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Interactive Audio Lesson
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Current Mirrors in Common Emitter Amplifiers
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Today, we'll be discussing how current mirrors function in common emitter amplifiers. Can anyone explain what a current mirror is?
I think it's a circuit that provides a constant current... but how does it work in amplifiers?
Good question! A current mirror takes a reference current and mirrors it to other parts of the circuit. This helps in maintaining stable biasing. For example, in our amplifier circuit, we can ensure that certain transistors are always biased properly.
So, are the transistor configurations important for this to work?
Absolutely! The characteristics of the transistors, such as their β value and Early voltage, play a crucial role in determining how accurately the current is mirrored. Can anyone tell me how we calculate the collector current given these parameters?
Isn't it based on the biases and resistances connected to the transistors?
Exactly! The collector current can be determined by the bias voltages and resistances in the circuit, allowing us to design efficient amplifiers. Remember, precision is key in achieving the desired performance.
To summarize, current mirrors are vital for biasing in amplifiers, ensuring stable operation, especially in differential amplifiers. Understanding their function leads to improved circuit design.
Calculating Collector Current
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Now, let's calculate the collector current in our example. Can anyone tell me the relationship between the collector current and the biasing resistances?
I believe there's a formula that involves the base current and the resistances connected.
Correct! Using the β value and assuming negligible base current losses, we can derive the collector current. We set the collector current of specific transistors equal to each other for balance.
The example mentioned 2 mA for the collector current; how did that value come about?
Great observation! The 2 mA was derived through the biasing resistors, which ensured adequate current flow. Can anyone mention the resistor values leading to this current?
Wasn't it 570 kΩ for each biasing resistor?
That's right! Such calculations enable us to achieve a well-defined operating point for our amplifier, ultimately impacting its performance.
In summary, knowing how to calculate the collector current precisely contributes to the effective design and application of amplifiers.
Output Resistance and Voltage Gain
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Next, let's address the output resistance and voltage gain of our amplifier circuit. What do you think is the output resistance derived from?
I think it comes from the combination of resistances through the transistors, right?
Exactly! It’s influenced by the small signal model of the transistors in the active region. Can anyone recall what typical value we derived for the output resistance?
Wasn’t it 25 kΩ?
Correct again! A higher output resistance typically leads to better voltage gain, which in this case was approximately 1923. Why do you think such high gain is expected?
It’s because of the active load from the current mirror?
Spot on! The current mirror effectively increases the gain by providing high output impedance. Remember, output characteristics are crucial for designing effective amplifiers.
In summary, the output resistance and voltage gain interplay significantly in amplifier design, impacting overall performance.
Effects of Early Voltage
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Now let’s delve into the effects of Early voltage on our current mirror configuration. How does this affect our calculations?
Doesn’t it impact the collector current accuracy?
Absolutely! A mismatch in the Early voltage between transistors can lead to inaccuracies in the expected collector current. Can someone relate this to the DC output voltage?
I suppose it can cause the output voltage to deviate from expected values?
Exactly! This is critical for high-impedance nodes where even small deviations can cause significant voltage changes.
So we need to minimize mismatches in real applications?
Yes, ensuring matched transistor parameters is crucial. In summary, taking Early voltage into account is essential for precise circuit design, especially in analog applications.
Introduction & Overview
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Quick Overview
Standard
In this section, we explore the application of current mirrors in common emitter amplifiers, examining biasing, output resistance, and voltage gain. It discusses numerical examples that illustrate the relationships between various circuit elements and demonstrates how to calculate critical values, such as collector current and voltage gain, under certain assumptions.
Detailed
In Lecture 88, Professor Pradip Mandal presents a detailed analysis of common emitter amplifiers utilizing current mirrors for biasing. The lecture begins with a discussion of identical transistors Q1 and Q2, examining how their configuration and parameters, like beta (β) values and Early voltage, affect performance. The relationship between the collector current and bias resistors is outlined, leading to specific calculations for achieving desired collector currents. The section evaluates small signal output resistance and voltage gain, demonstrating how they relate to the active region of operation for the devices involved. A DC output voltage calculation is performed, noting the importance of considering Early voltage and potential mismatches in components. Subsequent examples illustrate applications, highlighting the significance of precision in current mirrors for differential amplifiers and other circuits. Overall, this section provides a comprehensive understanding of utilizing current mirrors in analog circuits, emphasizing the impact of transistor properties on circuit behavior.
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Audio Book
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Introduction to Current Mirrors and Common Emitter Amplifiers
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Dear students, welcome back after the break. So, we are going through different numerical examples and now we are going to talk about one common emitter amplifier which is using current mirror and particularly to bias the active load say Q. We are using current mirror and transistor-1; it is the amplifying device then we are assuming that Q and Q they are identical and also we are assuming that whatever this Q and Q are also identical.
Detailed Explanation
In this chunk, we are introduced to the topic of common emitter amplifiers. We learn that a common emitter amplifier is a configuration that amplifies voltage signals using a transistor (here referred to as transistor-1) and a current mirror. A current mirror is a circuit that replicates a current through one active device by controlling the current through another active device of a circuit. The idea here is to keep certain components, specifically transistors Q1 and Q2, identical to ensure predictable behavior in the circuit.
Examples & Analogies
Think of a common emitter amplifier as a microphone in a concert. The microphone captures the sound (the input signal) and amplifies it, much like the transistor amplifies the input voltage. The current mirror acts like a manager who ensures that sound levels are consistent across all speakers (transistors) by adjusting them based on the performance of one particular microphone.
