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Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Current Mirrors in Emitter Amplifiers
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Today, we are going to explore how current mirrors work in common emitter amplifiers. Who can tell me what a current mirror does?
It helps to control the current flowing through the transistors, keeping it stable.
Exactly! It allows us to mirror a specific current in another part of the circuit. This is particularly useful in amplifier designs. Now, can anyone explain why we want our transistors in the circuit to be identical?
If they are identical, they will provide the same characteristics, like current gain and voltage drops.
Absolutely correct! We assume they have the same Beta value for our calculations. Speaking of calculations, who recalls the Beta value we mentioned?
The Beta value is the current gain, right? You mentioned it could be a value like 100.
Great memory! Now let's calculate a resistor value to achieve a collector current of 2 mA...
Calculating Resistances in the Circuit
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In our previous discussion, we established the need for an I value of 2 mA. How do we calculate the necessary resistance R1?
We would need to use Ohm's law, right? R = V/I?
Yes! And given that our Beta is 100, how would we express the relationship between bias current and collector current?
The base current would be Ic/Beta, which means I_B = 20 Β΅A here.
Correct! Given this, how do we find the value for R1 to ensure the base current supports our collector current?
I remember the value you calculated in both transactions was 570 kβ¦, right?
Exactly! Good recall. R1 and R2 must both equal 570 kβ¦, ensuring equal base currents through Q1 and Q2.
Output Resistance and Voltage Gain
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Now, let's shift our focus to small-signal analysis. What do we mean by output resistance in this context?
It's how much the output voltage changes with respect to the output current, right?
Exactly! We derived R_out as the combination of r_o1 and r_o4. Can anyone recall what that value was in our example?
It was 25 kβ¦, right?
Correct. Now, how do we use that to find the voltage gain of our amplifier?
We calculate via A_v = -g_m * R_out. So if g_m was, say, 0.5 mA/26 mV, we multiply it with our output resistance.
Spot on! The voltage gain we computed was nearly 1923. Can anyone explain what this means for our amplifierβs performance?
That means it has a very high gain with the active load configuration.
Impact of Non-Ideal Behaviors
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We mentioned that we can ignore certain base current losses. Why is this an important consideration for our DC output voltage?
It allows us to simplify our calculations, but we must remember that mismatches can occur based on current characteristics.
Right! If the Beta changes in transistors Q3 and Q4, how would that alter our output voltage?
It would lead to variations in currents, potentially dropping the output voltage. We calculated that variation to be 0.45 V.
Excellent insight! The heatmap details such variations, so we must consider these in practical circuit design.
Differential Amplifier Applications
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Next, letβs discuss the application of current mirrors in differential amplifiers. What advantage does it provide?
It helps bias the transistors accurately which improves performance.
Indeed! In our current mirror arrangement, a ratio of 2:1 is established. Can anyone explain how that affects the currents?
It ensures that one branch gets double the current from the reference signal, allowing for better amplification.
Exactly! Itβs crucial to maintain transistor matching for the best performance. Letβs work through some numerical calculations for this application next.
Introduction & Overview
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Quick Overview
Standard
In this section, we delve into the practical application of current mirrors in common emitter amplifiers, specifically focusing on numerical examples that demonstrate the calculation of collector currents, output resistance, and voltage gain. Additionally, we explore how variations in transistor characteristics impact circuit performance.
Detailed
Detailed Summary
In this section, Prof. Pradip Mandal from IIT Kharagpur discusses the utilization of current mirrors in common emitter amplifiers. The lecture includes step-by-step numerical examples to elucidate the functioning of circuits with active load configurations. Key calculations begin with the setup of identical transistors Q1, Q2, Q3, and Q4, aiming for a collector current (Ic) of 2 mA. To achieve this, the resistances are calculated based on the transistor's Beta value and early voltage.
Key Points Covered:
- Circuit Configuration: Introduction to common emitter amplifiers using current mirrors.
- Numerical Calculation: Calculation of biasing resistors (R1 and R2) to achieve collector current.
- Output Resistance and Voltage Gain: Derivation of small-signal output resistance and voltage gain using small-signal parameters and derived equations.
- Impact of Non-Ideal Behaviors: Exploration of early voltage effects and beta mismatch in determining the DC output voltage.
- Differential Amplifier Application: Discussion on further applications of current mirrors in differential amplifier circuits, highlighting the importance of transistor matching for circuit performance.
The section emphasizes practical applications alongside theoretical designs, engaging students with numerical exercises and conceptual understanding.
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Audio Book
Dive deep into the subject with an immersive audiobook experience.
Common Emitter Amplifier with Current Mirror
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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 Q1 and Q2 are also identical.
Detailed Explanation
In this chunk, the instructor welcomes students back and sets the context for the discussion on a common emitter amplifier. This amplifier uses a 'current mirror' for biasing the active load. A current mirror is an arrangement of transistors designed to copy (mirror) a current flowing through one active device into another, maintaining the same current. The instructor makes an assumption about the transistors, indicating they are identical, meaning they have the same parameters which help simplify calculations.
Examples & Analogies
Think of the current mirror like a team of identical twins working together. If one twin lifts a weight, the other can easily do the same. Here, the current mirror ensures that two different parts of the circuit behave as if they have the same characteristics, making it easier to design and predict circuit behavior.
Collector and Bias Current Equations
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So, to get the Ic current of transistor-1 and collector current or transistor-4 equal, we want the current flow through transistor-2 to be equal to current flow through transistor-1.
Detailed Explanation
This chunk discusses how specific currents must be equal to ensure the circuit operates correctly. The collector current (Ic) of one transistor needs to match that of another for the amplifier to function as intended. This matching is crucial for maintaining the performance and stability of the amplifier.
