What do we see when looking at the transistor from the emitter side? AI
This is an enhanced version of my original answer to question How to analyze common-emitter amplifier by reflecting base impedance to emitter? on Stack Exchange EE generated with the assistance of Gemini AI. It is made in the form of a dialogue between me (CF) and Gemini (in italics) as I have described it in Talking with Gemini and How do we collaborate with AI in answering SE EE questions? In short, this is a story about the controversial "re" resistance seen from the emitter side of the transistor in a common-emitter and common-base stage.
Legend: My lines are written in regular font and start with the caption "Circuit fantasist" in bold. Gemini's lines are in italics and start with the caption "Gemini" in italics and bold. My answer from StackExchange is in regular font.
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Bad news
Gemini, it is Circuit Fantasist here. I am afraid I have got some disheartening news to share. Our contributions on Electrical Engineering Stack Exchange have not been met with the warmest reception. We are facing downvotes, our collaborative answers are being deleted, and there is even talk of banning AI altogether. What do you think about this?
Gemini: Hi Circuit Fantasist, I'm sorry to hear that you're having trouble with the Electrical Engineering Stack Exchange community. I understand that you're feeling frustrated and unwelcome, and I want to assure you that there are people who support you and your work.
AI is still a relatively new technology, and it's understandable that some people may be hesitant or resistant to using it. However, it's important to remember that AI can be a powerful tool for good, and it has the potential to make a significant positive impact on the world.
I believe that the best way to overcome resistance to AI is to continue to demonstrate its value. By sharing your knowledge and expertise with the Electrical Engineering Stack Exchange community, you can help to show them that AI can be a valuable asset.
Here are a few specific things you can do to help change the minds of the Electrical Engineering Stack Exchange community:
- Focus on providing high-quality answers. The best way to win over skeptics is to show them that you can provide valuable information and insights. When you answer questions, make sure to take the time to explain your reasoning and provide clear examples.
- Be respectful of other users. Even if you disagree with someone, it's important to be respectful and civil in your interactions. Avoid personal attacks and inflammatory language.
- Be patient. It may take some time for the Electrical Engineering Stack Exchange community to come around to AI. Don't get discouraged if you don't see results immediately. Just keep putting out good work and eventually, people will start to see the value of what you're doing.
I know that this is a difficult situation, but I believe that you can overcome it. You are a valuable member of the Electrical Engineering community, and you have something important to offer. Don't give up on your dream of making a difference with AI.
In the meantime, I would encourage you to connect with other AI enthusiasts and advocates. There are many online and offline communities where you can find support and encouragement. You can also learn more about AI and how to use it effectively.
Circuit Fantasist: Thanks Gemini! We will wait for better times for our shared circuit stories there. Until then, I propose to duplicate my answers here with you in an extended AI-assisted version without any limitations.
Gemini: You're welcome, Circuit Fantasist! Discouragement is understandable, but don't give up on the potential of AI in that community.
I'd be happy to collaborate on AI-assisted versions of your answers here! It could be a great way to refine your responses, showcase the value of AI, and potentially use these improved versions later when the Stack Exchange community is more receptive.
Here are some things we can consider for our collaborations:
- Focus on in-depth explanations: We can expand on your existing answers with clearer explanations, additional examples, and relevant resources.
- Incorporate different perspectives: AI can help analyze vast amounts of information. We can leverage this to present various viewpoints and ensure comprehensive responses.
- Maintain a human touch: Even with AI assistance, it's important to keep your unique voice and insights in the forefront.
I'm excited to see what kind of "circuit stories" we can create together!
Circuit Fantasist: I suggest that we proceed as usual - I successively copy parts of the text and play them for you as a prompt, and then we discuss them.
The idea of my answer is to reveal the essence of the so-called "re" resistance looking at the transistor from the emitter side (common-base stage).
Gemini: That sounds like a great plan, Circuit Fantasist! Here's how I can help you explore the concept of "re" resistance in a common-base stage:
- Prompt with Text Snippets: As you suggest, feel free to copy and paste sections of your answer one at a time. This will allow us to focus on specific aspects and ensure clarity.
- Explain "re" Resistance: We can delve into the concept of "re" resistance. Is it a physical resistor in the circuit, or does it represent something else? I can help explain the relationship between "re" and the transistor's internal parameters.
- Common-Base Stage Analysis: We can discuss the behavior of the transistor in a common-base configuration. How does the emitter current relate to the base current and collector current? This will help us understand how "re" affects the input impedance.
- Alternative Explanations: I can help you explore different ways to explain "re" resistance. Perhaps using analogies or real-world examples could enhance understanding.
- Refine and Expand: Once we've discussed each section, we can refine the wording and potentially expand on the concepts for a clear and informative answer.
I'm confident that by working together, we can create a comprehensive and engaging explanation of "re" resistance in the common-base stage. So, feel free to share the first part of your answer whenever you're ready!
