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84.1.6. Improvements on Common Mode Gain
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Today, we will explore how current mirrors can enhance amplifiers' performance, particularly in common collector configurations. Can anyone explain what a current mirror does?
Isn't it used to provide a stable reference current?
Exactly, a current mirror generates a controlled current. In common collector amplifiers, we can set a reference current, which helps improve common mode gains. Why do you think that might be important?
It might help reduce noise and distortion in the output?
Correct! Lower noise and improved stability lead to clearer signals. Let's compare using passive loads versus active loads. Does anyone recall the general advantage of using active devices?
Active devices can provide higher performance metrics, right?
Exactly. They improve both the differential mode and common mode gains, which we will delve deeper into now.
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Before diving into specific examples, can someone explain what we mean by small-signal equivalent circuits?
It’s a method to simplify complex circuits to analyze their behavior under small signal variations?
Exactly! In our case, for a common collector amplifier, we focus on the small signal parameters from the current mirror which don't carry any signal current. What parameters are we particularly interested in?
We often look at the output resistance and voltage gain.
Perfect! The output resistance will influence how much gain we can achieve. Let’s look at how we derive these parameters from our examples.
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Create a free accountNow, let’s shift gears to differential amplifiers. Can anyone tell me how current mirrors can affect the performance of these amplifiers?
They can replace tail resistors with active loads to improve the common mode gain?
Exactly! By making this change, we allow the amplifier to maintain a more consistent output, even when the common mode input varies. What’s the consequence of this for the gain?
It increases both the differential and common mode gain, but with a better balance than using passive loads.
Right! And when we examine the small signal equivalents of such a configuration, we can quantify these gains. Could someone elaborate on how that works?
You replace the resistors with transistors and calculate using their transconductance?
Exactly! Well done. Let's now see practical circuit examples that illustrate these concepts.
Overview
Short Summary
This section discusses the utilization of current mirrors to enhance the performance of common collectors and differential amplifiers by improving their common mode gain.
Medium Summary
The section elaborates on how current mirrors are applied in common collector and differential amplifiers, focusing on their role in enhancing performance metrics like common mode gain. Key concepts are illustrated through circuit explanations, emphasizing the advantages of using active loads instead of passive resistors.
Detailed Summary
Improvements on Common Mode Gain
This section primarily addresses the various ways current mirrors can be implemented in amplifiers to enhance their common mode gain. Initially, it discusses their application in common collector amplifiers, where the current mirror configuration can effectively set the reference current for transistors and influence their collector currents. The section explains how the relationships between the reference current and collector currents of transistors can be approximated when certain non-ideality factors are minimized.
Next, the discussion shifts to common drain amplifiers where the principles are visually mirrored through similar analogies with transistor configurations. Again, the emphasis on drawing equivalences in small-signal analyses highlights the impacts of current mirrors on voltage gain and common modes. The section concludes with an extensive evaluation of current mirror applications in differential amplifiers, showcasing their advantages in improving both differential and common mode gains by replacing tail and load resistors with active devices. Detailed examples illustrate the practical implementations, ensuring that learners can grasp the theoretical and real-world significance of these concepts.
Reference YouTube Videos
Audio Book
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Create a free accountSo in this circuit what we are what we mentioned here it is essentially this R we are replacing by a current mirror. And this current mirror it is of course, it is current it is said by the DC supply voltage and then R and whatever it is dimension we do have. In fact, similar to the previous case here again if I say that this is the reference current I ; then you can say V = I × R + V .
Detailed Explanation
In this chunk, the original tail resistor in a differential amplifier circuit is replaced with a current mirror. The role of this current mirror is to maintain a stable current that is determined by a reference DC voltage and a resistor (R). By doing so, the amplifier can improve its performance, particularly in managing common-mode signals.
Examples & Analogies
Think of a current mirror as a traffic signal that controls traffic flow on multiple roads. Just like a traffic signal ensures that cars move smoothly by managing the flow, a current mirror ensures a consistent flow of current in the circuit, preventing fluctuations that could lead to errors in amplification.
