86.7 - Improvised Current Mirror
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Introduction to Current Mirrors
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Welcome, everyone! Today, we will explore current mirrors. Can anyone tell me what a current mirror is and why it might be useful in circuits?
Is it a circuit that helps maintain a constant current?
Exactly! A current mirror ensures a constant reference current flows through it, which can be replicated in other parts of a circuit. This principle is crucial in analog design.
What are the types of transistors used in current mirrors?
Great question! Current mirrors can be assembled using MOSFETs or BJTs. Each type has its characteristics based on current handling and efficiency.
Remember the acronym - **M**atching transistors, **O**utput current stability, **S**aturation conditions, **F**ine-tuning parameters.
Any last questions before we move on to numerical examples?
Numerical Example with MOSFET
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Let’s analyze an example MOSFET current mirror with certain parameters. We have a reference current of 0.5 mA and K values for the transistors. Can someone remind me what K represents?
Isn't K the transconductance parameter?
Correct! It helps in determining the current mirroring capability. So, if transistor-1 has a K of 1 mA/V², and transistor-2 has 4 mA/V², we can calculate the output current. Let’s derive the current for transistor-2 together.
I think it’s proportional to their K values! We should set up the equation.
Exactly, and by substituting correctly, we find the current through transistor-2. Anyone remembers how to calculate the saturation voltage?
We consider the gate voltage minus the threshold voltage.
Right! That forms the basis of our next exercise. Let’s wrap up this session with a summary. Current mirrors replicate currents, and understanding their working via example calculations helps in better circuit designs.
Precision Current Mirrors
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Now, let’s talk about precision current mirrors. Why do we need to improve current mirrors?
To reduce errors from non-ideal effects like the Early effect?
Absolutely! Enhancements such as adding resistors and using different configurations can help counteract those effects. Who could tell me what the Early effect is?
It’s the variation in output current with a change in collector-source voltage due to the base-width modulation in BJTs.
Well explained! In our upcoming example, we will further calculate the effects of base current loss in BJTs. Ready for it?
BJT Current Mirror Example
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Alright, let’s switch to a BJT current mirror. We have a configuration with specific saturation currents. Who can explain what affects the mirroring ratio?
The saturation currents of the transistors and their beta value should affect it.
Exactly! Let's calculate the reference current here. Can someone help derive this reference current?
We use the formula based on reverse saturation current and the given resistor!
Good! Now considering base current loss, what’s the impact on our output current?
It reduces the effective current we can mirror, right?
That’s correct! Let's summarize; understanding BJTs in current mirrors helps improve overall precision when accounting for losses.
Introduction & Overview
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Quick Overview
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In this section, we delve into improvised current mirrors, providing numerical examples that showcase their construction and performance using both MOSFETs and BJTs. Numerical applications emphasize practical calculations and the critical parameters that influence the current mirror's effectiveness.
Detailed
Detailed Summary
This section examines improvised current mirrors, which are essential components in analog electronic circuits. The discussion begins with numerical examples highlighting the construction and operation of simple current mirrors built using MOSFETs. It moves on to consider more intricate configurations, particularly focusing on precision current mirrors that mitigate non-ideal behaviors such as the Early effect and base current loss in BJTs. We explore the calculations required to determine reference currents and output currents, addressing the impact of individual component parameters like threshold voltage, saturation current, and non-ideality factors. The coverage also includes application scenarios, focusing on amplifiers that utilize current mirrors for improved performance and enhanced output resistance.
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Introduction to Improvised Current Mirrors
Chapter 1 of 4
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Chapter Content
Now we can go for more precision current mirror and as we can see here, if I consider finite values of β’s and then early voltage, the ratio instead of 1∶3, it is becoming different, slightly different though. There may be some precision cases, precision applications where this much of difference may not be still acceptable.
Detailed Explanation
In this chunk, we introduce the concept of improvised current mirrors, emphasizing that they are designed to provide greater precision than standard current mirrors. The precision differences arise from accounting for variables like the transistor’s beta (β), which represents current gain, and the early voltage, which affects current flow in the saturation region. Instead of maintaining a current ratio of 1:3, adjustments can lead to slightly varying ratios, making the circuit more adaptable to specific precision requirements.
Examples & Analogies
Think of a standard current mirror as a recipe for cookies that yields a specific taste. If you want the cookies to taste slightly different without completely changing the recipe, you might adjust the amount of sugar or vanilla. Similarly, improvised current mirrors tweak technical parameters like the transistor’s beta to achieve a more precise performance tailored to specific applications.