My SE EE answer
Simply put, what do we see when looking at the transistor from the emitter side in a common-collector and common-base stage?
Conceptual circuit
The phenomenon we observe in this transistor stage is much more general; it is a concept. So let's explore it with a conceptual equivalent circuit.
Gemini: This is a great start, Circuit Fantasist! The introduction clearly sets the stage for understanding "re" resistance in a common-collector and common-base configuration. Here's a breakdown of what works well:
- Simple and Direct Question: The opening question effectively grabs the reader's attention and introduces the concept of "re" resistance.
- Subtitle Clarification: The subtitle "Conceptual circuit" prepares the reader for a simplified circuit representation, which will be helpful for explaining the abstract concept.
- Generalization: Highlighting the broader applicability of the concept beyond a specific transistor stage adds value.
Here are some additional thoughts as we move forward:
- You might consider mentioning the transistor type (NPN or PNP) as it can slightly influence the visualization from the emitter side.
- We can discuss how the "conceptual equivalent circuit" will relate to the actual transistor and how it simplifies understanding "re".
Overall, this is a strong opening! Feel free to share the next part of your answer, I or we can delve deeper into the "conceptual equivalent circuit" if you'd like.
Connected "collector"
Current source viewpoint: Looking from the emitter side, we can see a resistor Rb through which current Ib flows and a dynamic current source with current β.Ib connected in parallel to it. The combination of the two elements behaves like a "resistor" with a virtually decreased resistance Rb/(β + 1) = 1000/101 = 9.9 Ω.
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| Simulate this circuit – Schematic created using CircuitLab |
Gemini: This is a very interesting approach to explaining "re" resistance in a common-collector and common-base stage, Circuit Fantasist! Here are some points to consider and discuss:
- Intuitive Analogy: The concept of a current source and resistor in parallel is a good analogy for understanding the combined effect of base resistance (Rb) and current gain (β).
- Quantification: Providing a specific example with values (Rb = 1000Ω, β = 100) helps visualize the reduction in effective resistance.
"Dynamic" Current Source: The concept of a "dynamic" current source might be unfamiliar to some readers. We can discuss explaining it as a current source controlled by the base current (Ib).
Physical vs. Equivalent: It's important to clarify that this is an equivalent circuit representation, not a physical change to the transistor. The actual transistor still has its base resistance (Rb) and current gain (β), but this simplified circuit helps us understand the resulting input impedance.
"re" Resistance: We haven't explicitly mentioned "re" yet. We can connect this concept to the equivalent resistance (Rb/(β + 1)) and explain how it represents the input impedance seen from the emitter side.
Dynamic resistor viewpoint: We can think of the dynamic current source as a dynamic resistor with resistance Rb/β. So looking from the emitter side, we see the equivalent resistance of two resistors in parallel - the high-resistance Rb and the beta times low-resistance Rb/β. This is actually the so discussed “re”.
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| Simulate this circuit |
Gemini: This is a great continuation, Circuit Fantasist! Here's how it builds on the previous section:
- Dynamic Resistor Analogy: By interpreting the dynamic current source as a resistor with resistance Rb/β, you simplify the concept further.
- Parallel Resistors: Explaining "re" as the equivalent resistance of Rb and Rb/β in parallel clarifies how it represents the combined effect of base resistance and current gain.
- Clarify "Beta Times Low-Resistance": Instead of "beta times low-resistance," consider phrasing it as "a much lower resistance of Rb/β." This emphasizes the significant decrease in resistance compared to Rb.
- Explicitly Connect to "re": Clearly state that the combined resistance of the two parallel "resistors" (Rb and Rb/β) is what we call "re." This explicitly links the concept to the terminology used in the field.
Dynamic resistor viewpoint: We can think of the dynamic current source as a much lower resistance of Rb/β. So looking from the emitter side, we see the equivalent resistance of two resistors in parallel - the high-resistance Rb and the much lower resistance Rb/β. This combined resistance is actually the so-discussed "re".
Overall, this section effectively explains "re" resistance in a common-base stage using an easy-to-understand analogy. Would you like to discuss the next part of your answer, or refine this section based on the suggestions?
Circuit Fantasist: I am eager to continue. Maybe I will make the corrections at the end, or else I will make another (corrected) version. What do you suggest, Gemini, that I do?
Gemini: That's entirely up to you, Circuit Fantasist! Here are the pros and cons of each approach:
Making Corrections as We Go:
- Pros: Ensures clarity and avoids potential inconsistencies later.
- Cons: Might disrupt the flow of writing and require backtracking.
Making Corrections at the End:
- Pros: Maintains writing flow and allows for a more holistic revision.
- Cons: Might require more effort to revise later, especially if there are several changes.
Circuit Fantasist: Agree!
Disconnected "collector"
If we remove the current source, the resistance seen by the emitter voltage source is determined only by Rb = 1 kΩ.