Examples & Analogies
Imagine two connected water tanks. The water level in one tank must match that of the other to maintain balance; otherwise, one would overflow while the other runs dry. Similarly, here, transistor currents must be balanced for optimal performance.
Bias Resistance Calculation
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And since, Q1 and Q2 are identical having the same Ξ² value of 100, the value of this bias resistance R1 should be identical to this transistor R2. So, that the base current here and base current here, DC base current they should be equal.
Detailed Explanation
In this section, the instructor explains how to calculate the bias resistance for transistors involved in the circuit using the concept of current gain (Ξ²). If the transistors are identical and have a constant Ξ², their operating conditions can be easily derived from each other, simplifying the design process.
Examples & Analogies
Consider cooking two identical dishes where each requires the same amount of seasoning. If you season just one dish without matching the other, they will taste different. Similarly, by ensuring resistances are equal, both transistors are balanced, providing a uniform response in the amplifier circuit.
Finding the Collector Current
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So, let us try to find what will be the value of this resistance to get the collector current Ic = 2 mA. So, since the Ξ² is 100 so, the Ib should be = 20 Β΅A. To get the value of this R1 to get 20 Β΅A, the R1 should be = 570 kβ¦.
Detailed Explanation
Here, the instructor calculates the specific values needed in the circuit. The collector current is specified, and using the transistor's current gain (Ξ²), the base current (Ib) can be derived. This then allows for the determination of the necessary resistance value to achieve the desired biasing condition.
Examples & Analogies
Picture a recipe where you need to add 2 cups of water to make the perfect stew. If you know how much is needed based on the portions (like Ξ²), you can then figure out how much more salt (resistance) to add to enhance the flavor without overwhelming the dish. This is similar to achieving the correct bias current through resistance.
Matching Collector Currents
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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 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
This part explains the outcome of the previous calculations. It reinforces that, under the previously stated assumptions (such as ignoring base current loss), the collector currents of various transistors in the circuit remain equal, indicating effective current mirroring. This consistency is vital for the amplifier's operation.
Examples & Analogies
Imagine a teacher explaining concepts to two identical classes. If the explanation is clear and balanced across both, both classes will understand the material equally well. Similarly, here, equal current flow ensures all transistors are synchronized for optimal function.
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
At this stage, the focus shifts to calculating the amplifier's small signal output resistance and voltage gain. These parameters are crucial as they determine how the amplifier will react to small variations in input, and thereby its overall performance. The instructor describes how the resistances for output are combined to provide total resistance figures.
Examples & Analogies
Think of tuning a musical instrument. Just as you combine various tunings to get the perfect pitch (output), here, combining resistances will determine the amplifier's responsiveness and clarity of the signal it amplifies.
DC Output Voltage Considerations
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Now let me clear the board and then again we will talk about the DC voltage. So, the current flow should be equal and if you see it carefully the DC voltage here it is defined by this Vcc - VBE drop.
Detailed Explanation
In this part, the instructor indicates a further examination of the DC output voltage in relation to current currents. The relationship between the collector voltage (Vcc) and the voltage drop across the base-emitter junction (VBE) is emphasized. This becomes relevant in understanding how the output behaves at DC levels.
Examples & Analogies
Consider a car engine needing fuel (current) and ensuring it runs efficiently (DC voltage). The gas tank level (Vcc) should always be enough above the engine needs (VBE drop) for it to function correctly; otherwise, the car won't perform optimally.
Mismatch and Non-Ideality Effects
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If the values of early voltage or in case we cannot ignore the base current and then of course, the corresponding current here and here there will be a mismatch.
Detailed Explanation
This section highlights how real-world imperfections, such as variations in early voltage or base current losses, can lead to mismatches in current flow across transistors. Such mismatches can affect the output voltage and amplifier performance, illustrating that theoretical designs need to consider practical variations.
Examples & Analogies
Imagine cooking with slightly different ingredients each time (like salt levels or spice potency). Despite following the recipe (theoretical design), the final dish might vary in taste. Similarly, small variations in transistor performance can lead to notable differences in the output voltage.
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 circuit that allows the copying of current from one branch to another while maintaining a stable design.
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Collector Current (IC): The current in the collector of the transistor, crucial for defining device operation in amplification.
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Small-Signal Resistance: Important for determining output voltage changes in response to varying inputs.
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Voltage Gain (Av): A key performance metric for amplifiers indicating how much a signal is amplified.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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Example calculation of R values using knowledge of Beta. By determining R1 and R2 for a target current of 2 mA, we analyze complete circuit functionality.
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Using numerical values to determine output voltage gain and small-signal output resistance from a given circuit configuration, elucidating real-world performance.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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To keep the current stable; let transistors be equal, with mirrors in the table.
π Fascinating Stories
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Imagine a team of identical twins - they mirror each other in actions just like transistors in a current mirror circuit, keeping the current in check.
π§ Other Memory Gems
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CATS: Current mirrors Activate Transistor Stability.
π― Super Acronyms
BEAM
- Beta
- Early voltage
- Amplifier design
- Matching transistors.
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 produces an output current that is proportional to an input current, allowing stable operation of transistors.
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Term: Collector Current (IC)
Definition:
The current flowing through the collector of a transistor, typically a key parameter in amplifier circuits.
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Term: Voltage Gain (Av)
Definition:
The ratio of output voltage to input voltage in an amplifier, indicating how much the amplifier increases a signal's voltage.
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Term: Beta (Ξ²)
Definition:
The current gain of a transistor, representing the ratio of output current to input current.
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Term: SmallSignal Output Resistance
Definition:
The output resistance looking into the output terminals of a transistor circuit when small signal analysis is considered.