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Create a free accountOn the other hand I = I and we know this I and V it is having known relationship. So, ( ) . We can drop the ( ) part and again utilizing these 2 equations we can find the reference current as we have discussed earlier.
Detailed Explanation
Here, we see that the reference current (I_REF) being used is derived from relationships between the voltages and resistances in the circuit. By using the equations relevant to the circuit design, the current can be accurately calculated and maintained, providing the necessary stability for the amplifier.
Examples & Analogies
Imagine you are filling multiple same-sized buckets with water from a tank. If you know the flow rate of the water and how much each bucket can hold, you can determine how long it will take to fill each bucket without overflowing. Similarly, the equations help to maintain stable current levels in the circuit.
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Create a free accountIn contrast to this circuit which you have discussed before, if you change the DC voltage or common mode DC voltage in the circuit coming here and here if you vary this DC current this node voltage the source node voltage also changes.
Detailed Explanation
This chunk discusses how changes in common mode voltages can affect circuit performance. The tail current from a tail resistor can be sensitive to these changes, leading to variations in output that could reduce overall amplifier performance. However, with a current mirror, the effect on the tail current is minimal, thus enhancing stability.
Examples & Analogies
Think of this as a juggling act. If the juggler is trying to maintain a steady rhythm while the ground beneath them shifts, it becomes difficult to keep all the balls in the air. Using a current mirror is like solidifying the ground, allowing the juggler to maintain a consistent rhythm despite fluctuations.
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Create a free accountNow in comparison with this if you consider this circuit having the tail current mirror suppose we do have a common DC voltage at both the inputs again if we vary this common DC voltage then the voltage here it will vary.
Detailed Explanation
In this part, the common mode gains of circuits with a tail resistor and with a tail current mirror are compared. It is recognized that with the current mirror, the response to common DC voltage changes is significantly more stable, thus improving the common mode gain.
Examples & Analogies
Imagine two different styles of playing a piano. One pianist adjusts their playing every time someone in the audience coughs, while another continues flawlessly regardless of minor disturbances. The second pianist represents a circuit with a current mirror, showcasing how it better maintains its output consistency during variations.
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Create a free accountSo, now if you replace the load part by active current mirror then what will be getting is the improvement of common mode and differential mode gain.
Detailed Explanation
This chunk explains how replacing passive load resistors with active current mirrors enhances both common mode and differential mode gains. The active current mirrors allow for better current management in the circuit, improving overall performance.
Examples & Analogies
This can be likened to upgrading from a traditional bicycle to an electric bike. The electric bike adjusts itself to offer better resistance, allows for a smoother ride, and handles various terrains more effectively. Similarly, replacing passive components with active ones in a circuit can lead to better performance.
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
Current Mirrors: Essential for setting reference currents in amplifier designs, allowing better control over output.
Common Mode Gain: A critical performance metric that reflects how well an amplifier can mitigate unwanted signals common to both inputs.
Differential Mode Gain: Represents the amplifier's ability to amplify the difference between two input signals.
Active Loads: Using active devices instead of passive components can significantly enhance performance metrics.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
In a common collector configuration, replacing the tail resistor with a current mirror minimizes the effect of common mode signals on the output.
By using active loads in differential amplifiers, we can achieve higher differential and common mode gains compared to normal passive resistors.
Memory Aids
Interactive tools to help you remember key concepts
Stories
Flash Cards
Glossary
Current Mirror
A circuit that provides a constant reference current, enabling precise current control in different parts of a circuit.
Common Collector Amplifier
Also known as an emitter follower, it is an amplifier configuration where the output is taken from the emitter of the transistor.
Common Mode Gain
The gain experienced by a signal that is common to both inputs of a differential amplifier.
Differential Amplifier
An amplifier that amplifies the difference between two input signals while rejecting common signals.
Active Load
A load in an amplifier circuit that uses active devices (like transistors) rather than passive components (like resistors) to improve performance.