86.4 - Continuation of Current Mirror Example
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MOSFET Current Mirror Basics
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Today, we'll start with understanding the basics of MOSFET current mirrors. Can anyone tell me why current mirrors are essential in circuits?
They help maintain the same current in multiple branches of a circuit, right?
Exactly! And they ensure that even with voltage variations, the current remains consistent. Now, let’s explore a simple MOSFET current mirror example.
What about the values of current and voltage in that example?
Good question! We'll calculate those shortly, so remember that the threshold voltage is vital to keep the transistors in saturation.
In this case, can anyone recall the equation for the output current in a current mirror?
I think it involves the reference current and the transistor 'K' factors?
Absolutely! The output current, I_DS2, is influenced by I_REF and the ratio of the K values. Let's make this memorable with the acronym KIR – Keep It Referenced.
To summarize, we discussed the basics of MOSFET current mirrors, their importance in maintaining current consistency, and began our example where we’ll calculate specific current outputs.
Calculating Output Current
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Let’s jump into calculating the output current for our MOSFET current mirror example. Can you remind me what the reference current is?
It was 0.5 mA, right?
Great recall! Now, applying our equations, if we have K values of 1 mA/V^2 and 4 mA/V^2 for our two transistors, how do we compute I_DS2?
We can use the equation involving the reference current and the K ratio.
Exactly! So, if we find I_DS2 = 2 mA, what does that suggest about our current mirror's performance?
It suggests good matching, since the output is higher than the reference, indicating it can handle more load.
Spot on! Now remember, when we vary the V_DS2, the current changes too. Let’s ensure we grasp this concept clearly by doing an example problem together.
In summary, we calculated the output current using K ratios and discussed performance insights based on our examples.
Non-ideality Factors in BJTs
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Now let's shift our focus to BJTs. What do you think are the typical differences we should anticipate when moving from MOSFETs to BJTs?
There might be differences in how we handle non-ideality, especially concerning base current effects.
Correct! In BJTs, we also need to consider reverse saturation current and Early voltage. Who remembers how these factors affect the current mirror’s output?
The Early voltage affects the output resistance and can lead to variations in current for each branch.
Excellent point! Let’s evaluate an example with a BJT, focusing on calculating the reference current and considering base current losses. Remember: For BJTs, we can often neglect the base current losses if the beta is high.
As we summarize, BJTs need careful consideration regarding non-ideality factors like the Early voltage and how it influences performance. We’ll be applying these concepts in our next numerical examples.
Beta-Helper Circuits
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Today, we’re going to learn about Beta-helper circuits. Who can remind me why beta-helper circuits are used in current mirrors?
They help reduce base current losses, which can affect the accuracy of our current mirrors.
Exactly right! When we introduce a Beta-helper transistor, we are essentially nullifying a portion of these losses. Can anyone recall how we would adjust our circuits with a Beta-helper?
We need to readjust the resistor values to ensure the reference current stays at 0.5 mA.
Yes! It keeps our reference current stable, allowing for better mirroring ratios. Let’s work through an example of how to calculate the new non-ideality factors with the Beta-helper in place.
To conclude, we reviewed how adding a Beta-helper circuit can significantly enhance performance through reduced non-ideality, effectively stabilizing our outputs.
Summary and Importance of Current Mirrors
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Let’s wrap up our discussions on current mirrors. Can anyone summarize why current mirrors are essential in analog circuits?
They provide stable bias currents and consistency across multiple devices!
Correct! As we’ve seen through both MOSFET and BJT examples, variations in output current and voltage are crucial factors to analyze. We also discussed how to manage non-ideality through techniques like Beta-helper configurations.
Will these concepts apply when designing amplifiers using current mirrors?
Definitely! Current mirrors are widely used in differential and single-ended amplifier configurations. Remember the acronym SMC: Stability, Matching, and Consistency.
