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Today we're discussing differential amplifiers. Can anyone tell me what a differential amplifier does?
It amplifies the difference between two input voltages.
Exactly! It's essential for removing common noise from signals. Now, what do we mean by common mode gain?
Isn't that the amplification of signals that are common to both inputs?
Correct! Think of it as the unwanted noise that you want to minimize in your signal processing. It's crucial to differentiate this from differential mode gain, where we're only concerned with the difference. Let's move on to how we calculate these gains.
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To enhance our differential amplifier, we can use current mirrors. Who can explain how they work?
They maintain a constant current in a circuit, right?
Exactly! By using current mirrors to set the tail current, we ensure greater stability and efficiency. Remember the idea of matching currents? This is vital for our common mode rejection ratio. Can anyone tell me why this is important?
It helps reduce the impact of noise signals on the output.
Precisely! The current mirror improves the performance of both common mode and differential mode gains.
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Let's dive into calculations. When we draw the small signal equivalent circuit, what do we need to consider?
We should include all the resistances and any transconductance factors.
Good! The differential mode gain, for instance, can be calculated using the transconductance and output resistances. Can anyone give me the formula for this?
Itβs A_d = g_m * R_o.
Exactly right! And the common mode gain is somewhat different. Remember, for good amplifiers, we want the ratio of differential mode gain to common mode gain to be highβthis is our common mode rejection ratio.
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Now, letβs talk about active loads versus passive loads. Why might we prefer an active load in a differential amplifier?
Active loads can provide greater gain because they can increase the output resistance.
Exactly! Active loads help in achieving higher differential and common mode gains compared to passive loads. Does anyone remember the difference in gain formulas with active loads?
Yes, with active loads, the output resistance is significantly higher which increases the overall gain!
Correct! Understanding these concepts helps us design better amplifiers.
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The section discusses the concept of differential amplifiers and the role of common mode and differential mode gains. It includes insights on configuring these amplifiers with active loads, specifically current mirrors, to enhance performance in terms of signal amplification and common mode rejection.
In this section, we delve into the fundamental concepts of common mode and differential mode gain as they pertain to differential amplifiers constructed using both BJTs and MOSFETs. We discuss the need for advanced configurations such as active loads and current mirrors that help improve the amplifier's characteristics. The common mode gain and differential mode gain are defined and formulas for calculating both types of gain are provided. Furthermore, diagrams illustrating the small signal equivalent circuits help solidify the understanding of these configurations. The interplay between differential and common mode gains, and the importance of current matching for high performance in these circuits, are emphasized.
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So, just for completeness we shall visit to those circuits which let me go to the next slide to start with replacement of the tail resistor.
The section begins by introducing the concepts of common mode gain and differential mode gain in the context of differential amplifiers. Common mode gain refers to the amplification of signals that are common to both inputs of the amplifier, while differential mode gain refers to the amplification of the difference between the two input signals. Understanding these gains is crucial for analyzing and designing circuits.
Think of a differential amplifier like a microphone that can pick up sounds from two different directions. The common mode gain is like the microphone picking up background noise that sounds the same from both sides, while the differential mode gain is focused on capturing the unique sound from one direction, ignoring the background noise.
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Now, here instead of R_T, of reference current I_REF, it is coming from V_CC.
In differential amplifier designs, traditionally passive elements like resistors are replaced with active components like transistors. This replacement improves performance by allowing for better control over the tail current, which plays a critical role in achieving the desired gain. For instance, the reference current is set by a voltage source, enhancing stability and accuracy in the amplifier's response.
Imagine altering a water pipe system where you initially used simple valves (passive elements) to control water flow. By replacing them with electronically controlled valves (active elements), you gain the ability to precisely adjust the flow rate under varying conditions, leading to a more reliable system.
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Now again similar to the previous case here if you analyse we can find the common mode gain.
To evaluate common mode gain, the circuit's response to identical inputs at both terminals is analyzed. This gain can typically be approximated by analyzing the small signal equivalent circuit. The formulas derived from these analyses allow engineers to determine how effectively the circuit amplifies common signals.
Consider a karaoke machine that amplifies the voice of a singer. If both the left and right speakers of the system are playing the same voice equally, the sound you hear is the common mode gain. If one speaker plays a different sound, the system's ability to amplify that unique sound would indicate a high differential mode gain.
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Now here also we can get the differential mode gain.
The differential mode gain is calculated similarly but focuses on the ability of the amplifier to process signals that are different between inputs. By analyzing the small signal model, engineers derive the differential mode gain, which is crucial for applications where noise rejection is paramount. The resulting gain often depends on the specific configurations and components used in the circuit.
Think of a conversation happening in a crowded cafe. The differential mode gain is like how well one can hear a friend's voice amidst all the chatter. The clearer the friend's voice becomes, the higher the differential mode gain, as it effectively distinguishes the desired signal from the noise.
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So, this load part it is having these current mirror and it is reference current of course, this reference current it is also getting from the same common reference current.
In advanced differential amplifiers, instead of using passive loads, active loads like current mirrors are employed. These active loads help maintain consistent current flow, leading to improved gain characteristics and better performance overall. By mirroring currents, these configurations offer significant benefits like higher output resistance and increased gain compared to traditional passive loads.
Imagine using a network of synchronized clocks (current mirrors) instead of individual clocks to maintain time. Each clock mirrors the time of a reference clock, ensuring that they all show the same time accurately, thereby enhancing coordination across multiple timekeeping devices.
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To summarize we have replaced both the active load part and the tail part it is getting in fact, current mirror.
The section concludes by summarizing the key modifications made to the differential amplifier design, mainly the introduction of current mirrors for both the tail current and the load. This comprehensive approach improves both the common mode gain and differential mode gain, highlighting the circuit's enhanced ability to reject noise and amplify the desired signals effectively.
This conclusion can be likened to implementing an efficient team structure in a workplace. By relying on a structured method (current mirror load), the team can effectively manage their tasks (amplifying signals) while minimizing disruptions (noise). The outcome is a noticeably improved workflow and productivity.
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Key Concepts
Common Mode Gain: The amplification of input signals that are common to both inputs.
Differential Mode Gain: The amplification of the difference between the two input signals.
Current Mirror: A method to provide a stable bias current that helps maintain the performance of amplifiers.
CMRR: A key indicator of an amplifier's effectiveness in rejecting noise and interference.
See how the concepts apply in real-world scenarios to understand their practical implications.
A differential amplifier using BJTs with current mirrors can provide a higher gain due to improved biasing techniques.
When constructing a differential amplifier, using active loads instead of passive loads enhances both the differential and common mode gains.
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When signals seem the same, don't let them take the fame; differential seeks the gap, amplifying what they tap.
Imagine a color mixing paint. If two colors are the same, you just get one color. But if they're different, you create something new and bursting with life! That's how differential amplifiers work - enhancing the unique differences while ignoring sameness.
For common mode gain, remember 'CC' - 'Common Cause' to recall it talks about shared input signals.
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Review the Definitions for terms.
Term: Differential Amplifier
Definition:
An amplifier that amplifies the voltage difference between two input signals.
Term: Common Mode Gain
Definition:
The amplification of signals that are common to both inputs of a differential amplifier.
Term: Differential Mode Gain
Definition:
The amplification of the difference between two input signals in a differential amplifier.
Term: Current Mirror
Definition:
A circuit configuration that allows a current to be copied from one branch of the circuit to another with high accuracy.
Term: Common Mode Rejection Ratio (CMRR)
Definition:
A measure of a circuit's ability to reject common mode signals, defined as the ratio of differential mode gain to common mode gain.