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Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Introduction to Current Mirrors
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Today we're going to talk about BJT current mirrors. Can anyone explain what a current mirror does?
A current mirror takes a reference current from one transistor and replicates it in another.
Exactly! It helps maintain a stable current in other parts of the circuit. Why do you think this could be useful?
It helps in keeping the current constant, which is important for biasing transistors.
Great point! We often use it for biasing and loads in differential amplifier circuits.
Understanding Simple BJT Current Mirror Configuration
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Let’s look at the configuration. Can someone describe how the two BJTs are set up in a simple current mirror?
Q1 is connected as a diode, and its collector is connected to its base.
And Q2 has its base connected to Q1, so they have the same base-emitter voltage.
Correct, and this matching allows us to mirror the current effectively. Remember, we need matched pairs to maintain consistency.
Calculating the Reference Current
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Now let's calculate the reference current. If we have a VCC of 12V and a VBE of 0.7V, how do we find RREF?
We can use the formula IREF = (VCC - VBE) / RREF.
That's right! If I want IREF to be 1 mA, what should RREF be?
It would be RREF = (12V - 0.7V) / 1mA, which gives us approximately 11.3kΩ.
Excellent! Choosing a standard resistor value would likely give us 11kΩ or 12kΩ. This rounding is common in practical circuits.
Output Current Behavior
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Now, let's discuss the output current. Ideally, what should IOUT equal?
It should be approximately equal to IREF.
Correct! However, there are nuances. What do base currents do to our output current?
Base currents can reduce the output current slightly, especially if beta is low.
Right! We need to take that into account as it affects current mirroring. That’s why we often refer to that equation IOUT = (1 + 2/β) * IREF.
Practical Applications and Limitations
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What applications do you think require current mirrors?
They are used in amplifiers for consistent biasing.
Absolutely! But there are limitations. Can anyone share what affects the performance of our current mirror?
One limitation is the Early effect, where the output current changes slightly due to variations in VCE.
Good observation! This variation can cause significant issues if the load changes, affecting how well our mirror functions.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The section explains the fundamental principles of a simple BJT current mirror, including its configuration, calculations for setting a reference current, and evaluating the output current. It highlights the significance of favorable transistor characteristics for accurate current mirroring.
Detailed
Simple BJT Current Mirror Design
Overview
A BJT current mirror is a crucial circuit used in various electronic applications, allowing stable current replication from one active device to another. This section focuses on the design of a simple BJT current mirror, specifically with matched NPN transistors (e.g., BC547).
Configuration
The basic layout consists of two BJTs, Q1 and Q2, with their bases interconnected and the collector of Q1 shorted to its base, which ensures it operates in the active region. The reference current (IREF) is determined by a resistor (RREF) connected to the supply voltage (VCC) minus the base-emitter voltage (VBE).
Key Principles
- Current Reference Calculation: The target reference current is defined by the formula:
IREF = VCC - VBE / RREF.
For instance, to achieve a target IREF of 1 mA, substituting the known values yields the value for RREF. - Output Current: The output current (IOUT) ideally equals the reference current, with slight variations due to base currents, described by the equation:
IOUT = (1 + 2/β) * IREF, where β is the transistor's current gain.
Practical Implications
Designing the current mirror effectively requires careful selection of matched transistor pairs and considerations of biasing and loading conditions. This design offers insights into ensuring output current remains stable despite variations in load conditions.
Audio Book
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Given Parameters for Design
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● Transistors: Two matched NPN BJTs (BC547)
● Supply Voltage: VCC =12V
● Target Reference Current (IREF ): 1mA
● Assume VBE =0.7V, βDC =100.
Detailed Explanation
In this section, we are given the basic parameters needed for designing a simple BJT current mirror. We will use two matched NPN BJTs, specifically the BC547 model. The supply voltage is set at 12 volts, and our target reference current IREF is set to 1 milliampere. Additionally, we assume a base-emitter voltage (VBE) of 0.7 volts and a direct current current gain (βDC) of 100. These parameters are fundamental in setting up our current mirror circuit.
Examples & Analogies
Think of the BJT current mirror as a pair of identical twins (the matched BJTs) that are controlled by their parents (the voltage and current settings). Just like twins may have similar traits, these BJTs share characteristics, ensuring they behave the same under similar conditions.
Calculating Reference Resistor (RREF)
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- Calculate RREF :
○ IREF = RREF (VCC − VBE)
○ RREF = IREF (VCC − VBE) = 1mA(12V − 0.7V) = 1mA(11.3V) = 11.3kΩ.
