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Understanding Current Mirrors
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Today we are going to discuss current mirrors and their importance in analog electronic circuits. Can someone tell me what a current mirror does?
A current mirror creates a current that is a replica of a reference current.
Exactly! However, current mirrors can exhibit non-ideal behavior. What do you think might cause these non-idealities?
I think base current loss might be one reason.
That's correct! Base current loss is a significant issue. To address it, we can use a technique called 'Beta-helper.'
What does Beta-helper do?
Good question! It helps in improving the effective current gain by compensating for the base current.
To remember these concepts, think of Beta as a superhero helping our current mirror to avoid losses!
In summary, current mirrors are essential in circuit design, and addressing non-idealities improves their performance.
Cascode Configuration
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Now, let's discuss how we can enhance the output resistance of the current mirror. Does anyone know what a cascode transistor is?
Is it a transistor placed on top of another transistor to provide better performance?
Precisely! By adding a cascode transistor, we can significantly increase the output resistance of the current mirror. Why do you think that is?
Because it improves the overall voltage drop across the output?
Correct! It isolates the output from the variations in input voltage, stabilizing the current mirror. Can anyone give me a scenario where this is particularly useful?
In amplifiers where we want to maintain constant current while changing the voltage.
Exactly! Cascode configurations are frequently used in modern amplifier designs. Remember: cascode for resistance!
In summary, cascode transistors improve output resistance and performance, especially in amplifiers.
Small Signal Analysis
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Next, let's tackle small signal analysis. Why is it important in circuit design?
It helps us understand how circuits respond to small variations in signals.
Exactly! To derive the small signal output resistance of a current mirror, we need the small signal model for each transistor. Who remembers how to represent a transistor in this model?
We use transconductance and small signal parameters like r_o.
Very good! The output resistance R_out can be computed as multiple resistances appearing in parallel. Can anyone recall that formula?
R_out = g_m * r_o, right?
Close! Itβs actually R_out = g_m * (r_o1 || r_o2 || ...). Let's simplify using given parameters to find an example resistance.
To recap, understanding small signal parameters is key in analyzing the stable operation of our designs.
Introduction & Overview
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Quick Overview
Standard
The section elaborates on the numerical examples of current mirrors, exploring methods to improve their performance by considering factors like base current loss and early voltage. It describes the implementation of cascode transistors to enhance output resistance and includes both MOSFET and BJT examples.
Detailed
DC and Small Signal Analysis
This section covers the numerical examples on current mirrors as part of the analysis of DC and small signal behaviors in analog electronic circuits. The primary focus is on enhancing the output resistance of current mirrors using cascode transistors to address non-ideality factors such as base current loss and early voltage effects that may arise from transistor characteristics.
The analysis starts with a MOSFET version of a current mirror and extends to discuss BJTs, illustrating how adjustments in component sizes, threshold voltages, and bias resistor values affect performance. The discussions also delve into practical calculations to find associated voltages and current flows through different branch circuits, culminating in a thorough understanding of how to calculate small signal output resistance. Therefore, the significance of current mirrors in advanced analog circuit applications is emphasized, showcasing their role in achieving higher performance specifications.
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Audio Book
Dive deep into the subject with an immersive audiobook experience.
Introduction to Cascode Current Mirrors
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So, in the next example, we are starting with MOSFET version and then after that we will go to the BJT version. And so, here transistor-3 it has been added on the main current mirror. So, the main current mirror it is constructed by M1 and M2 and whatever the reference current it is coming, it is mirroring here.
Detailed Explanation
In this example, the focus is on enhancing a current mirror circuit by adding an additional transistor, referred to as transistor-3. The primary goal of this addition is to improve the mirrored output current's accuracy and stability. The current mirror is initially built using two MOSFETs, labeled M1 and M2. When a reference current is applied, the circuit ensures that this current is mirrored efficiently through the additional transistor. This setup not only boosts performance but also aids in overcoming issues such as variations in output due to transistor base current losses.
Examples & Analogies
Think of the current mirror like a photocopier that duplicates a document. In this case, if the photocopier includes a better quality scanner (transistor-3), the duplicated documents (output current) will be clearer and more consistent, reflecting the original (reference current) more accurately.
Understanding Operating Regions
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To have meaningful operation of this circuit, we require its gate voltage V_BIAS, denoted here. V_BIAS should be sufficiently high so that transistor-2 and maybe 3 also are in saturation region of operation.
