84.1.2 - Common Collector Amplifier
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Understanding the Common Collector Configuration
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Welcome, class! Today we'll explore the common collector amplifier, also known as the emitter follower. Can anyone tell me why it's called a common collector?
Is it because the collector terminal is common to both the input and output?
Exactly! The common collector configuration allows for high current gain while maintaining unity voltage gain. Now, let's discuss how current mirrors play a role in this configuration.
How does a current mirror actually work in this context?
Great question! A current mirror uses transistor pairs to maintain a constant current, setting the operating point for the amplifier. Think of it as a stabilizing force. Remember the acronym 'CIR' — Current, Integrity, and Reference!
Transistor Relationships in Gain Calculation
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Now let's look deeper into the transistor relationships. If we denote Transistor-2's collector current as I_C2, what is its relationship to the reference current I_REF?
Isn't it supposed to be I_C2 = β × I_REF for ideal cases?
That's close! It approximates to I_C2 ≈ I_REF considering matched transistors. The approximation simplifies our calculations significantly. Always keep in mind the equation I_E1 = I_REF!
What happens if the transistors aren't perfectly matched?
Excellent point! Non-ideal factors, like early voltage effects, can introduce errors, which we account for in more advanced analyses.
Analyzing Small Signal Equivalent Circuits
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To analyze the gain, we need to draw the small-signal equivalent circuit. Can someone explain what elements we need to consider?
We should focus on the output resistance and the small-signal model of Transistor-1.
Exactly! The output is connected to AC ground, and we can drop irrelevant AC signals during analysis. How do we calculate gain from the model?
Would it be Av = V_out / V_in using the derivative equations?
Yes! The gain is expressed as Aᵥ = r_o2 / (r_o2 + R_Bias), revealing how load resistances influence performance.
Applications Beyond the Common Collector
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Now, let's consider where else current mirrors might be used beyond common collectors. Can anyone suggest an application?
What about in differential amplifiers?
Spot on! Current mirrors can replace tail resistors in differential amplifiers to improve common mode gain! Remember this: 'Current mirrors equalize.'
So improving the common mode gain decreases common-mode signals' impact, right?
Exactly! It makes them less sensitive to noise, improving overall performance. Understanding these applications is vital in design!
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The common collector amplifier, also known as the emitter follower, utilizes a current mirror to provide stable biasing and improve performance. It focuses on how the collector current can be approximated by a reference current and includes analyses of small-signal equivalent circuits to find the voltage gain.
Detailed
Common Collector Amplifier
The common collector amplifier, frequently referred to as the emitter follower, is instrumental in analog electronic circuits. This section discusses how current mirrors can enhance the performance of this amplifier design by providing stable reference currents and biasing.
Key Points Covered:
- Current Mirror Configuration: The current mirror constructed with transistors helps establish a reference current (I_REF) that is crucial for defining the operating points of the amplifier.
- Transistor Operation: It highlights the functioning of the main amplifying transistor (Transistor-1) and its interaction with the current mirror (Transistors-2 and -3). The collector current of Transistor-2 can be approximated as a multiple of I_REF, illustrating the use of constant current sources in the design.
- Small Signal Model: The analysis of the small-signal equivalent circuit shows that the output of the current mirror directly impacts the gain (A), assessed through resistor values in the model.
- Practical Applications: The section touches upon how the common collector configuration is used with MOSFETs and differential amplifiers, providing real-world applications for these concepts.
In summary, understanding the common collector amplifier's structure and its reliance on current mirrors is essential for improving performance and ensuring stable operation in electronic circuits.
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Transistor Configuration
Chapter 1 of 5
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Chapter Content
So, here we do have the transistor level circuit transistor-1 it is the main amplifying transistor input port is here. And then the output port it is here and earlier what we said is that the current of this transistor it can be set by one reference current.
Detailed Explanation
In this section, we start with the transistor configuration. The common collector amplifier is based on a transistor (transistor-1) that acts as the main amplifying unit. The input and output ports of this transistor are clearly defined. A key point mentioned is that the current flowing through this transistor can be controlled using a reference current, establishing a fundamental principle of operation for the amplifier.
Examples & Analogies
Think of the transistor like a water valve in a piping system. The valve controls the flow of water (current) based on a reference point, much like how the amplifier regulates its output based on the input current.
