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88.3.3. Additional Current Requirement
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Today, we are diving into the role of current mirrors in amplifiers, particularly how we can leverage them for effective biasing. Can anyone explain what a current mirror does in a circuit?
Is it used to provide a constant current in different parts of the circuit?
Exactly! A current mirror is designed to copy a current through one active device to another while maintaining the same voltage. Think of it as a way of creating a reference that can be used to keep currents stable across transistors.
Why do we have to ensure that the transistors are identical in characteristics?
Great question! Identical characteristics allow us to predict the behavior of the mirror accurately. When they are not matching, we might see discrepancies in the currents. Keeping this in mind is crucial as we compute other values.
Let's keep this foundational knowledge in mind as we analyze the common emitter amplifier circuit with our current mirror.
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Now, let's calculate the collector current for one of the transistors in our circuit. Assuming we want to achieve a collector current of 2 mA, how do we start?
We need to consider the base current, right? We can calculate it using the relation with beta.
Absolutely! For a β of 100, the base current here would be 20 µA. Knowing that, how can we calculate the required bias resistance?
If we know the total collector current and the base current, we can use Ohm's Law to find the resistance!
Correct! Following the calculations, we derived that a biasing resistance of 570 kΩ would maintain that collector current of 2 mA, assuming negligible base current loss.
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Create a free accountLet’s discuss the significance of output resistance and gain. What are some initial ideas on how they connect to our amplifier's performance?
Higher output resistance typically means better voltage gain, right?
Exactly! For the small signal model, we found that the output resistance of our configuration was 25 kΩ, yielding a voltage gain around 1923. Can anyone summarize what impacts the gain?
It's impacted by the product of transconductance and load resistance, but we also have to account for factors like early voltage.
Perfect! As we progress, understanding how all these features play together will help us with more complex amplifiers like differential amplifiers.
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Create a free accountNow, let's calculate the DC output voltage. Given our earlier assumptions, how do we determine the final value?
We subtract the base-emitter drop from our DC supply voltage, right? So it's 12 V minus 0.6 V?
Exactly! Thus, yielding a DC output voltage of 11.4 V. But we must also consider variations when parameters change. Why is the early voltage important in this calculation?
It can affect the current matching and ultimately the output voltage, especially in high-impedance cases.
Right! Even slight mismatches can lead to significant voltage changes in practical circuits, emphasizing the need to analyze all operating conditions.
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Create a free accountWe’re moving toward differential amplifiers now, which also use current mirrors. Can anyone outline how we'd adapt what we've learned?
We’d need to maintain matched conditions across multiple transistors to ensure proper performance.
And if we ensure that our mirrors are well designed, they’ll maintain reference currents effectively.
Exactly! The consistency of reference currents allows differential amplifiers to improve performance in a variety of applications. We’ll look into specific numerical examples and further details soon.
Overview
Short Summary
This section focuses on analyzing the current mirror circuit within a common emitter amplifier to understand the biases and outputs involving numerical examples.
Medium Summary
In this section, we delve into the numerical analysis of a common emitter amplifier utilizing a current mirror to bias the circuit. By assuming transistor identities and early voltages, we calculate key parameters like collector currents and output voltage, whilst also examining the implications of varying transistor characteristics.
Detailed Summary
Analysis of a Common Emitter Amplifier with Current Mirror
In this section, we engage with the application of a current mirror in a common emitter amplifier, particularly in biasing arrangements involving transistors. We assume that the transistors involved (Q1, Q2, Q3, and Q4) are identical and have a current gain (β) of 100. The analysis begins with determining the current through the first transistor, linking it to the collector current of the last transistor in the configuration. Given a target collector current of 2 mA, we establish a bias resistance R1 around 570 kΩ. The section further explores the relationships between output resistance, voltage gain, and introduces the concept of DC output voltage, ultimately culminating in numerical outputs based on assumed operating conditions. The final part of the section anticipates variations in output voltage in real-world scenarios, setting the stage for differential amplifiers using current mirrors in future examples.
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Create a free accountSo, to get the I current of transistor-1 and collector current or transistor-4 equal, we want the current flow through transistor-2 to be equal to the current flow through transistor-1. And since, Q and Q are identical having the same β value of 100.
