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Introduction to Current Mirrors
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Today, we're going to discuss current mirror circuits, both BJT and MOSFET versions. Who can tell me what a current mirror does?
A current mirror is used to provide a constant current in a circuit, mimicking a reference current!
Exactly! It uses a reference current to generate an output current that ideally mirrors this reference. Can anyone explain how this works for a MOSFET current mirror?
In a MOSFET current mirror, the first transistor is diode-connected, maintaining a voltage that is passed to the second transistor.
Great point! We set the VGS of the first transistor to establish a reference for the second. This reference current is paramount in determining the output current I2.
And it sounds like if the output voltage changes, the current mirror still tries to keep the I2 stable, right?
That's right! This leads us to the concept of output resistance, which we will discuss next. Let's recap: current mirrors help in providing a constant current by mirroring a reference current. Any questions before we move on?
Output Current Expression for MOSFETs
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Let's derive the output current expression for the MOSFET current mirror. Can anyone remind me how we start this derivation?
We assume that the transistor is in saturation and that VGS is equal for both transistors.
Correct! The output current I2 ultimately depends on the reference current I1 and the aspect ratio of the transistors. What do we need to consider about Ξ» or the channel length modulation?
It adjusts the current based on the effect of VDS on the output, right? The output current gets influenced by the non-ideality factor.
Exactly! This non-ideality factor can increase or decrease the current based on how VDS2 relates to VDS1. So, the expression for I2 becomes I1 times this factor. What is this factor again?
Itβs (1 + Ξ»(VDS2 - VDS1)).
Perfect! Remember that if VDS2 is smaller than VDS1, this factor can shrink, affecting our output current. Any questions on this derivation?
Understanding Output Resistance
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Now, let's talk about output resistance, denoted as R_out. Why is this parameter important for the current mirror?
Higher output resistance means the current is less dependent on changes in output voltage.
Exactly! A high output resistance helps maintain a stable I2 despite varying load conditions. Can anyone explain how we achieve high output resistance?
By keeping the transistors in the saturation region and ensuring proper voltage levels are maintained, like VDS being greater than V(th).
Well said! Thus, the circuit needs to be designed to maintain an adequate voltage across the mirrors to keep them operational in saturation. Recapping, a higher R_out means better current stability. Questions?
BJT Current Mirrors Compared
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Let's shift gears to BJTs. How does the BJT current mirror compare to the MOSFET current mirror we've just discussed?
They both aim to mirror currents, but the BJT versions have to consider base currents which affect the equality of I1 and I2.
Right! In BJTs, I2 is not directly equal to I1 due to base current losses. What can we write to account for this in our expressions?
We should include the base currents in our calculations, which can be expressed in terms of I2 and I1.
Exactly! This complicates the model slightly but allows us to maintain similar current mirroring principles. Remember, the goal is to keep the transistors ideally in their active region. Good recap here!
Introduction & Overview
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Quick Overview
Standard
In this section, we delve deeply into current mirror circuits, specifically focusing on the output current expressions, output resistance characteristics, and the non-ideality factor. We establish expressions that describe these relationships for both MOSFET and BJT configurations, illustrating how changes in voltage can affect current output in practical applications.
Detailed
Detailed Summary
In this section, we analyze current mirror circuits constructed with both BJTs and MOSFETs. The output current, noted as I2, can be expressed in terms of a reference current I1 and the aspect ratio of the transistors involved. For MOSFETs, the output current expression assumes the device is in saturation and is influenced by the transconductance and various voltage parameters, leading to the concept of the non-ideality factor.
Key Points:
- Current Output Expressions: The expressions for output current of the current mirror rely on the saturation operation of the transistors and their respective threshold voltages.
- Non-Ideality Factor: This factor reflects the real-world performance deviation from ideal behavior, represented as (1 + Ξ»(VDS2 - VDS1)). Depending on whether VDS2 is greater or lesser than VDS1, this factor can either increase or decrease the output current.
- Output Resistance: Defined as the output resistance of a current mirror, it is a critical parameter affecting the circuit's performance, especially when defining the range of output current with respect to varying voltage levels in the load.
- BJT vs. MOSFET: The BJT current mirror showcases similar principles where the collector currents relate through the base current losses and saturation characteristics. Both designs illustrate the impact of output resistance, where higher output resistance leads to improved current stabilization in feedback applications.
In conclusion, this section highlights both theoretical foundations and practical considerations in designing robust current mirrors, emphasizing the importance of maintaining transistors within their saturation regions.
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Understanding Output Current Expression
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So, in summary what you are saying is that the expression of the application circuit current or I2 it is given by its nominal value multiplied by a plus the additional component which is defined by the rds. In fact, if you see this expression this part is the . So, this = {1+ Ξ»( )}.