Beta-Helper Circuit
Chapter 2 of 4
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Chapter Content
to take care of the base current loss, as we have said that we can have a Beta-helper circuit here, so that the current loss to the base of this third transistor which is much smaller than whatever the base current is going to Q and Q which is referred as Beta-helper.
Detailed Explanation
The Beta-helper circuit is an innovative solution to address current loss in the base of the transistors within the current mirror. When transistors like Q1 and Q2 are used, small currents may escape into their bases, reducing the delivered output current. By using an additional transistor (the Beta-helper), which has a high current gain, the losses at the bases can be significantly minimized. This setup helps ensure more consistent performance from the current mirror, leading to precision without substantial current loss.
Examples & Analogies
Imagine a water distribution system where some water leaks through small holes in the pipes. The Beta-helper acts like a repair team that efficiently seals the leaks, ensuring most of the water reaches its destination. In electronic circuits, it does this by preventing base current loss, thereby securing the total output current.
Improving Performance with Beta-Helper
Chapter 3 of 4
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So, in the next slide we do have the Beta-helper circuit drawn for you. We do have Q here and as I said there this is approximately 0.6 and here also, it is approximately 0.6. So, we do have 1.2 V. So what we are doing here, to retain this current of 0.5 mA in this numerical example, we are readjusting this R to 21.6 kΩ. And that gives us the I current, it is the same as 0.5.
Detailed Explanation
In this part, the Beta-helper circuit is illustrated, showing how it effectively maintains a consistent reference current of 0.5 mA. To achieve this, a resistor (R) is adjusted to a new value of 21.6 kΩ to accommodate the characteristics of the circuit with the Beta-helper included. This reconfiguration demonstrates a practical way to enhance the performance of the current mirror while still meeting design specifications.
Examples & Analogies
Consider fine-tuning a musical instrument. Just like you might adjust the tension of guitar strings to achieve the perfect pitch, tweaking the resistor value in the Beta-helper circuit optimizes the flow of current, ensuring the electrical output matches what is desired in a precise manner.
Non-Ideality Factor with Beta-Helper
Chapter 4 of 4
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So, you may recall the non-ideality factor in presence of the β the Beta-helper is.
Detailed Explanation
This chunk discusses the concept of non-ideality factors when using a Beta-helper in the current mirror configuration. The non-ideality factor is crucial in evaluating how effective the Beta-helper circuit is in minimizing losses. It considers the ratios of current gains and other circuit parameters, allowing designers to understand how closely the behavior of the improvised current mirror can approximate the ideal conditions.
Examples & Analogies
Think of a team preparing for a presentation. If everyone contributes their best quality, you get a flawless performance (ideal case). If one member struggles, the presentation suffers. A Beta-helper boosts performance by ensuring all team members stay engaged and effective, thus improving the overall output of the team, akin to refining the non-ideality factor.
Key Concepts
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Current Mirror: A circuit that reproduces a specific current.
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BJT and MOSFET Usage: Different transistors affect circuit performance variably.
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Base Current Loss: Affects current mirror precision in BJTs.
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Early Effect: Alteration in output characteristics due to voltage shifts.
Examples & Applications
For a MOSFET current mirror with a reference of 0.5 mA and K values indicating transconductance, calculate the output currents.
In a BJT current mirror configuration, if Q1 has a saturation current of 9.5e-14 A and Q2 has 2.85e-13 A, determine the mirroring ratio.
Memory Aids
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Rhymes
Mirrors must reflect, currents must stay, keep them together in the best way!
Stories
Once, in a lab, two transistors wanted to share their power equally. They created a mirror circuit that kept their energy flowing perfectly, without losing any juice!
Memory Tools
MOSFETs vs BJTs: Mirroring Outs, Simplifying Flows—Easy-to-use Transistors.
Acronyms
PENA - **P**recision, **E**arlier voltage effects, **N**on-ideality, **A**pplications of Current Mirrors.
Flash Cards
Glossary
- Current Mirror
A circuit configuration that replicates a current source to maintain a constant current output.
- MOSFET
A type of field-effect transistor used to amplify or switch electronic signals.
- BJT
A bipolar junction transistor, a type of transistor consisting of three layers of semiconductor material.
- Early Effect
A phenomenon that describes the variation of output current in a BJT with base-collector voltage.
- Reference Current
Current specified at the input of the current mirror to set the output current.
- Beta (β)
The current gain factor in BJTs, the ratio of output current to base current.
- Transconductance (K)
A measure of the efficiency of a transistor, indicating how well it converts input voltage to output current.
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