basic concepts before tackling more complex applications.
Examples & Analogies
Imagine climbing a ladder. You start with the bottom rung (simple current mirrors) and steadily move up to higher rungs (more complex designs). Each level offers a better view of how current mirrors work, helping you understand their full potential in circuits.
Application Circuits for Current Mirrors
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Subsequently, we will be talking about numerical examples on amplifiers which are using current mirror. Our main focus is on the current mirror. So, we may not be going into the aspect of the amplifiers, but primarily what are the advantages we can get using current mirror and what is the corresponding calculation we do?
Detailed Explanation
This chunk emphasizes that while the main discussion revolves around current mirrors, the connection to amplifiers will be briefly explored. The focus is on understanding the benefits of using current mirrors in amplifiers through practical calculations, enhancing the student's comprehension of their functionality in electronic circuits.
Examples & Analogies
Consider a toolbox where the current mirror is a multipurpose tool. You often won't need to know every detail about the toolβs function (like the amplifier), but understanding its advantages in fixing a problem makes you a more effective technician.
Example: Simple Current Mirror
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So, here we do have the example circuit where M1 and M2 are forming current mirror. We do have a reference current here and then we do have the application circuit here.
Detailed Explanation
In this step, a basic example of a current mirror circuit is introduced. The two transistors (M1 and M2) illustrate how a current mirror operates. This setting provides essential context for students to understand how the components work together to maintain a constant current based on a reference input.
Examples & Analogies
Think of M1 and M2 as two workers in a factory, where M1 sets the pace and M2 mirrors their output. If M1 produces 100 units, M2 is designed to replicate that production level. This mirroring effect is crucial for consistent performance in electronic circuits.
Calculating Voltages and Currents
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Let me start with the calculation of V_GS1 for I_REF = 0.5 mA so, that is I_DS = 0.5 mA. But then, V_GS1 = V_TH1 + 2.5 V.
Detailed Explanation
In this section, the calculation of gate-source voltage (V_GS) for the first transistor is initiated. The reference current is defined, and the equation is set up to find the voltages across the transistors based on their characteristics. This illustrates how to apply theoretical understanding in practical calculations.
Examples & Analogies
Imagine you're balancing your budget. You have a fixed income (reference current) and you want to see how much you can spend (V_GS). By adding your essential expenses (threshold voltage), you can figure out how much extra (the current) you have to work with.
Output Current Calculation and Impact of Ξ» Effect
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In terms of I_DS2 = I_DS1 Γ K_n2 / K_n1. So, that gives us I_DS2 current flow here it is 2 mA. So, now we can find next part it is that we need to find what is the minimum value of this V_DS2.
Detailed Explanation
Here, the calculation of output current for the second transistor is derived from the current flows and transistor characteristics. The relationship between the currents emphasizes the importance of transistor parameter K in determining how output current behaves, including the consideration of the Ξ» effect (channel length modulation) on the output voltage for effective circuit function.
Examples & Analogies
If I_DS1 is the weight one can lift and K_n2/K_n1 determines how efficiently that weight can be supported (like the pulleys in a lifting system), this chunk explains how efficient transmission of power (or current) is critical in design and must consider variables like Ξ» to ensure strong output.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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MOSFET and BJT current mirrors: Different configurations serve similar purposes but require different calculations.
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Output resistance calculation: The output resistance influences the performance of the current mirror in a circuit.
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Reference current setup: Establishing a reference current is crucial for the effective functioning of the current mirror.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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For a MOSFET current mirror with K values of 1 and 4 mA/VΒ² and a reference current of 0.5 mA, the output current is calculated based on the ratio of the K factors.
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In a BJT current mirror, the saturation currents determine the output current ratio, and incorporating a Beta-helper can minimize losses.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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In circuits with currents to track, a mirror's commit is to bring them back.
π Fascinating Stories
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Imagine a river (current), flowing steadily regardless of rain (voltage changes). The current mirror is the dam keeping the water level consistent.
π§ Other Memory Gems
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To remember key concepts: C.M.I. - Constant Current, Mirror, Inputs.
π― Super Acronyms
K.I.T. - K factor, Input current, Transistor configuration.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Current Mirror
Definition:
A circuit that maintains a constant output current regardless of voltage variations.
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Term: K Factor
Definition:
A proportionality constant related to the transconductance of a MOSFET, indicating how much the drain current varies with gate voltage.
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Term: Vgs
Definition:
The voltage difference between the gate and source terminals in a MOSFET.
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Term: Betahelper
Definition:
A transistor configuration used to reduce base current losses in BJT current mirrors.
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Term: Output Resistance
Definition:
The resistance seen by the load connected to the output of the current mirror, influencing current source characteristics.