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Introduction to Current Mirrors in Differential Amplifiers
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Today, we'll focus on current mirrors and their essential role in differential amplifiers. Can anyone tell me what a current mirror does?
Is it used to maintain a constant current in a circuit?
Exactly! A current mirror ensures that the current in one branch of a circuit is mirrored in another branch. This is crucial for biasing in differential amplifiers.
How do we calculate the DC current through those transistors?
Good question! We can calculate it using the resistor values and the supply voltage. For instance, if we have an 11.4 kΞ© resistor with 12V, we can find the current flowing through it.
So, does that mean if the resistor value changes, the current also changes?
Yes, that's correct! The current is inversely proportional to the resistance. Letβs go ahead and replace the resistor value and see how it affects the current.
What happens to the current through the current mirror when the input varies?
Excellent point! The current mirror must adapt to that variation by providing an equal mirrored output, which brings us to small signal analysis and gain calculations.
In summary, current mirrors are key for stabilizing currents in differential amplifiers, particularly important for biasing transistors.
Calculating the Differential and Common Mode Gains
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Letβs now move on to calculating the differential mode gain. Who remembers how we go about this?
Is it the output voltage divided by the input voltage?
Exactly! The differential mode gain can be expressed as A_d = g_m * (r_1 || r_2), where g_m is the transconductance. Can anyone tell me what the '||' signifies?
It means we take the parallel resistance of r_1 and r_2.
Yes! Now, how does common mode gain differ?
In common mode, the input signals are the same at both input terminals, right?
Correct! Therefore, common mode gain is much smaller as external variations are effectively canceled out. Let's calculate it using the given resistor values.
So, the common mode gain tells us how well the circuit rejects noise?
Exactly! This rejection is vital for signal integrity. Letβs summarize.
In summary, understanding differential and common mode gains is essential for evaluating amplifier performance.
Introduction & Overview
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Quick Overview
Standard
The section focuses on the role of current mirrors within differential amplifiers, illustrating how they operate to mirror currents and maintain biasing. Included are numerical examples to calculate DC currents, differential mode gain, and common mode gain relevant to these circuits.
Detailed
Detailed Summary of Analog Electronic Circuits
In this section, Prof. Pradip Mandal introduces the fundamentals of current mirrors and their application within differential amplifiers, particularly in creating active loads. The key focus is on the operation and analysis of a differential amplifier that employs a current mirror configuration, specifically detailing the interactions between multiple transistors and the significance of mirrored currents. The analysis begins with determining the direct current (DC) levels through various transistors based on circuit parameters such as resistor values and voltage supplies.
Key Points Discussed:
- Current Mirror Functionality: The role of transistor-7 and transistor-8 in mirroring current to maintain balanced output.
- DC Calculations: Establishing the current flow through transistors based on given resistances (e.g., 11.4 kΞ© resistor yielding 1 mA current).
- Gain Calculations: Differentiating between differential mode gain and common mode gain, through small signal models.
- Sensitivity to Inputs: Discussion on how input common mode signals are managed through mirrored gains and output nodes.
- Numerical Examples: Engaging with the quantitative aspects including differential mode and common mode gains with provided examples emphasizing situations with identical transistor assumptions and BJT/MOSFET equivalences.
- Conclusion: Summarizing the importance of understanding current mirrors for effective circuit design in analog electronics.
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Audio Book
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Introduction to Differential Amplifiers and Current Mirrors
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Yeah. Welcome back to our next example. So here we do have the differential amplifier and the load of course, it is active load, but internally we do have current mirror. So, what we are expecting here it is transistor-7 DC wise it will be mirroring its own DC current into transistor-8 and that is good. In fact, we want this current should also be same as this 1 from balance point of view. So, we do not have to put any external circuit to bias here and in addition to that this transistor-7 it may also mirror signal current.
Detailed Explanation
In this section, the focus is on a circuit configuration known as a differential amplifier which includes an active load realized by using a current mirror. The term 'current mirror' refers to a circuit that produces a current that is a mirror image of the input current, which helps ensure stability in the circuit's operation. Specifically, transistor-7 is expected to mirror its own direct current (DC) to transistor-8, allowing both to maintain the same current level without the need for extra biasing circuits. Additionally, this mirroring arrangement can also handle signal currents, providing versatility in the signal processing of the amplifier.
Examples & Analogies
Consider a water pipe where water pressure needs to be consistent at two exits. If the first exit (transistor-7) maintains a steady pressure (current), then the second exit (transistor-8) will automatically have the same pressure due to the structure of the system (current mirror). This automatic adjustment ensures that you donβt need an additional pump (biasing circuit) to maintain pressure at the second exit.
DC Current Calculations in the Circuit
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So, we do have to find the DC currents through all the transistors. Again the value of this resistor it is 11.4 k, so that makes this current = 1 mA. So, the current flow here it is also 1 mA and if we are applying meaningful input common mode DC then half of the current is flowing here and remaining half it is here. So, each one of them it is 0.5 mA so this 0.5 mA it is getting mirrored here.
