Small Signal Equivalent Circuit - 85.3.1 | 85. Usage of current mirror (Part-C) | Analog Electronic Circuits - Vol 4
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Interactive Audio Lesson

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Introduction to Differential Amplifiers and Current Mirrors

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0:00
Teacher
Teacher

Welcome class! Today we're going to discuss differential amplifiers made with BJTs and how current mirrors play a vital role in their design. Let's start by reviewing what a differential amplifier is.

Student 1
Student 1

A differential amplifier amplifies the difference between two input signals, right?

Teacher
Teacher

Exactly! And current mirrors help set up specific currents in these circuits. Who can explain how they do that?

Student 2
Student 2

They use one transistor's current to control another's, keeping the output stable.

Teacher
Teacher

Good job! This stability is critical for ensuring consistent performance. Remember the acronym 'ICS' for 'Input Control Stability' in current mirrors!

Student 3
Student 3

What's the significance of using BJTs over MOSFETs?

Teacher
Teacher

Great question! BJTs can provide specific characteristics preferable in certain applications. Let's proceed to analyze the small signal equivalent circuit.

Analyzing the Small Signal Equivalent Circuit

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Teacher
Teacher

Now that we understand the basics, let’s dive into deriving the small signal equivalent circuit. Can anyone remind us what 'small signal' means?

Student 4
Student 4

It refers to analyzing the circuit’s behavior under small perturbations, usually around an operating point.

Teacher
Teacher

Exactly! In our circuit, the small signal model helps in calculating gains easily. What components are essential in this model?

Student 1
Student 1

We need parameters such as transconductance 'g' and small signal resistances 'r'.

Teacher
Teacher

Correct! By replacing the active load with a current mirror, we improve the amplifier’s performance. What outcome do we observe when we analyze the differential mode gain?

Student 3
Student 3

It increases compared to circuits with passive loads.

Teacher
Teacher

Exactly! And this improvement is thanks to effective use of current mirrors. Let's summarize: 'PRAISE' - Performance of Resistors and Active Loads in Small Equivalents.

Understanding Gain Calculations

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Teacher
Teacher

Next, let's explore gain calculations in our small signal model. Can someone explain the difference between differential and common mode gain?

Student 2
Student 2

Differential mode gain focuses on the difference between two inputs, while common mode gain deals with the input signals being the same.

Teacher
Teacher

Correct! In our analysis, the expressions for both gains are crucial. What effect does replacing the load with a current mirror have on CMRR?

Student 4
Student 4

It helps to improve CMRR because the differential gain increases significantly, while common mode gain doesn't increase as much.

Teacher
Teacher

Exactly! To remember this, I suggest 'COT' - Common versus Output Trade-off in gains. Let’s ensure we can derive these equations clearly!

Practical Applications of Current Mirrors in Amplifiers

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Teacher
Teacher

Finally, let’s wrap up by discussing real-world applications of current mirrors. Why are they preferred in amplifier design?

Student 1
Student 1

Because they provide better performance and stability compared to traditional passive loads.

Teacher
Teacher

Exactly! They enable more efficient biasing and higher gain configurations. Can someone suggest another advantage?

Student 3
Student 3

They also help in reducing noise and improve overall linearity.

Teacher
Teacher

Well said! Remember 'FAIR' - Flexibility and Amplification in Real-world applications for current mirrors. Great session everyone!

Introduction & Overview

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Quick Overview

This section discusses the small signal equivalent circuit of differential amplifiers using BJTs and current mirrors, emphasizing modifications that enhance performance.

Standard

The small signal equivalent circuit of a differential amplifier employing BJTs and current mirrors is analyzed, highlighting the replacement of passive components with active loads. Key characteristics such as differential and common mode gains are derived, illustrating how the circuit design impacts overall performance.

Detailed

Detailed Summary

This section focuses on the small signal equivalent circuit of a BJT-based differential amplifier utilizing current mirrors for biasing the tail current and active loads. The initial part introduces the modification of active load circuits, replacing passive elements to improve performance. The section outlines how to derive the small signal model from the circuit, detailing components such as transistors and resistors, and emphasizing the importance of matching the tail current with the load current.

It presents analyses for both common mode and differential mode gains, demonstrating how replacing loads impacts these characteristics. The common mode rejection ratio (CMRR) is defined, highlighting the significance of these configurations in enhancing amplifier performance. Examples are provided to elucidate the concepts, reinforcing the importance of current mirrors in modern amplifier designs.

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

Audio Book

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Introduction to the Small Signal Model

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So, whenever we are applying a common DC voltage and if transistor-1 and transistor-2, they are identical then you can see half of these two currents together they are converging to I without any problem maintaining their corresponding V equal.