Current Source and Bias Resistor
Chapter 2 of 5
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Instead of having reference current this is what we do have the implementation of the current source. In fact, if you see that it consists of a bias resistor R maybe you can see bias. So, V to collector of transistor-B CC 3 we do have this R and based on the value of this R we can get a current here and we call this is I the reference current.
Detailed Explanation
The section describes the incorporation of a current source circuit alongside a bias resistor (R). This resistor plays a crucial role in setting the reference current (I) that allows proper functioning of the amplifier. The current source ensures a stable current output irrespective of variations in other circuit parameters.
Examples & Analogies
Imagine you are trying to maintain a steady flow of traffic at a busy intersection using traffic lights. The lights (current source) provide consistent direction to vehicles (current), while the speed limit signs (bias resistor) help control how fast cars can go. In this way, both elements work together to manage the flow effectively.
Current Mirror Configuration
Chapter 3 of 5
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Chapter Content
So, we can say that the collector current of transistor-2 I_C2 is well approximated by I_REF multiplied by a certain factor...
Detailed Explanation
The text emphasizes the relationship between the collector current of one transistor (transistor-2) and the reference current. Specifically, the current mirror configuration allows for the collector current to be proportional to the reference current, enhancing the circuit's efficiency by ensuring consistent performance between the paired transistors (transistor-2 and transistor-3).
Examples & Analogies
Think of a current mirror as a two-way mirror in a fashion show: what light or direction goes through one side (reference current), is also mirrored through the other side, ensuring both sides reflect the same styles or trends (current).
DC Operating Point and Gain Measurement
Chapter 4 of 5
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Now, once we get the DC operating point then to find the gain from input to primary output we can draw the small signal equivalent circuit...
Detailed Explanation
Once the DC operating point has been established, the next step is to analyze the circuit's performance for small signal variations to determine the amplifier's gain. By drawing the small signal equivalent circuit, students can better understand how the input signal translates into an amplified output signal. This foundational knowledge is crucial for mastering signal amplification using transistor configurations.
Examples & Analogies
Consider this like tuning an instrument. Once you have achieved the correct pitch (DC operating point), you can then play notes (small signals) to see how well each note resonates (determines the gain) from the instrument into the performance.
Overall Performance and Comparison
Chapter 5 of 5
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So, similar to common collector amplifier the current mirror can also be used for common drain amplifier which is the MOSFET counterpart of this circuit.
Detailed Explanation
The section concludes by drawing parallels between the common collector amplifier and the common drain amplifier, indicating that the principles of current mirroring are applicable across different types of amplifiers, including MOSFET configurations. This demonstrates the versatility of current mirrors in electronic circuits.
Examples & Analogies
Just as different types of vehicles might use similar navigation systems to guide them, diverse amplifier configurations can leverage the same underlying principles of current mirrors to optimize their performance.
Key Concepts
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Current Mirror: A circuit configuration that ensures a stable reference current in amplifiers.
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Common Collector Configuration: This layout allows for high input impedance and lower output impedance.
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Gain Calculation: The relationship between collector and reference currents is vital for determining amplifier gain.
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Small Signal Analysis: Analyzing the small-signal equivalent helps understand overall performance.
Examples & Applications
Example 1: In a design requiring high input impedance, a common collector amplifier can be effectively employed to minimize loading on previous circuit stages.
Example 2: A current mirror in a differential amplifier helps improve its common mode rejection, thereby enhancing signal fidelity.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
In a common collector, current readers are lean; keep I_REF steady, output's seen.
Stories
Imagine a busy library (the amplifier) relying on a librarian (the current mirror) to ensure all reference books (I_REF) are in place, allowing patrons (the signals) to read the right materials without difficulty.
Memory Tools
Remember 'GIR' - Gain, Input, Reference for understanding the roles in a common collector amplifier.
Acronyms
CIR - Current, Integrity, Reference to remember the aspects of a current mirror in amplifiers.
Flash Cards
Glossary
- Common Collector Amplifier
An amplifier configuration where the collector is common to both input and output, providing high current gain.
- Current Mirror
A circuit designed to copy a current through a device while maintaining a constant current in another branch.
- Reference Current (I_REF)
The current established by the current mirror, used to set the operating point for the amplifier.
- SmallSignal Model
A linear approximation of a nonlinear circuit, useful for analyzing amplifier gain and response.
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