Understanding how transistors work

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    Transistors Work
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Discussion Overview

The discussion centers around understanding the operation of transistors, particularly their roles as switches and amplifiers. Participants explore the principles behind transistor functionality, the types of transistors, and the necessary calculations for selecting appropriate transistors for specific applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants express confusion about how transistors work and seek a thorough explanation of their function as switches and amplifiers.
  • One participant suggests that when thinking about amplification, it may be more accurate to consider control rather than direct voltage transformation.
  • Another participant clarifies that a transistor cannot directly convert 5V to 12V but can modulate a higher voltage source based on a lower input signal.
  • There is mention of different types of transistors, including Field Effect Transistors (FETs) and bipolar junction transistors (BJTs), with emphasis on their operational similarities to valves.
  • Some participants recommend specific textbooks for deeper understanding, indicating that the level of detail required depends on the reader's goals.
  • One participant provides a simplified explanation of how a transistor operates, describing it as a voltage-controlled semiconductor with three terminals and how the conductivity between terminals changes with applied voltage.

Areas of Agreement / Disagreement

Participants generally agree on the basic functions of transistors but express differing views on the specifics of their operation and the best resources for learning. The discussion remains unresolved regarding the most effective way to amplify a specific voltage using a transistor.

Contextual Notes

Participants reference various models and textbooks, indicating a range of assumptions about prior knowledge and the complexity of the subject matter. Some discussions involve unresolved technical details about transistor operation and calculations for selecting transistors.

Who May Find This Useful

This discussion may be useful for individuals seeking to understand the fundamentals of transistor operation, those interested in electronics, and learners looking for resources to study semiconductor physics and applications.

Atriusbread
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I am not fully understanding how transistors work. It would be great if someone could explain to me more thoroughly. As I understand it and have read transistors can be used as switches or to amplify voltages. Say then I needed to amplify 5v into 12 volts. How would I do this with a transistor, and how would I calculate what type of transistor I need? (Is that what it means when I keep reading amplify?)
 
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When you read amplify, think control.

Transistors replaced vacuum tubes and tubes were sometimes called valves. They kind of do act like a valve.
 


A transistor can't turn 5V into 12V, but it could modulate a 12V source to turn an input 5V (peak-to-peak) signal into an output 12V (peak-to-peak) signal. I like Horowitz and Hill's (The Art of Electronics) model of "transistor man;" he can look at the base-emitter current, and he can adjust the effective internal resistance of the transistor to give a proportionally larger collector-emitter current, but he can't turn the resistance below 0 ohms (or even that far). Does this make sense?
 


I would study the triode valve first. Field Effect Transistors (FETs, MOSFETs) work in a similar way.

Then PN junctions.. diodes.

And then bipolar junction transistors.. PNP and NPN.. Which are current amplifiers really. A small base current produces a much larger collector cuurent.
 


Well it depends on what you need to understand. If you want the details of how they work, then you should grab a book on the physics of electronic devices. I used Semiconductor Physics and Devices 3rd Edn by Donald Neamen. Otherwise if you simply want to know how they are used in the construction of micro-circuit amplifiers, then a book like Sedra & Smith's Microelectronic Circuits would do fine for you.
 


Atriusbread said:
I am not fully understanding how transistors work. It would be great if someone could explain to me more thoroughly. As I understand it and have read transistors can be used as switches or to amplify voltages. Say then I needed to amplify 5v into 12 volts. How would I do this with a transistor, and how would I calculate what type of transistor I need? (Is that what it means when I keep reading amplify?)

http://en.wikipedia.org/wiki/Common_emitter_amplifier

.
 
 


Sounds like you need a basic textbook on transistors. I think this is the book that I study in the 70s, I got the book using the author "Malvino" and this came up. I looked through whatever they show in the book. Looked like it is the same book I thought. THis is a very good book for Bi-Polar transistors. I learn all the introduction of transistors from this book and I did move onto even bi-polar IC design in the 80s. I never even went to school except studying on my own. I see they sell this used as low as $7.00 on Amazon. You might want to take a chance.

https://www.amazon.com/dp/0028028333/?tag=pfamazon01-20

Good luck. After this book, you can move onto more advanced and modern books.
 


Atriusbread said:
I am not fully understanding how transistors work. It would be great if someone could explain to me more thoroughly. As I understand it and have read transistors can be used as switches or to amplify voltages. Say then I needed to amplify 5v into 12 volts. How would I do this with a transistor, and how would I calculate what type of transistor I need? (Is that what it means when I keep reading amplify?)

I will try to answer one question only, as simply as possible. "how transistors work".
Think of a transistor as a "voltage controlled semi-conductor, with 3 terminals B, C and E, whose conductivity between C and E varies in response to a (small) voltage applied to B".

How this conductivity is changed is what is the semi-conductor physics all about. That you can read in your textbooks - all the PN junction jazz, field effect, electron tunnel or whatever.

But basically, it boils down to altering the C to E conductivity from B's terminal voltage.

Say you have a typical NPN transistor, with E grounded, C connected to a DC bias and signal input at B: The increasing B's voltage makes C to E "path" more conducting and and falling B voltage makes the path less conducting. If there is a large voltage source at C, that will make large or small flow from C to E. As the voltage varies at B, the large current from C to E varies as the same waveform as the voltage at B. Now if you make this large current flow thru a resistance external to the transistor, you will get a large voltage waveform across the resistor...this is amplification.

If you push it to the extreme - using voltage at B if you make the transistor such that
if there is some minimum voltage at B the C to E path is totally conducting and it the voltage at is less, that will make C to E path totally non conducting...this is switching.

hope this can get you started to dig more deep...

sai
 

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