Simple BJT Current Mirror Calculations - 11.3 | Experiment No. 6: Design and Characterization of Oscillators and Current Mirrors | Analog Circuit Lab
K12 Students

Academics

AI-Powered learning for Grades 8–12, aligned with major Indian and international curricula.

Professionals

Professional Courses

Industry-relevant training in Business, Technology, and Design to help professionals and graduates upskill for real-world careers.

Games

Interactive Games

Fun, engaging games to boost memory, math fluency, typing speed, and English skills—perfect for learners of all ages.

Typing
Memory
Math
English Adventures
Knowledge

11.3 - Simple BJT Current Mirror Calculations

Practice

Interactive Audio Lesson

Listen to a student-teacher conversation explaining the topic in a relatable way.

Understanding BJT Current Mirrors

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

0:00
Teacher
Teacher

Today, we're going to learn about BJT current mirrors, which are crucial in various electronic applications. What do you all think a current mirror does?

Student 1
Student 1

I think it can duplicate a current in another part of the circuit.

Teacher
Teacher

Exactly! A current mirror can copy a reference current through one transistor and maintain it through another. This is important in integrated circuits. Remember, we define 'IREF' as the reference current. Can anyone tell me why transistor matching is vital in this context?

Student 2
Student 2

If the transistors aren’t matched, the output current won't be the same as the reference current.

Teacher
Teacher

Correct! When Q1 and Q2 are matched, their VBE values will be similar, allowing for accurate current mirroring. Let's keep this focus on 'matching' as we progress.

Calculating Reference Current

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

0:00
Teacher
Teacher

Now that we understand the basic operation, let’s calculate the reference current, IREF. Can anyone explain how we derive the formula for RREF?

Student 3
Student 3

Isn't it related to the voltage source and VBE?

Teacher
Teacher

Yes! The formula is RREF = (VCC - VBE) / IREF. Who remembers the standard value of VBE for a typical BJT?

Student 4
Student 4

It's usually around 0.7V, right?

Teacher
Teacher

Exactly. If we set IREF to 1 mA and VCC to 12V, let's calculate RREF together. What would that be?

Student 1
Student 1

Using RREF = (12V - 0.7V) / 0.001A, I get RREF = 11.3kΩ.

Teacher
Teacher

Great job! Remember, the choice of resistors must also consider standard values.

Understanding Output Current

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

0:00
Teacher
Teacher

Next, let’s look at the output current, IOUT. Why might IOUT differ from IREF?

Student 2
Student 2

Because of base currents drawn by the transistors, right?

Teacher
Teacher

Correct! The equation IOUT ≈ IREF - base currents shows that the actual output current can be slightly less than IREF due to these losses. What do we mean by 'Early effect'?

Student 3
Student 3

It’s when changes in collector-emitter voltage affect the base width, altering the output current.

Teacher
Teacher

Exactly! This effect can result in less stable output current, which is why simple BJT current mirrors are often not the best for precise applications. Let's take a moment to summarize our understanding so far!

Output Resistance and Limitations

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

0:00
Teacher
Teacher

Now that we've discussed the output current, let's evaluate the output resistance of the current mirror. Why is this measurement significant?

Student 4
Student 4

Higher output resistance means a better constant current source?

Teacher
Teacher

Exactly! The output resistance indicates how well the current remains constant under varying voltage conditions. Can anyone summarize what factors limit the performance of a simple current mirror?

Student 1
Student 1

Base current errors and Early effect are the main limitations.

Teacher
Teacher

Very well stated! Just keep these limitations in mind when using a simple current mirror, as they can significantly affect performance.

Practical Applications of Current Mirrors

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

0:00
Teacher
Teacher

To conclude, let’s discuss practical applications. Where do we commonly find current mirrors used?

Student 2
Student 2

They are used in biasing for amplifiers or active loads.

Teacher
Teacher

Exactly! And they are also valuable in differential amplifiers. What advantages do they have in these applications?

Student 3
Student 3

They help maintain consistent current, improving performance in circuits.

Teacher
Teacher

Right! Consistent current is essential for stability and performance in analog circuits. Great job today! Let's recap the main points we covered about BJT current mirrors.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section focuses on the design and calculations involved in constructing a simple BJT current mirror.