Detailed Explanation
To analyze the circuit further, one must determine the direct current (DC) values flowing through the various transistors involved. In this specific circuit, a resistor value of 11.4 kΞ© results in a total current of 1 mA. When a common mode DC voltage is applied to the circuit, it divides equally across two paths, leading to 0.5 mA flowing through each path. This current of 0.5 mA is then mirrored by the current mirror configuration, allowing both transistors (7 and 8) to share this current equally.
Examples & Analogies
Imagine a split water flow at an intersection where a main pipe (11.4 kΞ©) allows 1 liter of water to flow. When two secondary pipes branch off equally, each pipe takes 0.5 liters. This equal splitting is like the current in the electronic circuit, ensuring both parts receive the same input without needing extra interventions.
Voltage Levels in Transistor Configuration
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if you ignore this early voltage to calculate the nominal quiescent current... So, I should say DC voltage at output-1 and output-2 both are 11.4 V.
Detailed Explanation
In evaluating the nominal quiescent current in the circuit, it is assumed that the early voltage effects can be neglected. Consequently, the analysis simplifies as we focus on the direct DC voltage levels. Both output nodes, referred to as output-1 and output-2, are observed to maintain a voltage of 11.4 V. This consistent DC voltage aids in establishing a stable operating point for the transistors involved.
Examples & Analogies
Think of a leveling tool that guarantees two shelves (outputs) are at the same height (voltage level). Despite fluctuations or possible wobbling (minor influence of early voltage), both shelves remain firmly at the same height, ensuring balance and providing a reliable workspace.
Differential and Common Mode Gain Calculations
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So, yeah so now our next calculation it is we need to find differential mode gain and common mode gain.
Detailed Explanation
The next step in circuit analysis involves computing two important parameters: differential mode gain and common mode gain. Differential mode gain measures how effectively the amplifier amplifies the difference between two input signals. On the other hand, common mode gain indicates the circuit's ability to amplify signals that are common to both inputs. Through specific calculations involving small signal parameters and resistor values present in the circuit, these gains can provide insights into the performance and efficiency of the amplifier configuration.
Examples & Analogies
Imagine a microphone that only picks up voice differences effectively (differential mode gain), while ignoring background noise that is the same level across its two recording sources (common mode gain). The effectiveness of our microphone indicates how well it can differentiate desired signals from common unwanted sounds.
Conclusion and Summary of Findings
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So, yeah so this is and then you can probably you can find what will be the numerical value.
Detailed Explanation
In conclusion, the session discussed numerical examples centered around a current mirror, moving on to analyze different amplifier configurations like the differential amplifier using both BJTs and MOSFETs. Each example reinforced the practical aspects of calculating the quiescent current, differential mode gain, and common mode gain, showcasing key principles in analog electronic circuits.
Examples & Analogies
Think of a cooking class where an instructor illustrates different techniques (current mirrors and amplifiers) to prepare a meal (analog circuits). By practicing each method and measuring the outcomes (gains and currents), students learn the effective ways to achieve the best results in their recipes (electronic applications).
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Current Mirror: A vital component for mirroring currents in differential amplifier circuits.
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Differential Gain: Measures how well a differential amplifier amplifies the difference in signals.
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Common Mode Gain: Reveals how well a circuit rejects noises and interferences common to both input signals.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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Example of calculating DC current through a transistor using a known resistor value: Given a supply voltage of 12V and a resistor of 11.4 kΞ©, the current I = V/R gives approximately 1mA.
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Calculating differential mode gain using the transconductance and resistances; if g_m = 0.5 and r1 = r2 = 200kΞ©, then A_d = 0.5 * (200k || 200k).
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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Mirroring current, oh so bright; Keeps our circuits working right!
π Fascinating Stories
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Imagine a shopkeeper who wants to sell exactly a thousand apples each day. He looks at his original stock and makes sure to always sell the same amount from everyone else. This is how a current mirror functions, keeping everything balanced.
π§ Other Memory Gems
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C-D-C: Current Mirror - Differential - Common Mode. It reminds us of the three key concepts in this section.
π― Super Acronyms
G-C-R
- Gain
- Current
- Rejection. This highlights the essence of differential amplifiers.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Current Mirror
Definition:
A configuration used in analog circuits to replicate a current from one branch of the circuit to another, providing stability and biasing.
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Term: Differential Amplifier
Definition:
An amplifier that amplifies the difference between two input signals and rejects common-mode signals.
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Term: Transconductance (g_m)
Definition:
A measure of how effectively a circuit converts changes in input voltage to output current.
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Term: Common Mode Gain
Definition:
The gain experienced by common signals applied to both inputs of an amplifier, ideally minimized in differential amplifiers.
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Term: Small Signal Model
Definition:
A linear approximation of a non-linear circuit close to an operating point, used for small perturbations.