Simple BJT Current Mirror Characterization - 7.3 | Experiment No. 6: Design and Characterization of Oscillators and Current Mirrors | Analog Circuit Lab
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7.3 - Simple BJT Current Mirror Characterization

Practice

Interactive Audio Lesson

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

Basics of BJT Current Mirrors

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0:00
Teacher
Teacher

Today, we'll explore BJT current mirrors, a crucial component for creating stable DC currents in circuits. Can anyone tell me about the basic setup of a current mirror?

Student 1
Student 1

Is it just about two transistors working together?

Teacher
Teacher

Exactly! We use two matched NPN transistors, Q1 and Q2. Q1 sets the reference current and is configured so that its collector is connected to its base.

Student 3
Student 3

Why do they have to be matched?

Teacher
Teacher

Good question! Matching helps ensure that both transistors will have roughly the same VBE, meaning that if we know IREF through Q1, we can replicate it as IOUT through Q2.

Student 2
Student 2

What do we use as the reference current?

Teacher
Teacher

We use a resistor connected to the power supply, VCC, to set our IREF. This allows us to tailor our output current to our circuit’s needs.

Teacher
Teacher

To summarize: A BJT current mirror allows us to mirror a reference current through matched transistors, providing consistency in current values.

Current Measurement in BJT Current Mirrors

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0:00
Teacher
Teacher

Now that we understand the concept, let’s talk about measuring our currents. How do we measure IREF?

Student 4
Student 4

We can use a multimeter in series with the reference resistor, right?

Teacher
Teacher

Exactly! We disconnect the resistor from the circuit, and insert our DMM to measure the current flowing through it. What about IOUT?

Student 1
Student 1

We do the same thing but for Q2?

Teacher
Teacher

Right. Insert the DMM in series with Q2's collector while varying the load resistance connected to it. This helps us see how IOUT changes.

Student 3
Student 3

And what should we record?

Teacher
Teacher

You’ll record IOUT for different load resistances and the collector-emitter voltage (VCE) across Q2, which are essential for plotting our V-I characteristic.

Teacher
Teacher

In summary, measuring IREF and IOUT accurately with a multimeter allows us to evaluate the performance of our current mirror.

Challenges and Limitations

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0:00
Teacher
Teacher

Let's look at some challenges in using simple BJT current mirrors. What do you think affects the output current stability?

Student 2
Student 2

Maybe variations in current flow or mismatched transistors?

Teacher
Teacher

Correct, mismatched transistors lead to different VBE, causing IOUT to deviate from IREF. Another factor is the base current of both transistors, which can slightly decrease IOUT.

Student 4
Student 4

What’s this Early effect everyone mentions?

Teacher
Teacher

The Early effect is a phenomenon that causes changes in collector current with variations in collector-emitter voltage, which affects the output resistance of the current mirror.

Student 1
Student 1

How can we measure or address these limitations?

Teacher
Teacher

You can observe the V-I characteristics of your output to find these inefficiencies. And to minimize errors, using transistors with high β and keeping them thermally similar can help.

Teacher
Teacher

Remember, while simple BJT current mirrors are useful, understanding their limitations allows us to seek better configurations in advanced designs.

Introduction & Overview

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

Quick Overview

This section covers the characterization of simple BJT current mirrors, including their design, operation, and key performance metrics.

Standard

In this section, students learn to construct and characterize simple BJT current mirrors. It details the operational principles, configuration, limitations, and methods for measuring current and output resistance, emphasizing the importance of accurately setting reference currents.

Detailed

Simple BJT Current Mirror Characterization

A BJT current mirror is designed to replicate a current from one active device to another, often used in circuit biasing for consistent current sources. The section elaborates on the configuration of a simple BJT current mirror using matched NPN transistors, where a reference current (IREF) flows through one transistor (Q1), creating a corresponding output current (IOUT) through the second transistor (Q2).