To summarize, we've learned the fundamentals of current mirrors, their calculations, BJT impacts, and enhancement techniques. Keep these concepts in mind for your upcoming designs!
Introduction & Overview
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Quick Overview
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In this section, we explore numerical examples involving current mirrors, focusing on MOSFETs and BJTs. The discussion highlights significant calculations like determining output current and minimum voltage for saturation, as well as analysis of non-ideality factors and output resistance in different configurations.
Detailed
Detailed Summary
This section dives deep into the practical aspects of current mirrors used in analog electronic circuits. It first describes numerical examples related to simple current mirrors made from MOSFETs, focusing on computations for output currents, gate-source voltages, and thresholds. Key calculations are performed to illustrate how variations in supply voltage affect output currents, demonstrating the effects of non-ideal characteristics like the Early voltage and base current losses in BJTs.
The section further elaborates how to calculate the output current is derived using the equations for ideal current mirrors and includes practical examples that provide clarity on how to handle parameters. Additionally, designer considerations around base current and Early voltage are discussed, culminating in the exploration of Beta-helper circuits to mitigate losses and improve current mirror precision.
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Overview of Current Mirror Calculations
Chapter 1 of 6
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Chapter Content
So here we do have the example circuit where M and M are forming current mirror. We do have a reference current here and then we do have the application circuit here. So in this example, the of transistor-1 and transistor-2 along with the K factor, it is given. For transistor-1, we do have 1 mA/V2. For transistor-2, we do have 4 mA/V2. And let me assume that both the transistors are having equal threshold voltage of 1.5 and then we do have the reference current = 0.5 mA, and then supply voltage it is 12 V.
Detailed Explanation
In this initial segment, we're examining a circuit that includes two MOSFET transistors configured as a current mirror. The K factor represents the transconductance of the transistors, with one having a value of 1 mA/V² and the other 4 mA/V². A critical assumption made is that both transistors share the same threshold voltage (Vth) of 1.5 V, while the reference current is set to 0.5 mA and the supply voltage is 12 V. These parameters will influence the circuit’s performance and the current that flows through each transistor.
Examples & Analogies
Think of the current mirror as a water faucet. The reference current is like adjusting how wide you want the faucet to open, and the supply voltage is akin to how much water pressure you have available in the pipes. The two different K factors represent two types of faucets that can dispense water at different rates, affecting how much water flows (or current) when you open them.
Calculation of Output Current
Chapter 2 of 6
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Let me start with the calculation of V or for I = 0.5 mA so, that is the I = its corresponding K which is 1 mA/V2 by 2 × ( ). So note that for this calculation, we are ignoring ( ).
Detailed Explanation
Here, we calculate the value for the gate-source voltage (Vgs) for transistor-1, using the reference current of 0.5 mA and the given K factor for that transistor. The formula used involves relationships among the currents and the defined K values. It's important to note that we are initially ignoring the channel-length modulation effect (lambda), which simplifies our calculation. The result from this calculation provides us with the necessary Vgs to further explore how the current behaves in the circuit.
Examples & Analogies
This step is like figuring out how much you need to open the faucet (Vgs) to get the desired amount of water (current). Ignoring the lambda effect can be compared to assuming the water pressure is constant, simplifying your calculations for now.
Determining Minimum Drain-Source Voltage
Chapter 3 of 6
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Now we can find next part it is that we need to find what is the minimum value of this V of transistor-2, particularly for transistor-2 for proper operation of the circuit.
Detailed Explanation
In this section, we delve deeper into the operational requirement for transistor-2, specifying that it must remain in saturation for the current mirror to function properly. This necessitates a minimal voltage from the drain to the source (Vds) that must be greater than the gate voltage minus the threshold voltage. Calculating this value establishes the lower limit for Vds to ensure effective operation, which is set at 1 V in our scenario.
