Common Mode and Differential Mode Improvement
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Introduction to Common Source Amplifiers
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Today, we're discussing common source amplifiers. Can anyone tell me what a common source amplifier typically does?
It amplifies signals, right?
Exactly! It amplifies signals but can also have variations in gain depending on the load. We use diode-connected loads in some configurations to improve performance.
How do diode-connected loads work in this context?
Great question! A diode-connected load ensures that the current mirror operates effectively, maintaining output stability.
How does that help us with the common mode and differential mode signals?
When we improve the load in a common source circuit, we decrease the common mode gain. This is essential for better signal integrity.
To summarize, using these loads helps in achieving a much lower common mode gain, which enhances our differential signal processing.
Analyzing the Circuit Behavior
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Now let's analyze the circuit behavior when we apply small signals. What do you think happens to the output voltage?
The output voltage should reflect the input, but scaled by the gain?
Correct! We find that the output voltage is given by the equation V_out = -g_m1 * V_in_c. This highlights the relationship between input and output.
What if the load changes?
If the load changes, the gain also alters. This is where our current mirrors play a pivotal role by stabilizing the load.
To consolidate, using current mirrors leads to a lower gain in the common mode, improving performance for differential signals.
Applications in Biasing and Gain Improvement
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Let's now discuss how current mirrors are utilized as biasing elements in amplifiers. What does biasing mean for an amplifier?
Biasing sets the DC operating point to ensure the amplifier works properly!
Exactly! And with the current mirror, the biasing becomes more reliable. Can anyone explain the advantage?
It likely helps to reduce unwanted variations in gain?
Right again! This stabilization is crucial, especially in differential amplifiers, where accurate signal representation is essential.
Summarizing today, current mirrors not only improve biasing but also enhance common mode and differential mode performance. This enables better signal management.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The section outlines the configuration of a common source amplifier's circuit and its transformation to utilize current mirrors as loads. It explains how these changes impact common mode and differential gains, illustrating their significance in signal processing applications.
Detailed
Common Mode and Differential Mode Improvement
In this section, we examine how a common source amplifier can be designed with diode-connected loads, effectively using current mirrors to optimize circuit performance. The configuration starts with a typical common source setup but proceeds to analyze small signal behaviors and their implications on voltage gain.
A primary focus is the establishment of common mode gain, represented as A''_oC, which is found to be -g_m1 a. Importantly, the current mirror reduces this gain significantly relative to previous values, illustrating the potency of employing active current mirror loads. This design reduces the common mode gain drastically, leading to better common mode rejection rates.
We also substantiate how this improvement pertains to subsequent amplifier stages, where the common mode signals are vital for biasing PMOS transistors. The implications are significant: with enhanced common mode rejection, the circuit proves beneficial for differential signals that need conversion into single-ended outputs.
The section concludes by summarizing the utility of current mirrors in enhancing amplifier performance, revealing that their applications stretch across various amplifier types, improving overall signal integrity and biasing effectiveness.
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Common Source Amplifier with Active Load
Chapter 1 of 7
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Chapter Content
this part and if we analyse this circuit it becomes like a common source amplifier, where the load it is diode connected.
So, the circuit becomes like this. We do have g , we do have g connected to V and m1 m7 DD then this transistor half of the transistor we may call say M by 2. It is just a just a matter 3 of representation and at this point we are giving the small signal along with the DC.
Detailed Explanation
In this part, we analyze a circuit resembling a common source amplifier. The active load is diode-connected, meaning it functions like a diode in the circuit. The transistors g, g connected to a voltage source (VDD) are introduced, with one transistor referred to as half of another one (M/2). We also introduce the concept of superimposing a small signal onto a direct current (DC) to analyze how it behaves in this circuit.
Examples & Analogies
Think of a common source amplifier as a microphone input. When someone speaks into a microphone (the DC signal), there are also smaller background noises (the small signals) that influence the overall output. The microphone captures both, just like the circuit captures DC and AC signals.
Analysis of Small Signal Equivalent Circuit
Chapter 2 of 7
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Now, if you analyse this circuit or if you draw the small signal equivalent circuit, you can find that the voltage here v = ‒ g × . So, this into whatever input signal we are applying v . In fact, as I said the voltage here also it will be same.
Detailed Explanation
Upon analyzing the small signal equivalent circuit, we derive that the output voltage (v) is proportional to the negative gain (−g) multiplied by the input signal (v_in_c). This tells us how the circuit responds to small variations in the input. Notably, the voltage across specific points in the circuit remains consistent when switching between common mode and differential mode operations.
Examples & Analogies
Imagine tuning an audio equalizer where a small change in one frequency influences the overall sound output. Here, we denote how adjusting a small signal affects the output voltage, similar to how each small adjustment can significantly change our audio experience.
