Why do Junction Transistors Amplify Current and not Voltage

[@arastrosap]

Sir! I want to repeat this everyday: You are the best Electronics teacher in the whole world. Thank You very much!

[@michaelmccarty4765]

Very, very helpful! I'm a vacuum tube guy trying to understand transistors, be them a BJT, FET, JFET, MOSFET... and the others. I've never heard BJTs described this way before. It's a lot of food for thought. Thank you!

[@whatzause]

I've done a lot of work in electrical circuits, but thanks to you, today I found out (basically) how an air-conditioner works! Very simply explained. Thanks!

[@charlesschneiter]

Thanks for this latest refresher! You are a teacher of great talent 👍

[@stuartbruff8786]

The conventional viewpoint has bugged me for years, so it's nice to see someone articulating what I'd thought all along and caused me a lot of self-doubt. Nice summary.

[@adastra123]

Brilliant 👏 👏 👏 👏 , I finally get it !!!!! Thank you. That gave me more confidence than a year of therapy !! Worth waiting for

[@jnc5255]

Thank u sir for great explaination keep up the good job and godbless

[@LISANCACOM]

Thanks, really informative, clear and educative video.

[@flurng]

Another terrific explanation, Bob! Yet, if I may offer a different point of view, the way I always understood it, is that the base-emitter (input) current directly controls the collector-emitter (output) current. Thanks to Ohm's law, increasing base voltage causes a rise in base current, which in turn, raises collector current far higher. Finally, thanks again to Ohm's law, this rise in collector current causes a substantial drop in collector voltage. And, conversely, dropping base voltage would have the opposite effect on collector voltage! (P.S. - I should point out that this explanation applies specifically to common- emitter circuits!)

[@RexxSchneider]

Increasing the base voltage does indeed cause a rise base current, but it doesn't follow Ohm's Law. The relationship is exponential, and as a rule of thumb, you can expect the base current to double for every 17.5mV increase in base voltage.

[@patricklanra3507]

A very informative video - Thanks Sir

[@RexxSchneider]

Just a couple of corrections and a note: There is no linear region on the Vbe - Ic curve for a bipolar transistor under any practical conditions. The relationship between Vgs and Id for a FET is quadratic, not linear. It's worth remembering that for any pn junction, the current doubles for each 17.5mV increase in the voltage applied to it at room temperature.

[@MohammeHadishvideos4u719]

Very nice sir

[@OmarMohamed-ej5ek]

Wow thank u teacher to remind me the relation between current and voltage on bjt

[@ΚωνσταντίνοςΛαζαρίδης-ξ9ι]

Thank you sir! Really nice expanation

[@biseraciric454]

Thank you very much for this explanation! 👋🏻 😄

[@tvamos73]

My teacher told me that if I can imagine the BJT is a current controlled current source and MOSFET is a Voltage controlled resistor.

[@RexxSchneider]

The mosfet behaves a a voltage-controlled resistor only when the voltage between drain and source is small compared to its threshold voltage (the ohmic region). When the voltage between its drain and source is significantly larger, it acts as a voltage-controlled current source (active region).

[@ChrisHalden007]

Great video. Thanks

[@flurng]

As Bob so astutely pointed out, in order to get any measure of linearity from the input voltage variation, it must indeed be restricted to a very small operating range. This, combined with the highly linear relationship between base and collector currents, is quite probably why "they" say that BJT's amplify current, rather than voltage . Yet, I have always felt that this highly restricted input voltage range is the VERY REASON they are so useful as amplifiers, since this is PRECISELY what devices such as dynamic microphones, magnetic pickups and phonograph cartridges provide! After all, if they REQUIRED a much wider input range, something more on par with the output voltage range, then their gain would be rather insignificant, and thus, their usefulness as amplifiers would be greatly diminished!

[@RexxSchneider]

In a BJT, the relationship between collector current and base-emitter voltage is intrinsically non-linear. To get linear amplification it is necessary to apply some sort of feedback so that the non-linearity is reduced as a proportion of the output. In a common-emitter amplifier, this is often done by using an emitter resistor, lowering the gain and increasing the linearity. Even with small signal sources, such as those you mention, it remains important to achieve good linearity while attempting to maximise the gain in order to keep the circuit noise low compared with the signal.

[@michaelwise9363]

Do you have video for an rf npn amplifier (eg BFQ19) where a choke is used in the supply to the collector? How does this effect things? Thanks M

[@klmkt4339]

In the world we should value and look for linear relationship. What i learned from transistor

[@mr7911]

Will the current always be limited to the power source? Let's say (hypothetically) you had a 12v battery, but for some reason it was limited to picoamps, could a transistor, like the one you describe amplify such a low current up to something more useful even though the power source itself isn't capable?

[@khimroy3958]

Very very very Good explanation! 👽👍👍👍

[@Donquixote-qv7sp]

Wait..so in actuality the VBE voltage is the one that controls the current ......

[@jonahansen]

Short answer: Because that's how they work. There is always a relationship between voltage and current, and the operation of a BJT is best characterized by the current relationship.

[@Coolgiy67]

How does it actually amplify the current? Maybe you can use the hybrid pi model to explain ?

[@mikesradiorepair]

This is probably a oversimplification but here goes. The collector has a voltage applied to it. The emitter in the video example is tied to ground. The collector to emitter basically looks like a infinite resistance with no base voltage. Applying a small voltage to the base allows current to flow through the collector to emitter. The higher the base voltage the higher the collector to emitter current. So in a nutshell, the small variable base voltage controls a large collector to emitter current. Mike KC3OSD

[@flurng]

Not to get into the whole "Conventional current vs. Electron flow" debate, but the truth of the matter, is that electrons flow from negative to positive. So, with an NPN type transistor, as shown in the example, electrons actually flow IN to the emitter. From there, due to the atomic structure of the semiconductor layers, approximately 5% of those electrons flow OUT of the base, while the remaining 95% flow OUT of the collector. This proportion is maintained regardless of how many electrons (thus, how much "current") are actually flowing at any given time. As a result, a small change in the base current creates a much larger change in collector current, thereby amplifying the current.

[@RexxSchneider]

In an NPN transistor, the base can be considered to be a thin layer sandwiched between the emitter and the collector (the actual geometry doesn't matter), When biased normally, the base-emitter junction is forward biased and the collector-base junction is reverse-biased. As the voltage on the forward-biased base-emitter junction increases, electrons begin to cross from the emitter into the base, where they can flow out of the base towards the more positive voltage (Vbe). However, because the base layer is very thin, a larger number of electrons are attracted towards the significantly higher voltage at the collector (Vce) and are able to cross the reverse-biased junction into the collector and are drawn out of the collector by its higher voltage. It turns out that there is an approximately linear relationship between the number of electrons collected by the collector and those gathered by the base. This ratio is the transistor β and is often in the 100s. As long as Vce is larger than Vbe, the collector collects almost all of the electrons that cross the C-B junction and the collector current under those circumstances does not depend on the collector voltage. That describes the main features of the BJT.

[@андрій-ч7ч6ы]

Common collector,common emitter,common base..what is difference?