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Understanding Current Mirrors
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Today, we'll dive into the concept of current mirrors. Can anyone explain what a current mirror does?
A current mirror replicates a current through a load by using a reference current!
Exactly! And what can negatively impact how accurately the current is mirrored?
Base current loss could be one factor. It affects the output!
Right! That's where Beta-helper comes in to enhance our circuit performance. Beta helps us mitigate these non-idealities.
Are there other non-idealities we should consider?
Yes, we also look at Early voltageβthat's why we use cascode transistors! They help improve output resistance.
Letβs summarize: Current mirrors replicate current, but base current loss and Early voltage limit performance. Enhancements like Beta-helper and cascode transistors help us achieve better results!
Calculating Output Resistance
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Now, letβs discuss output resistance! Who wants to share how we calculate the small signal output resistance in a current mirror setup?
I think it involves the transconductance and output resistances of individual transistors?
Precisely! The formula is given by R_out = g_m3 * (r_o3 + r_o2). What do these parameters mean?
g_m is transconductance, and r_o represents the output resistance of the transistors!
Great! Remember to use the parameters assignedβin this case, we used 0.01 V^β1 for Ξ». Letβs calculate an example together!
So if g_m = 4 and each output resistance is 50 kβ¦, whatβs our total output resistance?
That would be R_out = 4 * (50k + 50k) = 10.1 Mβ¦!
Excellent! Remember, high output resistance is critical for accurate current mirroring.
Practical Applications of Current Mirrors
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Now, letβs talk about where we use current mirrors in real circuits! Can someone provide an example?
I know they are used in common source amplifiers as active loads!
Exactly! The current mirror works to maintain a stable bias current. Why is stability important here?
Stability helps ensure consistent performance, especially under varying load conditions!
Well said! These mirrors also manage to minimize sensitivity to voltage variations, enhancing amplifier efficiency.
As a takeaway, remember that current mirrors enhance both voltage gain and output resistance in applications, which is vital for analog circuits!
Introduction & Overview
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Quick Overview
Standard
In this section, numerical examples of current mirrors using MOSFETs and BJTs are explored. The examples illustrate how to address non-idealities in current mirrors, particularly due to Early voltage, by introducing cascode configurations. Key calculations include determining bias voltages, currents, output resistance, and practical applications in amplifiers.
Detailed
Detailed Summary of Lecture - 87
This lecture discusses the implementation of current mirrors, emphasizing numerical examples to solidify understanding. It begins with an introduction to the challenges posed by non-ideal factors in current mirrors, particularly focusing on the effects of base current loss and Early voltage.
In the first example, a MOSFET version of a current mirror is analyzed. The introduction of a cascode transistor is highlighted as a solution to improve output resistance and outcomes in the mirroring process. Practical calculations are illustrated, including the determination of gate voltages and the essential biasing components needed to ensure saturation in the transistors involved.
Parameters like the transconductance (b) and channel length modulation (Ξ») are specified to examine their roles in determining output characteristics like small signal output resistance. After determining the appropriate voltage and current outputs based on the configured circuit, the results demonstrate how a current mirror can effectively replicate the desired currents, overcoming typical challenge problems such as voltage inputs in saturation.
This leads to examples with a BJT current mirror, comparing the effects and output characteristics with those derived from MOSFET arrangements. Each example is framed in an operational context, demonstrating relevant applications, such as common source and common emitter amplifiers, illuminating the importance of current mirrors in analog electronic design.
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Introduction to Current Mirror Examples
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So dear students welcome back after the break. So, before the break we are talking about the numerical examples of the current mirror. As you can see here, and there we have used Beta-helper to improve the non-ideality factor coming due to the base current loss namely, this base current loss. So in the next example, what we will see that improvisation of the numeric current mirror circuit to take care of the non-ideality factor due to early voltage.
Detailed Explanation
In this section, the instructor welcomes the students back and summarizes the previous discussion about current mirrors and the challenges associated with them, specifically focusing on the non-ideality factors. One such factor is the 'base current loss' in transistors. The upcoming example will illustrate how to improve current mirror circuits by addressing another non-ideality factor known as 'early voltage' by adding a cascode transistor.
Examples & Analogies
Think of a current mirror as a lamp that imitates another lamp's brightness. The base current loss is like a dimming effect due to an obstruction that makes one lamp appear less bright. By optimizing the circuit, we can ensure both lamps shine with even brightness, similar to fixing the obstruction.
Cascode Transistor Operation
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To be more precise, we like to place one cascode transistor here and that improves the output resistance of the current mirror.
Detailed Explanation
The instructor explains that adding a cascode transistor to the current mirror configuration enhances its performance by increasing the output resistance. This is essential in maintaining a stable current output despite fluctuations in voltage, ensuring consistent performance in various conditions.
Examples & Analogies
Imagine using a multi-tiered stand to display different sizes of fruits. Each tier is like a transistor: the stand gives better visibility and stability to the fruits, akin to how the cascode transistor boosts output resistance, providing a more stable current in the circuit.
Transistor Specifications
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Now coming to the different sizes of different transistors given here, we do have for transistor-1, we do have the it is 1 mA/V2. On the other hand for transistor-2 and transistor-3, we do have = 4 mA/V2.