Examples & Analogies
Imagine that for your faucet to properly dispense water, there has to be a certain amount of pressure in the pipes. If the pressure drops too low (in this case, the Vds being lower than the calculated minimum), the faucet won’t work correctly.
Continuing with Finite Channel-Length Modulation
Chapter 4 of 6
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Chapter Content
So, in the next slide we do have, so, what we have here it is all the other parameters remaining same. In fact, it is continuation of the same example, but we are considering λ = 0.01 V‒1.
Detailed Explanation
This next part focuses on re-evaluating our previous calculations by including a finite value for the channel-length modulation factor (λ). This adjustment takes into account the fact that when Vds changes, it can affect the output current slightly. We explore how different values for Vds lead to variations in the output current, and adjust our equations accordingly for two specific cases.
Examples & Analogies
This is like realizing that the water pressure from the main supply line isn't constant, and calculating how slight changes in pressure affect the flow out of your faucet. By considering channel-length modulation, we're better modeling the real-world behavior of the circuit.
Calculating Output Resistance
Chapter 5 of 6
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So, small signal output resistance at the output of the current mirror R . So, that is the voltage difference and we know that ΔV it is 3 V and the corresponding variation of this ΔI it is 0.06 mA.
Detailed Explanation
The small signal output resistance is defined as the ratio of the change in voltage (ΔV) to the change in current (ΔI) resulting from small variations around the operating point. By measuring how the output current responds to variations in output voltage, we can calculate a resistance value that indicates how stable the current mirror is under small-signal conditions. In this specific case, we found it to be 50 kΩ.
Examples & Analogies
This output resistance can be likened to how sensitive a faucet is to the pressure in the water pipes. A higher resistance means the water flow changes less with fluctuations in pressure, indicating a more stable faucet.
Introduction to BJT Current Mirror
Chapter 6 of 6
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So now, if we continue this exercise for say, other condition. So, let we see in the next slide the third part of this, no now we are going to BJT.
Detailed Explanation
Having covered the workings of a current mirror using MOSFETs, this transition introduces a current mirror built using BJTs (Bipolar Junction Transistors). We will follow similar concepts but must also consider the unique parameters and characteristics of BJTs, such as their different operational principles and parameters for calculating current flow.
Examples & Analogies
Switching from MOSFETs to BJTs is like moving from one style of kitchen faucet to another; they're both designed to dispense water, but they operate on different mechanics and designs that need careful consideration in calculations.
Key Concepts
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Current Mirror Functionality: A current mirror supplies a constant output current despite changes in voltage.
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Parameter Analysis: Understanding K factors and reference currents is crucial for accurate outputs.
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Non-Ideal Effects: Consideration for Early voltage and base current in BJTs affects accuracy.
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Performance Enhancements: Techniques such as Beta-helper transistors improve current mirror precision.
Examples & Applications
Example 1: Output current for a MOSFET current mirror with known K values.
Example 2: Calculating changes in output current using different V_DS values.
Example 3: Analyzing a BJT current mirror considering base current losses.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
A current mirror, stable and bright, keeps currents aligned, just right.
Stories
Once there was a master mirror who reflected light, but it also needed to keep the current flowing right, ensuring every part of its realm felt equal light.
Memory Tools
Remember 'MIRROR' - Maintain Individual Reference, Resist Output Resistance.
Acronyms
KIR - Keep It Referenced (for maintaining current in mirrors).
Flash Cards
Glossary
- Current Mirror
A circuit that produces a constant current that is independent of voltage.
- MOSFET
Metal-Oxide-Semiconductor Field-Effect Transistor; used to control electronic signals.
- BJT
Bipolar Junction Transistor; a type of transistor that uses both electron and hole charge carriers.
- Output Resistance
The resistance looking into the output of a circuit, which affects current delivery.
- Betahelper
A transistor used in current mirrors to mitigate the effect of base current loss.
- Early Voltage
A parameter in BJTs that reflects the output characteristics and impacts saturation behavior.
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