Continuation of BJT Current Mirror Example - 86.6 | 86. Numerical examples on current mirror and its applications (Part-A) | Analog Electronic Circuits - Vol 4
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Interactive Audio Lesson

Listen to a student-teacher conversation explaining the topic in a relatable way.

Introduction to Current Mirrors

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0:00
Teacher
Teacher

Today, we'll explore the concept of current mirrors. Can anyone tell me what a current mirror is and why it's useful?

Student 1
Student 1

I think a current mirror is used to copy a current from one branch of a circuit to another.

Teacher
Teacher

Great! It essentially allows us to maintain a constant current, which is crucial for biasing transistors in amplifiers. We can think of it as a 'current source' which maintains consistent performance.

Student 2
Student 2

What are some practical applications of current mirrors?

Teacher
Teacher

Excellent question! Current mirrors are used in differential amplifiers and operational amplifiers among other applications. Remember this acronym: 'DAC,' which stands for 'Differential Amplifier Circuit.'

Student 1
Student 1

Got it! So it’s really about helping transistors maintain their operation consistently.

Teacher
Teacher

That's correct! Let's summarize: Current mirrors are important for maintaining constant currents. They help in precision biasing. Now, let's look at a simple current mirror example using MOSFETs.

Numerical Example with MOSFET Current Mirror

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Teacher
Teacher

In our first numerical example, we have two MOSFETs forming a current mirror. If we set a reference current of 0.5 mA and know the K factors, what do you think we need to calculate first?

Student 3
Student 3

We should find the output current of the second transistor based on the reference current.

Teacher
Teacher

Exactly! Given that K is different for both transistors, let’s see how it affects the output current. Can someone help me compute the voltage at the gate-source for the first transistor?

Student 4
Student 4

I think it would be Vgs1 = Vth1 + 1.5 V!

Teacher
Teacher

Correct! Now, using that voltage, how would we find I_DS2?

Student 1
Student 1

We can use the equation I_DS2 = I_REF * K2/K1 to find the output current.

Teacher
Teacher

That’s spot on! And once we calculate I_DS2, we can assess the saturation limits, which is crucial for validity.

Student 3
Student 3

What are the implications if the transistor is not in saturation?

Teacher
Teacher

Very important point! If a transistor is not in saturation, the current cannot be defined accurately, leading to distortion in circuits. Let’s ensure we often check the saturation conditions.

Current Mirrors with BJTs

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Teacher
Teacher

Now let’s shift gears and explore current mirrors using BJTs. Who remembers how BJTs function in current mirrors?

Student 1
Student 1

They use base-emitter voltage and have transistors mirroring the current like MOSFETs.

Teacher
Teacher

Correct! With BJTs, we have to consider the reverse saturation current among other parameters. Can anyone explain the effect of beta on current output?

Student 2
Student 2

If beta is not high enough, the output current will drop, right?

Teacher
Teacher

Absolutely! Lower beta means more base current loss. We often use a non-ideality factor to account for that. Remember the formula: I_C = I_REF / (1 + I_base/I_C).

Student 4
Student 4

And that explains why we need to consider early voltage too!

Teacher
Teacher

Exactly! Both these factors help us to understand the accuracy of our current mirrors considerably. Let’s now apply these concepts to a problem.

Student 3
Student 3

What kind of problems will we be solving?

Teacher
Teacher

We’ll calculate currents based on saturation voltages and see how they change. Ready to dive in?

Practical Applications and Enhancements

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0:00
Teacher
Teacher

As we finalize our understanding, what improvements do you think we can make to our simple current mirror designs?

Student 1
Student 1

Using a beta helper circuit to account for base current losses could improve accuracy.

Teacher
Teacher

Excellent! Utilizing the beta helper can indeed minimize base current errors. Can anyone summarize the steps we take to enhance precision?

Student 2
Student 2

First, we calculate the current, then adjust thresholds, and also use cascode configurations if needed.

Teacher
Teacher

Spot on! This systematic approach allows for enhanced output resistance and overall accuracy. Can we think of any specific scenarios where high precision in current mirrors is critical?

Student 3
Student 3

In sensitive amplifier circuits, where signal integrity is vital.

Teacher
Teacher

Exactly! Amplifiers for audio or RF applications often require precise current mirrors for optimal performance. Summary time: remember the enhancements, the critical nature of current mirrors, and how they can impact the circuitry!