Collector Current Matching
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So, to get the I current of transistor-1 and collector current or transistor-4 equal. We want the current flow through transistor-2 should be equal to current flow through transistor-1. And since, Q and Q are identical having the same β value of 100.
Detailed Explanation
This chunk discusses the importance of matching collector currents in a transistor circuit. The collector current of transistor-1 (I_C1) must equal the collector current of transistor-4 (I_C4). This is crucial for the circuit to function correctly, ensuring that the mirrored currents through transistors are equivalent. Since the transistors Q1 and Q2 are identical and have the same beta (β), which represents the current gain, it means they will respond the same way to voltage changes.
Examples & Analogies
Imagine you are at a relay race, where each runner (transistor) must pass the baton (current) to the next with the same timing and speed to ensure a smooth handoff. If one runner is much faster or slower, it could disrupt the race plan, similar to how mismatched currents can cause issues in an electronic circuit.
Determining Resistance for Desired Collector Current
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So, with that we do have this current is also 2 mA and if we assume that the base current loss it is ignorable, then you can say that collector current of transistor-3 it is also 2 mA which is getting mirrored to transistor-4.
Detailed Explanation
In this section, we identify the collector current of the transistors as 2 mA, which is critical for ensuring proper amplification. By assuming that the base current loss is negligible, we conclude that the currents in the relevant transistors (such as transistor-3) also reflect this 2 mA value. This means that transistors in the current mirror configuration effectively replicate the input current levels.
Examples & Analogies
Think of a factory assembly line where each worker (transistor) is responsible for assembling parts (current). If one worker successfully completes their part at a rate of 2 parts per hour, and all are identical and work at the same pace, others will replicate that output without issues, ensuring the production line operates smoothly at that level.
Calculating Output Resistance and Voltage Gain
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Now, with this information let us try to find the small signal output resistance and voltage gain of the amplifier.
Detailed Explanation
Here, we introduce the calculations for determining the small signal output resistance (R_out) and the voltage gain of the amplifier. The output resistance is derived from the r_o values of the transistors, which characterize how the transistors will respond to changes in output voltage. The voltage gain is calculated based on the transconductance (g_m) and the output resistance, typically resulting in a high amplification factor, indicating the ability of the amplifier to boost signal levels.
Examples & Analogies
Imagine you have a water pump system. The pump’s ability to increase water pressure represents voltage gain. The resistance to the flow, akin to output resistance, determines how efficiently the pump works against potential leaks or blockages. The more effectively a pump can build pressure (higher gain) while managing flow (output resistance), the more reliable and powerful the water delivery system becomes.
Understanding DC Output Voltage
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As I said that the current flow current flow here and here should be equal and if you see it carefully the DC voltage here it is defined by this V – V drop. So, I should say the voltage here it is 12 V. So, the voltage here it is 12 – 0.6 so, that is 11.4 V.
Detailed Explanation
This section emphasizes the importance of ensuring that current flow remains consistent across the transistors, which in turn affects the DC output voltage of the amplifier. The voltage drops are calculated based on certain parameters, resulting in a DC output voltage of 11.4 V. Understanding these voltage drops and currents within the circuit helps ensure that components operate within their intended ranges, thereby stabilizing overall performance.
Examples & Analogies
Think of a battery in an electronic device. The battery provides a specific voltage (the source voltage), and as current flows through various components, some voltage (energy) gets 'used up' (drops) due to resistance. If the battery voltage is too low or if too much energy is lost, devices may fail to operate properly, similar to how the DC voltage in the circuit needs to maintain a healthy level to function reliably.
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 configuration providing steady current outputs for transistors.
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Voltage Gain: The expected amplification factor in an amplifier circuit.
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Collector Current: Essential for biasing and signal amplification in transistors.
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Early Voltage: A crucial parameter influencing transistor performance.
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Output Resistance: Impacts voltage gain and overall circuit behavior.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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When calculating the collector current in a common emitter amplifier, if a 2 mA collector current is desired, appropriate biasing resistors should be used to achieve this value.
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In cases of mismatched Early voltages, the output voltage can deviate significantly, illustrating the need for precision in design.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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With a current mirror bright, the current stays just right!
📖 Fascinating Stories
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Imagine a steady stream of water flowing; a current mirror ensures it flows smoothly no matter the changes upstream.
🧠 Other Memory Gems
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C-V-A: Collector Current, Voltage Gain, and Early Voltage.
🎯 Super Acronyms
C-M-E
- Current Mirroring Efficiency.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Current Mirror
Definition:
A circuit that provides a reference current which is mirrored to other branches, maintaining a constant current across multiple transistors.
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Term: Common Emitter Amplifier
Definition:
A configuration of an electronic amplifier that allows the output voltage to be inverted and amplified while using a single-ended input.
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Term: Beta (β)
Definition:
The current gain factor for a transistor, indicating the ratio of the collector current to the base current.
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Term: Early Voltage
Definition:
The parameter that accounts for the variation in collector current due to changes in collector-emitter voltage in a transistor.
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Term: Collector Current (Ic)
Definition:
The current flowing out of the collector terminal of a transistor.
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Term: Output Resistance
Definition:
The resistance seen by the load connected to the output of the amplifier, affecting voltage gain.