How does a transistor act as an amplifier?

[Aurelius120]

Homework Statement

How does a transistor work?
What is the explanation for amplification of current?

Relevant Equations

NA

I was reading about transistors. There was one good video.. It said to treat the NPN transistor as two diodes.

And so I did. I remember learning that a forward biased diode can be treated as a cell that causes a potential drop equal to the barrier potential (while a reverse biased diode is an open circuit?)

But I don't know how to proceed further?

And the video explained how the current is amplified a little too fast towards the end. I didn't understand it. From the circuit diagram and the explanation in the video I can somewhat see that the current will be increased. But I don't understand how? And definitely don't see how it increases by multiplication instead of addition?(Like why is the collector current something times the base current and not something plus the base current?)

 

[DaveE]

Sorry, I didn't watch that video. Transistors are a bit more complex than just two separate diodes as you've drawn in your schematic. The key point is that they share a very thin base region, so it's a single combined device. Honestly, there is a ton of information on the web explaining them. You'll do better to study a bit more than have me give a worse explanation than what's already out there. I would start here:

https://www.khanacademy.org/science...r-working-class-12-india-physics-khan-academy

 

[Aurelius120]

Sorry, I didn't watch that video. Transistors are a bit more complex than just two separate diodes as you've drawn in your schematic. The key point is that they share a very thin base region, so it's a single combined device. Honestly, there is a ton of information on the web explaining them. You'll do better to study a bit more than have me give a worse explanation that what's already out there. I would start here:

https://www.khanacademy.org/science...r-working-class-12-india-physics-khan-academy

So in this video he said that no matter how powerful a battery you apply across the N and N of the transistors, current will not flow unless a second battery is used across N and P because there is a depletion zone at one NP junction.

But the P-region of NPN is very thin so can't the charge carriers simply cross it when the applied voltage is large enough.

Also why is the ratio of the currents preserved? Why is it only dependent on the smaller voltage?

 

[berkeman]

But the P-region of NPN is very thin so can't the charge carriers simply cross it when the applied voltage is large enough.

Sure. That is known as "letting the smoke out" of the transistor. Search on BVceo

 

[Aurelius120]

Sure. That is known as "letting the smoke out" of the transistor. Search on BVceo

You mean this video?
Here he says that if the collector circuit is open, all emitter current will flow through base and transistor will burn. But
If there is no collector current, then why will there be any amplification, the only current should be the small base current which flows through the emitter as well.
Why will attaching an open battery across the NN ends cause any effect on the circuit?

 

[nsaspook]

https://webhome.phy.duke.edu/~schol/phy271/faqs/faq15/node2.html

A more technical two diodes model that will likely require watching some of the previous material of the video series.
https://circuitcellar.com/resources/quickbits/the-ebers-moll-bjt-model/
The Ebers-Moll BJT Model

 

[berkeman]

You mean this video?
Here he says that if the collector circuit is open, all emitter current will flow through base and transistor will burn. But
If there is no collector current, then why will there be any amplification, the only current should be the small base current which flows through the emitter as well.
Why will attaching an open battery across the NN ends cause any effect on the circuit?

Much like @DaveE I don't generally watch YouTube videos as references. If you have access to the Khan Academy, you can view their instruction video on transistors that Dave linked to. You can also read the Wikipedia article, especially the section on Simplified Operation:

https://en.wikipedia.org/wiki/Transistor

For me, one of the best images to keep in mind when thinking about how an NPN bipolar transistor works in the "Common Emitter" current amplifying configuration is using this graph:

https://www.electronics-tutorials.ws/transistor/tran_2.html

That is also a good tutorial to read through to understand why that plot is so instructive.