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

The section elaborates on numerical examples, calculations, and applications of current mirrors primarily using BJTs and MOSFETs.

Standard

This section dives into numerical examples of current mirrors, focusing on both BJT and MOSFET configurations. It highlights how to calculate currents, voltages, and output resistances while addressing non-ideality factors like current gain and voltage differences that arise in practical applications.

Detailed

Detailed Summary

The section discusses practical applications of the current mirror concept in analog electronic circuits. It includes numerical examples demonstrating the workings of simple current mirrors constructed with both MOSFETs and BJTs. The calculations involve determining the reference current, output currents, and the minimum voltage needed for proper operation in saturation regions, factoring in non-ideal behaviors such as early voltage effects and base current losses. By illustrating these principles through specific examples, particularly those using real transistor parameters like saturation currents and thresholds, the section clarifies the important concepts of current mirroring and their significance in circuit design.

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Analog Electronic Circuits _ by Prof. Shanthi Pavan
Analog Electronic Circuits _ by Prof. Shanthi Pavan

Audio Book

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Continuation of the Numerical Example with Finite Ξ»

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Now, let us consider the next part of the same question in the next slide. So, that will be continuation of this problem. But probably, we can consider some finite value of Ξ»'s.

Detailed Explanation

In this segment, we are continuing the previous calculations while introducing a finite value for the channel length modulation parameter, Ξ». This parameter helps to understand the variations in the output current from the current mirror when transistors operate under non-ideal conditions.

Examples & Analogies

Consider Ξ» as a filter that allows only specific currents to pass through. By adjusting this filter, we can see how the output current changes under different conditions, similar to how changing a water filter can affect water flow and quality.

Impact of V_DS on Current Output

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Now for this case, V_DS2 = 2.5, both this V_DS and this V_DS1 are the same. So, and also the lambdas are equal. So, we can say that this part of this equation it is becoming 1 for 2.5. As a result, the corresponding current here it is coming. So this ratio it is 4 and I_REF as I said it is 0.5. So, that gives us 2 mA.

Detailed Explanation

In this chunk, we analyze how the drain-source voltage (V_DS) affects the current output in a current mirror configuration. By maintaining V_DS values as equal, we simplify the calculation of the output current, resulting in a calculated output of 2 mA based on earlier reference values.

Examples & Analogies

Imagine adjusting the pressure in water pipes. If the pressure is equal throughout, the flow (current output) remains stable. Similarly, equal V_DS values help maintain consistent output current in the circuit.

Finding Minimum V_DS for Proper Operation

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So to keep this transistor in saturation, we know that the drain voltage it should be higher than the gate voltage minus V_th, the gate voltage we know, it is 2.5 V.

Detailed Explanation

This chunk describes finding the minimum drain-source voltage (V_DS) necessary for maintaining a transistor in saturation mode, which is crucial for ensuring accurate and stable current mirroring. The calculations involve understanding the fundamental voltage relationships within the transistor.

Examples & Analogies

Think of this as ensuring a car has enough fuel to keep running smoothly. Just as a car needs a minimum amount of fuel (energy) to operate efficiently, a transistor requires a certain minimum voltage to function effectively.

Continuing Calculations with a Given Ξ»

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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 Ξ» = 0.01 V^-1.

Detailed Explanation

After determining the minimum voltage requirements, the subsequent calculations involve the introduction of a finite channel length modulation parameter (Ξ»). This adjustment allows for a more realistic evaluation of the output current based on practical transistor operation.

Examples & Analogies

Imagine tuning a musical instrument. Initially, it sounds perfect, but adjusting its pitch (like changing Ξ») helps improve its overall sound quality based on real conditions.

Calculating Output Current with V_DS Adjustments

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So, if I say V_DS2 = 5.5, for this, I_DS2 is equal to 2 mA multiplied by this non-ideality factor.

Detailed Explanation

This chunk discusses calculating the new output current when adjusting the drain-source voltage (V_DS2) to 5.5 V. It illustrates the relationship between V_DS and the calculated output current, demonstrating how variations affect transistor behavior.

Examples & Analogies

Consider filling a container with water. Increasing the height from which you pour (analogous to increasing V_DS) raises the water flow into the container; likewise, adjusting V_DS directly impacts current output.