The operation relies on the relationship between the base-emitter voltages (VBE) of the transistors. If both transistors are well-matched, IOUT ideally equals IREF. However, practical limitations such as base current consumption and the Early effect can impact accuracy, causing IOUT to be slightly less than IREF.

This section outlines the specific steps for constructing the circuit, measuring IREF and IOUT against varying load resistances, and plotting the current-voltage (V-I) characteristics to evaluate performance metrics like output resistance (Rout). By highlighting these aspects, students grasp the significance of current mirrors in integrated circuits and the factors influencing their efficacy.

Audio Book

Dive deep into the subject with an immersive audiobook experience.

Overview of BJT Current Mirror Setup

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  1. Collect Components: Gather two matched NPN BJTs (BC547, try to use transistors from the same batch if possible for better matching), and the resistor (RREF) as per Section 5.3 design.

Detailed Explanation

Before starting the experiment, you need to gather the necessary components. This includes two matched NPN BJTs, preferably from the same batch to ensure they have similar electrical characteristics, which is crucial for accurate current mirroring. Also, prepare the reference resistor (RREF) as calculated in the pre-lab design.

Examples & Analogies

Think of it like collecting identical pairs of shoes for a dance pair—if they are mismatched, one dancer may not perform as expected, just like a mismatched transistor pair leads to inaccuracies in current mirroring.

Circuit Assembly Instructions

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  1. Construct Circuit: Assemble the simple BJT current mirror on the breadboard as per your circuit diagram (Section 6.3). Initially, for Q2's collector, use a variable resistor (potentiometer, e.g., 10kOhm) as the load, or simply connect it to a DMM measuring current.

Detailed Explanation

Following the circuit diagram for the BJT current mirror, connect the components on a breadboard properly. Use a variable resistor as a load for the output transistor (Q2) to measure how the current changes with different loads. This allows you to see how the current mirror behaves under different conditions.

Examples & Analogies

Imagine building a model train track (the circuit) and using a switch (the variable resistor) to control how much power (current) is sent to the train. By adjusting the switch, you can see how the train performs differently depending on the track setup.

Connecting Power and Measuring Current

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  1. Power On: Connect the DC power supply (+12V).

Detailed Explanation

Once the circuit is assembled, power it with a DC supply at +12V. This voltage is essential to provide the necessary operating conditions for the BJTs to function correctly, allowing the current mirror to operate.

Examples & Analogies

It's like turning on the fuel supply for a car engine. Just as the engine needs fuel to run, the BJT current mirror requires power to mirror the currents successfully.

Reference Current Measurement (IREF)

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  1. Measure Reference Current (IREF):
  2. Break the connection between RREF and the base of Q1. Insert the DMM in series, configured for DC current measurement. Measure IREF. Record in Table 10.3.1.
  3. Reconnect RREF.

Detailed Explanation

To measure the reference current, disconnect RREF from Q1's base and connect a Digital Multimeter (DMM) in series. This will allow you to measure the current flowing through RREF, which you will record for analysis. After the measurement, reconnect the circuit to continue the experiment.

Examples & Analogies

Think of this step like measuring the flow of water in a pipe. By disconnecting the pipe and inserting a flow meter (the DMM), you can accurately determine how much water flows through, which in this case represents the current.

Output Current Measurement (IOUT)

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  1. Measure Output Current (IOUT) vs. Load Resistance (RL):
  2. Connect the DMM (in DC current mode) in series with the collector of Q2 (the output branch) and a variable load resistance (potentiometer set as a variable resistor) to ground.
  3. Vary the load resistance (RL) and for each RL value, measure the output current (IOUT) and the collector-emitter voltage of Q2 (VCE2). Record RL, IOUT, and VCE2 values in Table 10.3.2.

Detailed Explanation

Now, connect the DMM to measure the output current (IOUT) that flows through Q2. As you change the load resistance (RL), observe how IOUT varies. This step provides insight into the current mirror's performance and its ability to maintain a constant output under varying conditions.