Detailed Explanation
For the current mirror circuit to function correctly, it is critical that the gate voltage (V_BIAS) be set to an adequate level. This voltage controls whether the transistors are in the 'saturation' operating region, which is crucial for maximizing the efficiency of the current mirroring process. The saturation region is where the transistor can maintain a constant output current despite changes in output voltage, thus ensuring stable operation. Properly setting V_BIAS helps maintain this operational integrity for transistors 2 and 3.
Examples & Analogies
Imagine turning up the brightness on your computer screen. When itβs bright enough, you can clearly see and interact with everything without distortion. Similarly, in the circuit, having enough gate voltage (V_BIAS) ensures that the transistors operate clearly and correctly.
Calculating Currents and Voltage Drops
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So, we can write this V_GS - V_th = R_BIAS Γ I_DS. And on the other hand, we know that expression of I_DS of transistor-1 and then ( ) .
Detailed Explanation
To find the value of the gate-source voltage (V_GS) and its relationship with threshold voltage (V_th), we utilize Ohm's law to account for voltage drop across the bias resistor (R_BIAS). This equation helps establish how the current flowing through the transistor (I_DS) and the resistance of the circuit affect the voltage across the transistor. By manipulating these values, we can derive how changing one variable influences the others, vital for designing efficient circuits.
Examples & Analogies
Consider a water flow system. The water pressure (V_GS) has to be sufficient to push water through a pipe (R_BIAS). If the resistance of the pipe is too high (large R_BIAS), you need a higher pressure (V_GS) to maintain the same water flow (I_DS).
Finding Output Resistance
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So, we need to find what will be the small signal output resistance. And R_out = g_m3 r_o3 + r_o2.
Detailed Explanation
Once we have the current flowing in the circuit, the next step is to calculate the small signal output resistance (R_out). This is crucial as it affects the overall gain and stability of the circuit. The output resistance is determined by the transconductance (g_m) of transistor-3 and its output resistance (r_o3) in conjunction with the output resistance of transistor-2 (r_o2). This relationship helps designers understand how variations in input voltage can affect output current, thereby refining the circuit's performance.
Examples & Analogies
Think about a sponge used to soak up water. If the sponge can hold more water (high output resistance), it can handle fluctuations in water input without spilling. Similarly, in our circuit, a high output resistance means it can manage changes in input without significantly affecting the output current.
Challenges with Biasing and Control
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So that is how we can set this resistor so that the current here in the left branch it is 1 mA.
Detailed Explanation
Setting the bias resistor (R_BIAS) accurately ensures that the current flowing through the specified branch reaches the desired level (1 mA in this case). This is not only about selecting a resistor of the right value but also understanding how it interacts with other components and voltage supplies in the circuit. The calculations performed thus far enable you to adjust this resistor to achieve the necessary current, which is vital for the function of the entire current mirror.
Examples & Analogies
Consider a recipe where you need to add just the right amount of salt to a dish. Too little makes it bland, too much can ruin the flavor. Similarly, properly setting R_BIAS is essential for the current to be just rightβonly then can the circuit function as intended.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Current Mirror: A circuit that produces a current matching a reference current, essential in analog designs.
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Cascode Configuration: Enhances output resistance of current mirrors to improve circuit performance.
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Small Signal Analysis: A method to analyze the response of circuits to small variations, critical for stability.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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Example 1: Calculation of a MOSFET current mirror output current based on threshold voltage and bias conditions.
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Example 2: Using a BJT current mirror in a common emitter amplifier to demonstrate practical application and performance.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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Cascode up high, keeping currents nigh, improving the gain, resisting the pain!
π Fascinating Stories
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Imagine a superhero named 'Beta' helping current mirrors fight against base current loss. With a cape of ideality, they soar through circuits, ensuring steady currents.
π§ Other Memory Gems
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To remember the steps for DC analysis: 'Current Cascodes Surely Stabilize!' (C for Current, CS for Cascode, S for Stabilize)
π― Super Acronyms
DAS - DC and Small Signal Analysis
- D: for Derivation
- A: for Analysis
- S: for Stability.
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 produce a current that is proportional to a reference current.
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Term: Cascode Transistor
Definition:
A transistor configuration that improves the output resistance of a current amplifier by cascading transistors.
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Term: Betahelper
Definition:
A technique used to counteract base current loss in current mirrors by exploiting the transistor gain.
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Term: Small Signal Model
Definition:
An equivalent circuit used to analyze the behavior of a circuit for small variations around a bias point.
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Term: Transconductance (g_m)
Definition:
A parameter that measures the change in output current divided by the change in input voltage in a transistor.
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Term: Output Resistance (R_out)
Definition:
The resistance seen by the output terminal of a circuit, important for determining its ability to maintain constant output current.