Detailed Explanation
This chunk introduces the concept of current mirroring in transistors, crucial for achieving similar currents in different transistors. To ensure that the collector currents of transistor-1 and transistor-4 are equal, the current flowing through transistor-2 must match the current flowing through transistor-1. The assumption here is that both transistors Q1 and Q2 have the same current gain (β), which is 100, meaning they amplify their input currents to the same degree.
Examples & Analogies
Think of it like a relay in a race. If one runner is faster (like transistor-1), the relay must adjust to ensure the second runner (transistor-4) runs at the same speed. If they are equally skilled (identical β), they will match their speeds when properly set up.
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Create a free accountSo, to get the value of this R1 to get 20 µA, the R1 should be = 570 kΩ. So, the value of this resistance as well as this resistance they are = 570 kΩ.
Detailed Explanation
This section discusses how to calculate the biasing resistance (R1) needed to set the desired current in the circuit. Given that the base current needs to be approximately 20 µA, the calculated value for resistor R1 (and R2) is 570 kΩ. This resistance allows the correct biasing for the transistors to function optimally without excessive power loss.
Examples & Analogies
Imagine adjusting a tap to control the water flow to a garden. Each resistance value is like adjusting how much water (current) flows to ensure that all plants (transistors) receive the right amount without flooding or drought.
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Create a free accountNow, with this information let us try to find the small signal output resistance and voltage gain of the amplifier. So, we are assuming both the devices are in active region. So, the output resistance Rout = ro1 || ro4.
Detailed Explanation
This chunk delves into calculating the small signal output resistance and the voltage gain of the amplifier. By assuming that both transistors are operating in their active regions, the output resistance can be found by using the formula for parallel resistances (Rout = ro1 || ro4). This is an important step in determining how the amplifier will respond to small signals and its efficiency in amplification.
Examples & Analogies
Think of it as a team working together to lift a weight. If one person (ro1) can lift 50 kg on their own and another (ro4) can also lift 50 kg, together, their combined strength works efficiently just like the combined output resistance helps the circuit work effectively.
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Create a free accountSo, the gain of the amplifier of course, the voltage gain of this amplifier it is gm * Rout with a ‒ sign. So, the voltage gain it is coming 1923.
Detailed Explanation
This section outlines how to calculate the voltage gain of the amplifier. The formula used is voltage gain = gm * Rout, incorporating the transconductance (gm). In this example, the voltage gain calculated is approximately 1923, indicating a significant amplification effect due to the active load configuration.
Examples & Analogies
Imagine a microphone amplifying your voice. If your voice is the input signal (small signal) and the microphone is the amplifier, the gain (1923 times) is like your voice being broadcast loudly in a stadium, demonstrating how small inputs can lead to substantial outputs.
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Create a free accountNow let me clear the board and then again we will talk about the DC voltage. The DC voltage here it is defined by this Vcc - VBE drop...
Detailed Explanation
Here, the impact of Early voltage on DC output voltage is discussed. The collector-emitter voltage drop is examined, leading to the final DC output voltage, which is influenced by the assumptions about transistor current gains and circuit configuration. The chunk indicates how practical circuit behavior can cause variations in expected voltages, which engineers need to account for.
Examples & Analogies
Consider a water tank: the water level (DC output voltage) is affected by the amount of water flowing in versus the flow being drained. Variations in pipe size can represent changes like Early voltage, which affect how much flow can be maintained at a desired level.
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
Current Mirror: A device to copy current from one branch to another in a circuit.
Collector Current: The current that flows through a transistor's collector, determined by the base current.
Beta (β): Ratio indicating how effectively a transistor can amplify current.
DC Output Voltage: The steady output voltage of the amplifier after stabilization.
Early Voltage: A measure influencing the output characteristics of a transistor.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
Example 1: Calculate the required bias resistance if the target collector current is 2 mA, leading to 570 kΩ based on the assumption of negligible base current.
Example 2: Determine the small signal output resistance and voltage gain based on the calculated resistances.
Memory Aids
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Glossary
Current Mirror
A circuit that copies a current through one active device to another, maintaining a constant current flow.
Collector Current
The current that flows through the collector of a transistor, which is influenced by the base current.
Beta (β)
The current gain of a transistor, indicating the ratio of the collector current to the base current.
DC Output Voltage
The steady-state voltage output from a circuit when it has stabilized under direct current conditions.
Early Voltage
A parameter that indicates how the collector current changes with respect to the collector-emitter voltage, affecting transistor performance.
Voltage Gain
The ratio of output voltage to input voltage in an amplifier, indicating its amplification capability.