Standard

The section discusses the fundamental principles of BJT current mirrors, including their configuration, operation, and limitations. It also covers the necessary calculations for setting the reference current and measuring output current, along with the significance of parameters like output resistance and matching accuracy.

Detailed

In this section, we delve into the Simple BJT Current Mirror, a circuit designed to copy a current through one active device to another in a predictable manner, making it essential for stable biasing in circuits. A simple BJT current mirror consists of two matched NPN transistors (Q1 and Q2) where Q1 functions as a diode-connected transistor, establishing a reference current (IREF). The operation heavily relies on the uniformity of the transistors' characteristics, resulting in an output current (IOUT) that ideally mirrors IREF. However, due to factors like base current loss and the Early effect, IOUT may be slightly less than IREF. The section provides detailed steps for calculating the values of RREF needed to set IREF and discusses the limitations that affect the current mirror's performance, particularly focusing on accuracy and output resistance.

Audio Book

Dive deep into the subject with an immersive audiobook experience.

Configuration of the Simple BJT Current Mirror

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

A simple BJT current mirror consists of two matched NPN (or PNP) transistors, Q1 and Q2.
- Q1 is configured as a diode: its collector is shorted to its base. This forces Q1 into active region operation (or saturation if base current is too high, but usually active).
- A reference current (IREF) flows into the collector of Q1. This current is set by a voltage source (VCC) and a reference resistor (RREF).
- The base of Q1 is connected to the base of Q2. Since the transistors are matched, VBE1 = VBE2.
- The emitters of both Q1 and Q2 are connected to ground.
- The output current (IOUT) is taken from the collector of Q2, flowing into a load.

Detailed Explanation

In a simple BJT current mirror, we use two identical transistors, Q1 and Q2, to copy a current. Q1 is connected in such a way that its collector is shorted to its base, making it operate as a diode. This setup means that when we pass a reference current (IREF) through Q1, it establishes a voltage (VBE) across it.
Because both transistors are identical and their bases are connected together, the base-emitter voltages (VBE) are the same. Therefore, when the VBE for Q1 equals that of Q2, the current through Q2 (IOUT) becomes nearly equal to IREF. The output current is designed to flow into a load, making this configuration useful for applications requiring stable and predictable current.

Examples & Analogies

Think of this current mirror as two identical twins. If one twin, representing Q1, eats a specific amount of food (IREF), the other twin, representing Q2, will try to eat the same amount because they are identical and share the same eating habits (VBE). When resources (current) are required for different tasks, having two identical twins ensures that one can always mirror the actions of the other reliably.

Principle of Operation

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

  1. The reference current IREF flows through Q1, establishing a VBE1 across its base-emitter junction.
  2. Since Q1 and Q2 are matched and their bases are tied together (and emitters grounded), VBE1 = VBE2.
  3. Because VBE2 = VBE1, the collector current of Q2 (IC2 or IOUT) will ideally be equal to the collector current of Q1 (IC1). IREF = IC1 + 2IB (approximately, ignoring diode current of Q1 base-collector junction) IOUT = IC2 Ideally, IOUT = IC1 ≈ IREF.

Detailed Explanation

In the BJT current mirror, the reference current enters Q1 and establishes a voltage across its base-emitter junction (VBE1). Since Q1 and Q2 are matched devices, with their bases connected together, VBE for both transistors stays the same. This condition allows the collector current of Q2 (IOUT) to reflect the collector current flowing through Q1 (IC1). In an ideal situation, IOUT is equal to the reference current IREF. However, we must also keep in mind that there are slight inaccuracies due to the base current drawn by the transistors.
The equation helps us understand that the output current will slightly drop below the reference current based on the base current consumption.

Examples & Analogies

Imagine having a water pipe that feeds two identical buckets (the transistors). The first bucket (Q1) has water (current) flowing in, while it also leaks a small amount out (base current). The second bucket (Q2) receives water based on what the first contains. Over time, it also loses a bit through its own leak. As a result, both buckets end up with slightly less water than what initially flowed in. Thus, while they try to match (mirror) each other, the total amount keeps decreasing slightly due to the leaks.