Detailed Explanation
The instructor introduces the specifications of three transistors used in the current mirror example. The transconductance (K) values for these transistors indicate their ability to control current flow: transistor-1 has a standard K value, while the other two are designed to have higher values, enhancing their current handling capabilities. Keeping other operational parameters the same allows for easier calculations.
Examples & Analogies
Think of K value as the horsepower of an engine. Different engines (transistors) have different horsepower ratings, which affect how powerfully they can propel a car (current). By adjusting the horsepower, we can improve stability and efficiency in driving.
Calculation Process
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So to calculate that, in some of our previous examples we have done that this current which is I , it is also creating a drop here and after subtracting this drop, say from 12 V supply, it is giving us the V .
Detailed Explanation
This section details the calculation process for determining the gate voltage. The instructor highlights that the current through the circuit creates a voltage drop affecting the available voltage at the gate of the transistor. Hence, to find the gate voltage (VGS), one must subtract the drop due to the current from the total supply voltage.
Examples & Analogies
Consider a water tank. The water level at the tap (gate voltage) depends on how much water is flowing out (current) and the initial height of the tank (supply voltage). If too much water flows out, the tap will be low, similar to how the voltage drop affects the gate voltage.
Output Resistance Calculation
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So we need to find what will be the small signal output resistance. And R is g m3 r + r .
Detailed Explanation
Here, the small-signal output resistance of the current mirror is expressed using the transconductance (g) and output resistances (r) of the transistors. This form of resistance is crucial in understanding the output behavior of the circuit under small signal conditions. The combination of these resistances helps ensure the current mirror provides stable operation across a range of output voltages.
Examples & Analogies
Think of a flexible bridge (output resistance) that can support various weights. The sturdiness of each supporting column (g and r values) determines how much weight the bridge can safely hold when cars drive over it (small signals) without collapsing.
Current Flow Variations
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So, the corresponding current here at this point it is 2 mA + 3 Γ 10β7 A. So, we can say that this current = 2.0003 mA.
Detailed Explanation
The instructor calculates how the current flowing through the circuit changes due to the output resistance when the voltage is altered from 5 V to 8 V. The main current is 2 mA, but because of the high output resistance, only a tiny additional current influences the total current flowing through the branch, showing the circuit's stability despite changes in voltage.
Examples & Analogies
Picture a large river (2 mA) flowing steadily, and then a small stream (3 Γ 10β7 A) entering it. The overall flow increases, but the river's strong current means that the small stream's effect is barely noticeable, demonstrating stability even when slightly altered.
Improving Current Mirror Performance
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But then it is little inconvenient to get this voltage and to overcome this problem, what we can do we can put a transistor here which is also diode connected and that produces a voltage and that is normally that is what it is getting applied.
Detailed Explanation
The instructor offers a solution to the inconvenient voltage measurement by suggesting adding a diode-connected transistor to the circuit. This transistor will assist in generating a necessary voltage internally, reducing reliance on external measurements and simplifying the design.
Examples & Analogies
Imagine adding a water pump (diode-connected transistor) to a natural spring (circuit), which automatically keeps the water level consistent no matter the changes in the environment (output voltage) instead of needing constant manual checks.
Conclusion and Summary
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So that is how we can make the output current very less sensitive to the voltage for improved current mirror or which is referred as cascode commonly known as cascode current mirror.
Detailed Explanation
The instructor concludes this section by summarizing the improvements made to the current mirror design through the addition of the cascode configuration. This design effectively stabilizes the output current against voltage variations, thereby enhancing performance and reliability.
Examples & Analogies
Think of the cascode current mirror as a highway overpass. While the surface street may be affected by traffic lights (voltage changes), the overpass allows for a continuous flow of cars (current) without interruption, maintaining smooth operation regardless of obstructions below.
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 providing current replication.
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Base Current Loss: Loss of current due to transistor biasing imperfections.
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Early Voltage: Affects output impedance and current in transistors.
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Cascode Configuration: Enhances output resistance in current mirrors.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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In a common source amplifier, a current mirror is employed to provide an active load while maintaining the bias current stable.
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Adding a cascode transistor to a current mirror can improve output resistance significantly, keeping the current stable despite variations in supply voltage.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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A current mirror can show, replicate currents that flow.
π Fascinating Stories
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Imagine a factory where each worker mirrors the actions of one lead worker. If the lead worker is efficient, so are all others, much like a current mirror in a circuit.
π§ Other Memory Gems
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Remember 'C-MORE' - Current Mirror Offers Resistor Enhancement, depicting its function
π― Super Acronyms
CASCODE - 'Current Amplification Supporting Constant Output' helps recall why we use cascode transistors.
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 configuration that ensures a certain reference current is replicated in another branch of the circuit.
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Term: Betahelper
Definition:
A technique used to minimize base current losses by adjusting circuit parameters.
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Term: Cascode Transistor
Definition:
A transistor used to increase the output resistance in a current mirror by improving the saturation of other transistors.
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Term: Early Voltage
Definition:
A parameter that describes the variation of a transistor's output current with changes in collector-emitter voltage.
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Term: Transconductance (g_m)
Definition:
A measure of how effectively a transistor can control the output current with respect to its input voltage.