Common Mode Rejection Enhancement
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Overview of Common Mode Rejection
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Today, we're diving into common mode rejection, specifically its enhancement through current mirrors in amplifier circuits. Can anyone tell me what common mode rejection means?
It's the ability of an amplifier to reject input signals that are common to both inputs.
Exactly! Now, what happens if the common mode gain is high?
It could lead to distortion in the output signal.
Right again! The goal is to minimize this common mode gain, and that's where our main topic comes in—using current mirrors.
Analyzing the Common Source Amplifier
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In our modified common source amplifier setup, we connect a diode to create a current mirror. Can someone explain what a current mirror does?
It replicates the current in one branch of a circuit into another branch.
Exactly! By using it, we can drastically decrease the common mode gain. How do we express this mathematically?
It's defined by A′′ = ‒g × r, where g is the transconductance.
Great observation! This relation shows how the output directly links to the input signal and enhances our circuit performance.
Implications of Current Mirrors
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Now, let’s explore how this circuit affects DC biasing for PMOS transistors. Why is this important?
It ensures that they operate in the correct region and enhances stability.
Correct! The DC voltage can be obtained from the previous transistor's drain node. How does this help?
It simplifies our design and ensures efficient biasing while maintaining gain.
Excellent point! We see that current mirrors not only enhance gain but also support effective biasing in differential configurations.
Summary and Key Takeaways
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To recap today’s discussions, what are the main benefits of using current mirrors in amplifiers?
They reduce common mode gain and stabilize DC biasing.
They improve differential mode gain too!
Exactly! Enhanced common mode rejection is crucial for clearer signals in amplifier circuits. Great job, everyone!
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, we analyze a modified amplifier circuit where a current mirror is used, resulting in a noteworthy decrease in common mode gain. The relationship between input signals and output characteristics is detailed, showcasing advantages in design considerations for effective biasing in differential amplifiers.
Detailed
In this section, we explore the functionality of a circuit resembling a common source amplifier with a diode-connected load, effectively transforming it into a current mirror configuration. The analysis reveals that under common mode operation, the output voltage is equal to the input voltage times a gain factor defined by the transconductance 'g'. Notably, the presence of a current mirror significantly decreases the common mode gain, calculated as a negative product of transconductance and feedback resistances. This modification allows for enhanced performance in differential amplifiers by improving both common mode and differential mode gains while ensuring that crucial DC biasing conditions are met for subsequent stages of amplification. We also highlight the implications of utilizing current mirrors in biasing elements contributing to the circuit's overall efficacy.
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Understanding the Circuit Configuration
Chapter 1 of 5
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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 of representation and at this point we are giving the small signal along with the DC. Of course, this node it is getting the bias similar to this point and we like to know what will be the corresponding output here due to the small signal and this small signal it is v .in_c
Detailed Explanation
This chunk introduces the basic circuit configuration used in the study of common mode rejection. It describes a common source amplifier where a diode-connected load is utilized. The circuit includes transistors where specific labels (M1, M7) and connections (to DC voltage VDD) are mentioned. The text emphasizes that at this point, a small signal is superimposed on a DC bias. It’s crucial to note that this setup allows us to analyze how the circuit responds to small input signals in addition to the constant DC voltage.
Examples & Analogies
Think of this circuit like a water pipe system. Just as a water source provides a constant flow of water (DC bias) while also allowing for variations in water pressure from a hose connected (small signals), this circuit maintains a steady voltage while handling fluctuating input signals.
Small Signal Analysis
Chapter 2 of 5
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Chapter Content
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. So, we may say that v under common mode operation is also = v and that gives us v = v common mode.
Detailed Explanation
This section explains the process of small signal analysis by transitioning to the small signal equivalent circuit. It states that under this analysis, the output voltage (vo) can be represented as a function of the small signal gain (g), depending on the input small signal (vin_c). The relationship implies that during common mode operation—where both inputs are treated the same—the output for both nodes is equal. This reinforces the concept of common mode signals and how they affect the circuit's response.