○ Choose Standard Resistor Value for RREF : 11kΩ or 12kΩ. Let's use 11kΩ.
■ If RREF = 11kΩ, then IREF = 11kΩ/11.3V ≈ 1.027mA.
Detailed Explanation
We need to calculate the reference resistor (RREF) that sets the reference current (IREF) flowing through the first transistor (Q1) in the current mirror. Using Ohm's Law, we derive RREF from the current and voltage relationships. Starting with IREF equals the reference resistor multiplied by the effective voltage supply (VCC minus VBE), we find that RREF is approximately 11.3 kΩ. For practical purposes, we select a standard resistor value close to this calculation, namely 11 kΩ. This choice helps ensure that our reference current is as close as possible to the target of 1 mA.
Examples & Analogies
You can think of RREF as the amount of 'allowance money' given to each child (BJTs). The equation signifies that if you set the allowance based on how much the kids can spend (VCC), adjusting for what they must save (VBE), you can ensure they each have just enough to spend (IREF).
Expected Output Current (IOUT)
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- Expected Output Current (IOUT ):
○ Ideally, IOUT ≈ IREF .
○ Considering base currents:
IOUT = (1 + 2/β) IREF = (1 + 2/100) 1.027mA = 1.021mA ≈ 1.007mA.
Detailed Explanation
The output current (IOUT) of the current mirror ideally mirrors the reference current (IREF). However, we must account for a small error introduced by the base current in the transistors. The equation shows that for each transistor, a small fraction of the reference current is drawn to keep the transistors operating, which is described as 1 plus twice the inverse of the current gain (βDC). The outcome is that our actual output current, after the correction for the base currents, is slightly less than the ideal case, giving us approximately 1.007 mA.
Examples & Analogies
Imagine you’re trying to duplicate a favorite recipe (IREF). While you can follow it closely, some ingredients might slip away (base currents) before you serve it, meaning you'll end up with slightly less than you intended (IOUT). It’s a similar concept in the current mirror: we aim to duplicate the desired output, adjusting for those tiny losses.
Summary of Components for Current Mirror
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● Transistors: Q1, Q2 (BC547, matched)
● Reference Resistor: RREF = 11kΩ
Calculated Theoretical Reference Current: [1.027 mA]
Calculated Theoretical Output Current: [1.007 mA]
Detailed Explanation
In summary, the components necessary for building the simple BJT current mirror include two matched BC547 transistors and a reference resistor calculated to be 11 kΩ. The target reference current we aimed for was 1 mA, leading to a calculated theoretical output current of approximately 1.007 mA, taking into account the real-world adjustments needed for accuracy. This summary encapsulates the essential design choices for constructing our circuit.
Examples & Analogies
Think of this summary as packing your luggage for a trip. You know you need matching outfits (the transistors) and a budget for buying souvenirs (the reference resistor). Your ideal amount to spend is set (IREF), but you account for a little extra here and there, realizing you might just come under budget (IOUT). This understanding helps you plan effectively.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Transistor Matching: The necessity of using matched transistors for maintaining consistent output current in a current mirror.
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Reference Current Calculation: The method to calculate the reference current based on known supply voltage and VBE.
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Output Current Equation: Understanding how the output current relates to the reference current and the effect of base current.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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In a BJT current mirror, if Q1 is supplied with a reference current of 1 mA, Q2 should ideally also output close to 1 mA, assuming perfect conditions.
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If the supply voltage VCC is 12V and VBE is 0.7V, the resistor value calculated for a 1 mA reference current would be approximately 11.3kΩ.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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To mirror current, watch your beta, set your RREF, don’t let it fader.
📖 Fascinating Stories
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Imagine two friends, Q1 and Q2, always sharing their snacks. Q1 decides how much they need, and Q2 mirrors that. But if they share with too many friends, they'll each get less!
🧠 Other Memory Gems
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IREF -> RREF; Output current (IOUT) convenience.
🎯 Super Acronyms
BJT
- Breathe Just Transistors. Remember
- for a current mirror
- we need these friends!
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: BJT
Definition:
Bipolar Junction Transistor, a type of transistor that uses both electron and hole charge carriers.
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Term: Current Mirror
Definition:
A circuit configuration that allows a current to be copied or 'mirrored' from one active device (transistor) to another.
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Term: Reference Current (IREF)
Definition:
The predetermined current that is established to be replicated or mirrored in the current mirror output.
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Term: Output Current (IOUT)
Definition:
The resultant current flowing from the collector of the output transistor in a current mirror configuration.
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Term: Early Effect
Definition:
The phenomenon where the output current of a transistor changes due to variations in the collector-base voltage.