Detailed Explanation
This chunk explains how to express the output current in a current mirror. The output current (I2) is described as having a nominal value (the expected current based on ideal conditions) and an additional component that accounts for non-ideality (real-world factors). This additional component is represented by the factor Ξ», which relates to the output resistance (rds) of the transistors involved. Essentially, the output current can deviate from the ideal value due to changes in voltage, and this equation quantifies that deviation.
Examples & Analogies
Think of the output current like the speed of a car on a highway. Ideally, you'd expect the car to travel at a constant speed (the nominal value). However, due to wind resistance and road conditions (non-ideality factors), the actual speed can vary. Here, the nominal speed is akin to I2, while the factors causing changes in speed represent the additional component in the equation.
The Concept of Non-Ideality Factor
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So whenever, we will be talking about non-ideality factor is essentially that is 1 plus this additional component. Of course, this V if it is higher than V then this will be +ve; otherwise, it will be βve. So that is the expression of the output current.
Detailed Explanation
The non-ideality factor represents how real-world conditions have an impact on the expected behavior of the circuit. It's defined as 1 plus the additional component derived from output resistance. If the drain-source voltage (Vds) is higher than a specified value, it contributes positively to the current, otherwise negatively. This concept is crucial for understanding how variations in operating conditions affect performance.
Examples & Analogies
Imagine baking a cake. The recipe gives you an ideal time to bake (like the expected output current), but if your oven temperature is off (representing non-ideality), your cake may take longer or shorter to bake than expected. The adjustment based on the ovenβs actual performance relates to the non-ideality factor.
Determining Output Resistance
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So, we can say that this R_out is essentially influenced by the output resistance of the transistors. This r_ds is a crucial part, and output resistance is high only when the transistor is kept in saturation and should meet specific conditions.
Detailed Explanation
Output resistance (R_out) describes how well the current mirror maintains a consistent output current in the presence of varying load conditions. It's fundamentally linked to the transistor's output resistance (r_ds), which is maximized when transistors operate in a saturation region. For optimal performance, certain voltage conditions must be met so that the transistors remain in saturation. If these conditions fail, the output resistance can decrease, leading to inconsistent performance.
Examples & Analogies
Consider the output resistance like the stability of a tightrope walker. The tighter the wire (higher output resistance), the more stable the walker's performance (consistent current). If the wire is loose (lower output resistance), any slight disturbance can throw the walker off balance, just as varying loads would affect the output current in a circuit.
The Role of Saturation Voltage
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So, we can say that this voltage here should be at least higher than V_th to ensure that the output resistance remains high. Typically, its value it is 0.3 V.
Detailed Explanation
The saturation voltage (V_th) is the minimum voltage required across a transistor to keep it in saturation, where it operates most effectively. To achieve high output resistance, the voltage across the transistor must always exceed this threshold. For instance, a standard value for this saturation voltage may be around 0.3V. This emphasizes the need for proper biasing and voltage levels in circuit design to ensure reliable operation.
Examples & Analogies
Think of saturation voltage like a minimum height required to ride a rollercoaster. If a rider is not tall enough (voltage not high enough), they cannot go on the ride (transistor doesn't enter saturation), leading to an unstable or non-functioning thrill. Therefore, ensuring riders meet the height requirement mirrors ensuring voltages exceed the saturation threshold.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Current Mirrors: Circuits that replicate a reference current for consistency in outputs.
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Output Current: A critical measure reflecting the default performance of the mirror under varying loads.
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Non-Ideality Factor: Indicates how real-world conditions modify the intended mirror current.
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Output Resistance: The higher it is, the more stable the current output under electrical load conditions.
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Channel Length Modulation: Affects the performance of MOSFET mirrors at different signal levels.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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A MOSFET current mirror in a simple amplifier circuit where output current remains stable under load variation.
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Comparing the output performance of a BJT current mirror by demonstrating the effects of base current on total output.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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Current mirrors provide a steady flow,
π Fascinating Stories
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Imagine a factory where every machine needs the same amount of power. Just like a manager ensures every team runs on the same energy, current mirrors do the same for transistors, ensuring a steady supply regardless of changes in demand.
π§ Other Memory Gems
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For Remembering outputs: "More Current Requires Good Resistance" β denotes the need for higher output resistance for stable current.
π― Super Acronyms
CORN - Current Output Requires Non-ideality factor to be minimized.
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 designed to copy a current through one active device by controlling the current in another active device.
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Term: Output Current (I2)
Definition:
The current delivered by the current mirror, ideally equal to a reference current, adjusted by non-ideality factors.
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Term: NonIdeality Factor
Definition:
A term which accounts for real-world deviations from the ideal behavior in current mirrors, typically represented as (1 + Ξ»(VDS2 - VDS1)).
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Term: Output Resistance (R_out)
Definition:
The dynamic resistance seen by the load connected to the output of the current mirror, indicating current stability.
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Term: Channel Length Modulation (Ξ»)
Definition:
A phenomenon where output current changes with varying drain-source voltage due to the finite size of the MOSFET.