Examples & Analogies
Imagine tuning a radio to hear a specific station (the common mode signal). In this analogy, the adjustments in volume can reflect how much of the input signal you're amplifying. Just as the radio adjusts its output for a steady stream of music, this circuit adjusts its output voltage based on the uniform input signals.
Impact of Circuit Elements
Chapter 3 of 5
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Chapter Content
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. So, to summarize what we can see the advantage of having this current mirror in the load part.
Detailed Explanation
This chunk discusses the impact of changing resistance in the circuit. It indicates that the gain associated with common mode signals (denoted A) becomes smaller than what it was previously. The introduction of a current mirror in the load improves the circuit by allowing better control over the gain, resulting in enhanced performance in rejecting unwanted common mode signals.
Examples & Analogies
Consider a speaker using equalization to lower background noise while maintaining sound quality. Similarly, by using a current mirror, the circuit is fine-tuning its response to lessen the effects of common mode signals, letting the desired signals stand out.
Final Observations on Common Mode Gain
Chapter 4 of 5
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Chapter Content
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. So, we can say that differential input to single ended output gain it is same as whatever the gain earlier you obtained.
Detailed Explanation
This portion explains a critical consequence of incorporating an active current mirror load—it provides a means of keeping the differential input to single ended output gain unchanged despite modifications in the circuit. The paragraph implies that while adjustments are made to the current mirror's configuration, the effective gain derived is still consistent with prior calculations, reinforcing the reliability of the current mirror setup.
Examples & Analogies
Think of this as using a dependable translator who maintains the meaning of speech regardless of how fluently it is spoken. The current mirror acts as such a translator, keeping the core message (or gain) intact, despite various changes in the input's format.
Applications and Practical Implications
Chapter 5 of 5
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Chapter Content
So, this helps to improve the common mode rejection drastically. I think that is all to cover. In the next class we will be talking about numerical examples.
Detailed Explanation
This final chunk reveals the importance of the current mirror in enhancing common mode rejection in the circuit. It emphasizes the significance of practical applications in circuit design, potentially leading to improved audio systems, communication equipment, and more, all requiring effective signal processing capabilities. The instructor suggests that further understanding will continue in a subsequent class with numerical examples to highlight the concepts discussed.
Examples & Analogies
Visualize this as tuning out static noise while listening to a podcast during a morning commute. Just as filters help isolate the voice you want to hear, current mirrors in this circuit help to focus on the desired signal while minimizing background interference, greatly improving communication clarity.
Key Concepts
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Common Mode Rejection: The capability of an amplifier to suppress common input signals.
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Current Mirror: A configuration used to maintain the same current in a circuit, enhancing output characteristics.
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Transconductance: The parameter reflecting how input voltage variations affect output current in an amplifier.
Examples & Applications
Using a current mirror to bias a common source amplifier, leading to improved linearity and signal integrity.
A differential amplifier with reduced common mode gain using current mirrors results in clearer differential signals.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Common mode gain needs a low, To keep the output from reaping woe.
Stories
Imagine an amplifier trying to listen to a whispered secret. If everyone nearby whispers the same secret, it can’t differentiate. By lowering common mode gain, it can tune out the noise and pick up the actual message clearly.
Memory Tools
C-R-A-C: Common mode rejection is aided by current mirrors.
Acronyms
CME
Common Mode Enhancement using transconductance.
Flash Cards
Glossary
- Common Mode Gain
The gain of a signal that is common to both inputs of a differential amplifier.
- Current Mirror
A circuit that mirrors the current flowing in one active device by controlling the current in another active device.
- Transconductance (g)
A measure of how effectively a device can convert variations in input voltage into output current.
- Differential Amplifier
An amplifier that amplifies only the difference between two input signals.
- DC Biasing
The use of a DC voltage to establish the operating point of an amplifier.
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