Detailed Explanation

In this segment, the focus is on how transistors in a differential amplifier share currents when an identical DC voltage is applied. If two transistors (transistor-1 and transistor-2) are matched, they will effectively split the incoming current equally between them. This equal sharing helps maintain balance in the circuit, keeping the voltage (V_BE, the voltage across the base-emitter junction) at both transistors consistent. Hence, this forms the basis of their operation in a small signal model where the conditions are predictable and manageable.

Examples & Analogies

Think of this as two friends sharing a pizza evenly. If both eat an equal share, they can maintain a balanced mood and enjoyment from the meal. Similarly, transistors sharing current properly keep the circuit balanced, leading to better performance.

Common Mode and Differential Mode Gain

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Now here, similar to the common mode gain for differential mode gain we can we can draw the small signal equivalent circuit and then we can analyse the circuit to get the differential mode gain.

Detailed Explanation

The distinction between common mode gain and differential mode gain is crucial in amplifier design. Common mode gain refers to the amplification of signals that are common to both inputs of a differential amplifier, while differential mode gain pertains to the amplification of the difference between these inputs. The small signal model allows engineers to analyze these gains more easily by simplifying circuit components to their linear equivalents and extracting relevant parameters from the circuit. This analysis helps in designing amplifiers that maximize signal integrity while minimizing noise.

Examples & Analogies

Imagine you're listening to two speakers playing the same song at the same time (common mode) versus trying to listen to a conversation between two people who are speaking different things (differential mode). You want your audio system to effectively amplify the conversation while ignoring the background noise, just like an amplifier needs to enhance the differential signal over the common mode noise.

Circuit Modifications with Active Loads

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So, as we have seen earlier, this part can also be replaced by active load and that load current of course, should be consistent with whatever the current we do have flowing through transistor-3.

Detailed Explanation

This section discusses the enhancement of differential amplifiers by implementing active loads. An active load involves using additional transistor circuits to replace passive resistors, improving overall circuit performance. Active loads can provide higher gain and better linearity by ensuring that the load current mirrors the current flowing through key transistors. This enables the amplifier to operate more efficiently and accurately under varying conditions.

Examples & Analogies

Think of this as upgrading from a traditional car engine to a hybrid engine. The hybrid engine (active load) can optimize power better than a simple engine (passive load), allowing for improved performance, such as higher speeds and better fuel efficiency.

Current Steering and Signal Amplification

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So, here we do have the active load circuit. On the other hand, if you see the transistor-7 and transistor-8, both are having equal DC current. So, why not making this transistor like a diode connected transistor if it is having say half of the current flowing here then that half current or , that can be mirrored into transistor-8.

Detailed Explanation

In this part, the focus shifts to how transistors can be configured to enable efficient current steering for improved amplification. By configuring transistors (transistor-7 and transistor-8) so they mirror currents effectively, the amplifier can maintain a strong output signal. This way, as one transistor conducts, it ensures that the mirrored current in its counterpart guarantees that the system is balanced. The active load thus amplifies the input signal more efficiently, leading to better performance.

Examples & Analogies

Imagine a relay race where the runner (transistor-7) passes the baton (current) to another runner (transistor-8). If the first runner is efficient, the second runner can maintain or even boost the pace, ensuring that the overall performance of the team is enhanced. Similarly, transistor configurations work together to amplify signals more effectively.

Definitions & Key Concepts

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

Key Concepts

  • Differential Mode Gain: The gain related to the difference of two input signals in an amplifier.

  • Common Mode Gain: The gain related to identical signals applied to both inputs.

  • Current Mirror: A mechanism to set and stabilize current through active means, improving amplifier performance.

Examples & Real-Life Applications

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

Examples

  • In a BJT differential amplifier design, using current mirrors increases the precision of the biasing current, dramatically affecting gain.

  • Using an active load instead of a resistive load in a differential amplifier improves the speed and reduces power consumption.

Memory Aids

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

🎡 Rhymes Time

  • In circuits where signals shine, a differential amplifier sets high, oh so fine!

πŸ“– Fascinating Stories

  • Imagine two friends, Differential and Common, always arguing about who is louder. The wise Current Mirror settles their debates by ensuring only the loudest voice is heard!

🧠 Other Memory Gems

  • Remember 'Gains are Set on Mirrors' (GSM) for how current mirrors impact amplifier gains.

🎯 Super Acronyms

CMRR

  • C: for Common
  • M: for Mode
  • R: for Rejection
  • and R for Ratio.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Differential Amplifier

    Definition:

    An amplifier that amplifies the difference between two input signals.

  • Term: Current Mirror

    Definition:

    A circuit that produces a current equal to a reference current, thereby biasing other components.

  • Term: Common Mode Gain

    Definition:

    The gain experienced when the same input signal is applied to both inputs of a differential amplifier.

  • Term: Differential Mode Gain

    Definition:

    The gain experienced when the difference between two input signals is amplified.

  • Term: Common Mode Rejection Ratio (CMRR)

    Definition:

    A measure of an amplifier's ability to reject common mode signals.