Setting the Reference Current (IREF)

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

IREF = RREF (VCC − VBE1)

Detailed Explanation

Setting the reference current (IREF) is straightforward. We derive it using Ohm’s Law.
The formula IREF = RREF (VCC − VBE1) indicates that you can control the amount of current flowing through the current mirror by adjusting the value of the resistor (RREF) connected to the supply voltage (VCC). VBE1 is the forward voltage drop across Q1 when it is conducting. By choosing an appropriate RREF, you can set the desired IREF for your application.

Examples & Analogies

Think of VCC as the height from which water is poured into a funnel (RREF). The amount of water that drains into the barrel (IREF) can be controlled by changing the size of the funnel's opening (value of RREF). If you open it wider (increase RREF), more water flows through to the barrel, thus allowing you to control how much current you want to reflect in the current mirror.

Limitations of Simple Current Mirror

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

● Base Current Error: A portion of IREF is consumed by the base currents of Q1 and Q2.
IREF = IC1 + IB1 + IB2 = IC1 + βIC1 + βIC2. If IC1 = IC2 = IC, then IREF = IC (1 + 2/β). So, IOUT = IC = (1 + 2/β)IREF.
The output current is actually slightly less than IREF. This error is significant if β is low.

● Early Effect: As the collector-emitter voltage of Q2 (VCE2) changes (due to varying load resistance), its collector current (IC2) will change slightly due to the Early effect (base width modulation). This means the output current is not perfectly constant, and the output resistance is limited.

Detailed Explanation

There are key limitations to consider when working with a simple BJT current mirror. First, the base current errors arise because not all of the reference current (IREF) can flow into the output. Some is 'lost' to the base currents of the transistors — meaning that the output current (IOUT) is typically less than expected. If the transistor's beta (β) is low, this error is more pronounced.
Additionally, the Early effect can cause variations in output current with changes in collector-emitter voltage (VCE). If the load changes and VCE varies, the current flowing through Q2 will vary slightly due to the physical properties of BJTs. This instability in IOUT limits the performance of the simple current mirror.

Examples & Analogies

Think of a simple current mirror like a team of two pitchers trying to throw the same distance. If one pitcher uses a bit of energy to aim, they may not reach the target (base current error). Their strength depends on their ability (beta), and weaker pitchers (lower beta) will struggle even more. Additionally, if the pitching mound (collector-emitter voltage) is unstable or shifts in height, it affects their throw distance, meaning they might not hit the same target consistently (Early effect).

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Current Mirroring: The concept of duplicating a current from one transistor to another.

  • Reference Current: The initial current used to set the output current of the BJT current mirror.

  • Transistor Matching: The importance of having identical characteristics for accurate operation in current mirrors.

  • Output Resistance: Indicates how well the output current remains constant despite varying load or voltage.

  • Early Effect: Describes how changes in voltage can affect the current characteristics of BJTs.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • A simple BJT current mirror used in an amplifier circuit to provide a stable biasing current.

  • A differential amplifier configuration utilizing current mirrors to ensure balanced performance and accuracy.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • To mirror a current, precise must it be, / Match up those transistors so the flow's trouble-free.

📖 Fascinating Stories

  • Once there were two twin transistors, Q1 and Q2. Wherever Q1 went, Q2 followed closely. But oh, Q1 got distracted by a base current. Q2 tried to copy, but lacked accuracy! Once they learned to stay matched, their current was as good as gold.

🧠 Other Memory Gems

  • MIRROR - Match transistors, Identify Reference, Output Resistance.

🎯 Super Acronyms

IOUT - Individual Output, Uncovered Through Matching.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: BJT (Bipolar Junction Transistor)

    Definition:

    A type of transistor that uses both electron and hole charge carriers.

  • Term: Current Mirror

    Definition:

    A circuit that replicates a current flowing in one active component to another, used in various applications for biasing.

  • Term: IREF (Reference Current)

    Definition:

    The current set by the reference resistor in a current mirror circuit, serving as a baseline for output current.

  • Term: Output Resistance

    Definition:

    The resistance seen by the load that affects the stability of output current in current mirror circuits.

  • Term: Early Effect

    Definition:

    A phenomenon in BJTs where the output current varies with the collector-emitter voltage due to changes in the effective base width.