Examples & Analogies

Consider this step like testing a power generator that supplies electricity to different appliances. You observe how the generator handles different loads: turning on a light bulb (low load) versus a heater (high load) shows you its capacity to maintain consistent output.

V-I Characteristic Plotting

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  1. Plot V-I Characteristics: Plot IOUT (Y-axis) versus VCE2 (X-axis) using the data from Table 10.3.2. This will show how well the current mirror maintains a constant current despite varying VCE2.

Detailed Explanation

Using the collected data, create a graph of IOUT against VCE2. This graph represents how well the current mirror maintains its output current as the voltage across the output transistor changes. A stable line indicates good current mirror performance.

Examples & Analogies

This step is akin to tracking how well an athlete performs under different conditions. Just as you would analyze their performance across various terrains, the graph helps in understanding the current mirror's stability and reliability under load changes.

Output Resistance Measurement (Rout)

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  1. Measure Output Resistance (Rout):
  2. To find the output resistance, we need to measure the change in VCE2 for a small change in IOUT (in the active region where IOUT is relatively constant).
  3. From your IOUT vs VCE2 plot, pick two points in the flat, active region of the characteristic curve.
  4. Rout = ΔIOUT / ΔVCE2. Calculate this value. Record in Table 10.3.3.

Detailed Explanation

The output resistance is determined by observing how VCE2 changes when IOUT changes slightly in the active region. By selecting two stable points on your graph, you can calculate Rout, which indicates how well the current remains constant despite changes in load voltage.

Examples & Analogies

Imagine measuring the firmness of a spring under different weights. The stiffer the spring (higher output resistance), the less it compresses under load. Similarly, the output resistance shows how resistant the current mirror is to variations in collector-emitter voltage.

Circuit Power Off

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  1. Power Off: Turn off the DC power supply.

Detailed Explanation

After all measurements are complete, safely turn off the power supply. This ensures that all components can be handled without the risk of damage or electric shock.

Examples & Analogies

Turning off the power supply is like shutting down a computer safely—it prevents data loss and damage to the components. Always ensure circuits are powered down before making any adjustments.

Definitions & Key Concepts

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

Key Concepts

  • BJT Current Mirror: A circuit arrangement using two matched transistors to replicate a current.

  • Reference Current (IREF): The initial current used to set the output current in a current mirror.

  • Output Current (IOUT): The current from the second transistor in a current mirror, intended to match IREF.

  • Early Effect: The effect that causes a variation in current output due to changes in collector-emitter voltage.

  • Output Resistance (Rout): Indicates how stable the output current remains despite voltage changes.

Examples & Real-Life Applications

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

Examples

  • A current mirror is crucial in integrated circuits to provide bias currents uniformly across multiple transistors.

  • In operational amplifier circuits, current mirrors can set the biasing points for GBW (gain-bandwidth) accurately.

Memory Aids

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

🎵 Rhymes Time

  • Reflect the current from Q1 to Q2, that's the mirror's job to do!

📖 Fascinating Stories

  • Imagine two friends doing identical tasks in a relay; the first sets the pace, and the second mirrors it perfectly, fast and steady.

🧠 Other Memory Gems

  • I'M COLD: I (IREF), M (Match), C (Current) O (Output), L (Load), D (DMM).

🎯 Super Acronyms

MIRROR

  • M: (Matched transistors)
  • I: (IREF)
  • R: (Rout)
  • R: (Ready)
  • O: (Output current)
  • R: (Resistive load).

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, commonly used in current mirrors.

  • Term: Current Mirror

    Definition:

    A circuit that replicates a current through one active device to another, providing a stable bias current.

  • Term: IREF

    Definition:

    Reference current that sets the output current in a current mirror.

  • Term: IOUT

    Definition:

    Output current which is intended to match the reference current in a current mirror configuration.

  • Term: Early Effect

    Definition:

    The variation of collector current with changes in collector-emitter voltage due to modulation of the base width in BJTs.

  • Term: Output Resistance

    Definition:

    A measure of how well a current mirror maintains a constant output current despite variations in voltage.