Common Mode Operations and Gain
Chapter 3 of 7
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So, we may say that v under common mode operation is also = v and that gives us v = v common mode. And hence the corresponding common mode gain if I say A′′ which is defined by = ‒ g × .
Detailed Explanation
We define that under common mode operation, the output voltage (v) is equal to the input voltage (v), which leads to the common mode output voltage equating to the common mode gain (A′′). This is formulated as the product of the negative gain and some component of the input signal.
Examples & Analogies
Think of a seesaw: when both sides (input signals) are lifted equally (common mode), the center remains balanced—a representation of how the common mode gain functions, maintaining balance in the output despite equal inputs.
Improvements in Common Mode Gain
Chapter 4 of 7
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Now, again you can see here because the resistance now it is getting changed. So, this A it is much smaller than the previous common mode gain.
Detailed Explanation
The change in circuit resistance indicates that the common mode gain (A′′) is reduced compared to previous configurations. As a result, using an active current mirror load results in better performance by diminishing the common mode gain, which is favorable for reducing noise in the output.
Examples & Analogies
Consider a garden with different plants (signals) where some spread their roots farther, affecting others. By introducing a better barrier (active load), we can hinder those influences, improving the overall health of the garden (the signal output).
Differential Input to Single-Ended Output Gain
Chapter 5 of 7
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So, this is one consequence of having this active current mirror load and the gain at this point, gain at this point if I say that. So, that is becoming g ( ). In fact, you may say g or g both are same.
Detailed Explanation
The active current mirror load allows for converting differential signals to a single-ended output without altering the gain significantly. When we express the differential input to single-ended output gain (g), we observe it remains consistent regardless of how it is categorized.
Examples & Analogies
Think of a speaker producing stereo sound (differential) but you want to connect it to a single channel (single-ended). The output maintains richness while still staying true to the original sound—reflecting how this circuit preserves gain during conversion.
Practical Applications of Current Mirrors
Chapter 6 of 7
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So, we can say that DC voltage here it is V ‒ V and hence this voltage can directly be used to bias PMOS transistor in the subsequent stage.
Detailed Explanation
The voltage derived from the previous stages, calculated as the difference V – V, serves as a DC bias for the PMOS transistor in later amplifier stages. This highlights the utility of current mirrors in managing biasing effectively, ensuring that the transistor operates within its preferred range.
Examples & Analogies
This situation is akin to a relay race where each runner (transistor) passes the baton (voltage) to the next. The successful transfer ensures that every runner stays energized and at their peak performance through each segment of the race.
Summary of the Lecture
Chapter 7 of 7
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To summarize what we have done it is we have started with a small signal model of current mirror particularly, under DC condition as well as whenever it is carrying the signal current for both BJT and MOSFET versions.
Detailed Explanation
In the final summary, we encapsulate the key points discussed throughout the class. It includes the small signal model of current mirrors for both BJT and MOSFETs, their application as biasing elements in amplifiers, and the enhancement of common mode and differential mode gains using active loads.
Examples & Analogies
It's like wrapping up a cooking class by summarizing how each ingredient (topic) combined to create a delicious dish (final understanding of current mirrors and their applications). Each component plays a role, just as each concept contributes to the overall comprehension of the circuit design.
Key Concepts
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Common Mode Gain: The gain that results from common signals applied to both inputs.
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Differential Mode Gain: The gain generated by the difference between two input signals.
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Current Mirror: A device that maintains a constant output current under varying load conditions.
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Diode-Connected Load: This load configuration allows for stable output current by mimicking a diode's behavior.
Examples & Applications
Example of a common source amplifier with a diode-connected load, improving gain stability.
Illustration of how a current mirror stabilizes the biasing in a common source amplifier.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
In signals we thrive, watch the gains strive, lower noise, keep the clarity alive.
Stories
Imagine a gardener using mirrors to reflect sunlight onto plants while reducing shadow — similar to how current mirrors help amplify signals while minimizing interference.
Memory Tools
C-D-C: Common mode reduces, Differential mode confers clarity.
Acronyms
G-C-G
Gain from Current gain
decreased for Common Mode — Get it?
Flash Cards
Glossary
- Common Mode Gain
The ratio of the output signal to the input signal when both inputs are driven by the same signal.
- Differential Mode Gain
The ratio of the output signal to the difference between the two input signals.
- Current Mirror
A circuit that provides a constant current to a load independent of the load voltage.
- Common Source Amplifier
A type of field-effect transistor amplifier configuration where the source terminal is common to both input and output.
- DiodeConnected Load
A load configuration where the load device's terminals are configured such that it operates in a diode mode, ensuring the current remains constant.
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