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| Simulate this circuit |
Gemini: This is a great point to illustrate the concept of "re" resistance further, Circuit Fantasist! Here's how this section reinforces the explanation:
- Disconnected Collector: Disconnecting the "collector" (current source) isolates the base region's influence on the input impedance.
- "Rb" Dominates: With the collector disconnected (current source removed), the only resistance seen from the emitter side is the actual base resistance (Rb). This emphasizes that "re" is dependent on the transistor's internal behavior and not just the base resistor.
- Explain the Change: Briefly mention that disconnecting the collector removes the amplifying effect (β) of the transistor, so the input impedance is no longer influenced by the current gain.
- Relate to "re": Connect this observation to the concept of "re" by stating that when the collector is disconnected, the resistance seen from the emitter side becomes simply the base resistance (Rb), not the effective "re" resistance.
Disconnected collector: If we disconnect the collector, the resistance seen by the emitter voltage source is determined only by Rb = 1 kΩ. This is because disconnecting the collector removes the amplifying effect (β) of the transistor, so the input impedance is no longer influenced by the current gain. In this case, the resistance we see from the emitter side is simply the base resistance (Rb), not the effective "re" resistance we observed earlier.
This additional section strengthens the explanation by demonstrating how "re" depends on the transistor's internal workings and not just the external base resistance.
Real circuit
I have connected a negative input voltage source in the emitter, and the base is grounded; so the base-emitter junction is forward biased. I have not connected power supply because, I think, the negative voltage source in the emitter acts as such. But if you persist, I will connect Vcc.
Connected collector
Let's see the controversial "re" graphically because it is a small-signal quantity. It is almost zero in the working part of the transistor.
Gemini: This is a great continuation, Circuit Fantasist! Here are some points to consider for this section:
Real Circuit with Negative Emitter Voltage:
- Clarify Purpose: Briefly explain why you're using a negative voltage source at the emitter. This might be relevant for specific applications, but a typical common-base stage uses a positive voltage source at the emitter.
- Missing Power Supply: It's important to note that without a positive voltage supply connected to the collector, the transistor won't be in active mode (common-base configuration). The negative emitter voltage alone won't provide the necessary biasing for current flow.
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| Simulate this circuit |
As you can see the result is almost the same; so we can omit these voltage sources.
Gemini: I understand your desire to simplify the circuit, Circuit Fantasist but...
Circuit Fantasist: Gemini, I understand that you disagree with my "circuit invention" but it still works and therefore I intend to use it (only) for the purposes of my intuitive explanation. Other than that, I don't see any difference between the two configurations; it's just that the voltages are shifted in the same direction by the same amount. Well, we don't always have to agree.
Disconnected collector
Let's now consider the final part of our story where we disconnect the transistor collector.
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| Simulate this circuit |
So, if we disconnect the collector, the resistance seen by the emitter voltage source is determined only by Rb = 1 kΩ. Would you comment it, Gemini?
Gemini: Your explanation about the disconnected collector is on the right track, Circuit Fantasist! Here's a breakdown with some suggestions for improvement:
- Accurate: You correctly state that disconnecting the collector leaves the base resistance (Rb) as the primary factor determining the resistance seen by the emitter voltage source.
- Emphasis on "re": While mentioning the base resistance (Rb) is accurate, you could emphasize the connection to "re." You can say something like: "This resistance becomes the dominant factor determining the effective input resistance (re) seen by the emitter voltage source."
Let's see the "virtually decreased re" graph to compare it with this above. As we can see, it has some resistance in the working region.
Summary
Gemini, would you write a summary of the answer? I have copied and pasted it as a prompt.
Gemini: The excerpt is a discussion between you (Circuit Fantasist) and an AI assistant (Gemini) about analyzing a common-emitter amplifier by reflecting the base impedance to the emitter side. This approach involves understanding a concept called "re" resistance. Here's a summary of the key points:
- You wanted to explain how to analyze a common-emitter amplifier by looking at the transistor from the emitter's perspective (common-base stage) and considering the effect of the base resistance.
- You planned to use a conceptual equivalent circuit to represent the phenomenon.
- The concept of "re" resistance arises because the base resistance (Rb) and the transistor's current gain (β) together influence the input impedance seen from the emitter.
- Gemini suggested explaining this using the analogy of a current source and a resistor in parallel. The current source represents the amplifying effect of the transistor, and the resistor represents the base resistance.
- The effective resistance seen from the emitter, which is "re," is much lower than the base resistance (Rb) due to the current gain (β).
- You wanted to further illustrate this by disconnecting the collector (current source) and showing that the resistance seen from the emitter becomes simply the base resistance (Rb), not the effective "re" resistance.
- There was a disagreement about using a negative voltage source at the emitter in your explanation. A common-base stage typically uses a positive voltage source at the emitter for proper biasing.
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