Understanding Output Resistance from Current Changes

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To calculate the small signal output resistance, we can get the calculate the slope of this line and reciprocal of that is the small signal output resistance.

Detailed Explanation

In this section, we learn how to derive the small signal output resistance from the output current variations and the corresponding drain voltages. It's essential for analyzing the performance characteristics of the current mirror.

Examples & Analogies

Think of it as measuring the stretch of a rubber band: the tighter you pull (changes in current), the more you can determine how stretchable it is (output resistance).

Transitioning to BJT Current Mirror Example

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So, now we are going to BJT. So, we have considered this simple current mirror.

Detailed Explanation

This chunk introduces the transition from MOSFET-based current mirrors to those using Bipolar Junction Transistors (BJTs). This step is significant since BJTs have different characteristics and parameters affecting current mirroring.

Examples & Analogies

Imagine switching from using LED lights (MOSFETs) to incandescent bulbs (BJTs). While they both provide light (current), their power supplies, heat, and brightness control are unique, demanding distinct understandings and calculations.

Calculating the Output Current of BJT Current Mirror

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In this case, just for a change, instead of giving a reference current, we are giving a resistor here, supply voltage is given to us 12 V.

Detailed Explanation

This section covers the how-to of establishing a BJT current mirror by using a resistor to set the reference current rather than feeding a direct current value. It discusses the significance of setting up conditions for current flow with careful consideration of parameters like supply voltage.

Examples & Analogies

Imagine trying to balance on a seesaw by applying weight (current) precisely. In this case, controlling current through a resistor functions like adding weight to the seesaw ensures a balanced operation.

Considering Non-Ideality Factors and Early Voltage

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Now, if we consider these finite β values and early voltage, the ratio instead of 1∢3, it is becoming different.

Detailed Explanation

Here, we explore how non-ideality factors, including the finite beta (Ξ²) and early voltage, affect the current ratio within a BJT current mirror setup. Understanding these factors is crucial for achieving precision in applications.

Examples & Analogies

It's akin to baking a cake with fluctuating oven temperatures. Different settings (non-ideality factors) can lead to varying degrees of success (output results).

Introducing the Beta-Helper for Improved Current Control

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So to take care of the base current loss, we can have a Beta-helper circuit here.

Detailed Explanation

The chunk introduces the Beta-helper circuit, which mitigates base current losses in BJTs, enhancing the output current accuracy. This technique is especially useful for improving the performance of current mirrors in real-world applications.

Examples & Analogies

Think of the Beta-helper like an assistant at a job; it helps reduce the workload (current loss), ensuring tasks are completed effectively and efficiently.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Current Mirror: A circuit that duplicates current from a reference.

  • Saturation: Condition under which transistors maintain accurate current flow.

  • Non-Ideality Factors: Effects in current mirrors that deviate from ideal behavior, such as early voltage and base current losses.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Example calculating output current for a MOSFET current mirror given specific K factors.

  • Example demonstrating the design of a BJT current mirror while considering reverse saturation current.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎡 Rhymes Time

  • For mirrors of current, keep things aligned, transistors in sync, the right path to find.

πŸ“– Fascinating Stories

  • Imagine two friends passing a ball; one throws, the other catches, keeping the same size and weight in the current hall.

🧠 Other Memory Gems

  • Remember 'CAPS' for Current Mirrors: Constant, Accurate, Precise, Simple!

🎯 Super Acronyms

MCU

  • Maintain Current Uniform
  • reminds us of current mirrors' roles.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Current Mirror

    Definition:

    A circuit design that copies a current from one branch to another, maintaining constant current.

  • Term: MOSFET

    Definition:

    Metal-Oxide-Semiconductor Field-Effect Transistor, often used in current mirrors.

  • Term: BJT

    Definition:

    Bipolar Junction Transistor, another type of transistor used in current mirror designs.

  • Term: Saturation Current

    Definition:

    The maximum output current that can be delivered by a current mirror.

  • Term: Beta (Ξ²)

    Definition:

    The current gain of a transistor, indicating how much the collector current is amplified from base current.

  • Term: Early Voltage

    Definition:

    A parameter that represents the effect of the base-collector voltage on current gain, impacting current mirror performance.