Do transistors actually amplify the current?If yes,then how?

Thread Starter

AdityaS

Joined Aug 18, 2017
3
Hi, I am a beginner in electronics and learning the basics of it.
I am confused about whether a transistor actually amplifies the current as the literal meaning of 'Amplify' is to increase and the current depends on the the voltage applied and the resistance of the material used.
So,does it somehow increase the voltage or reduce the resistance or both?If yes,then how?

Thank You :)
 

dl324

Joined Mar 30, 2015
12,790
Depending on how it's connected, a BJT can give current and/or voltage gain.

Current gain is determined by transistor beta which is mainly a function of doping, geometry, collector current, collector-emitter voltage, and temperature.

Voltage gain is determined by the ratio of the collector and emitter resistors.
 

AlbertHall

Joined Jun 4, 2014
11,219
The transistor (BJT, MOSFET...) doesn't generate a larger current from a smaller current. It merely allows a low current to control the flow of a much larger current. The large current is provided by a power supply of sort connected to the transistor circuit.
 

crutschow

Joined Mar 14, 2008
26,994
So, as has been noted, the bipolar junction transistor (BJT) doesn't literally "amplify" the current into its base.
It just controls the current through the collector (from a power supply) with the collector current equal to the base current times the transistor current gain (beta or hfe).
This is called amplification.

A field-effect-transistor (FET) has a very high impedance input and controls its source current from its gate-source voltage with a voltage-to-current gain (transconductance or gm).
 

nsaspook

Joined Aug 27, 2009
8,262
A transistor is the rough electrical equivalent of a kitchen faucet in the coveted water analogy. A force at one point (the handle) has the potential to control a much larger force (water through the valve). The kitchen faucet handle also provides a mechanical advantage (possible force amplification similar to 'HFE) for opening and closing the valve with a given pressure on the valve from the water.

 

MrChips

Joined Oct 2, 2009
23,227
Nice. I like to use the fire hydrant analogy, to be more dramatic.
One small turn with a wrench creates a huge current flow.

upload_2017-9-5_12-55-46.jpeg
 

ScottWang

Joined Aug 23, 2012
7,004
@panic mode
You are the first one that I never heard anyone used dam to explain the current of bjt, almost using as nsaspook shown the water faucet, it's quite strange when I saw the picture of dam, because the dam is too big, and the water flow is too large, maybe we just learned and used the water faucet to explain for the ee beginners, but in another way, if using the dam to explain for the big current as 200A, maybe it is more match the situation.
 

Hanabi

Joined Aug 9, 2017
1
A transistor as a current amplifier is a simplified model. It works in most cases but not in all. A transistor really takes a voltage across the base emitter junction and causes a current to flow in the collector. Thus it is a voltage to current converter and depending on how you connect it up can have either voltage or current gain. An amplifier that has this charateristic, where gain is output over input is called a transconductance amplifier since a current (output) divided by a voltage (input) is a conductance (1/resistance). The mechanism of how this works can be found inmost basic electronics books so to keep the reply short I will leave that as an exercise to the student. But to understand many transistor configurations, you need to use a transconductance model not a current amplifier model. Hope this helps.
 

eddsewe

Joined Sep 4, 2017
4
The transistor (BJT, MOSFET...) doesn't generate a larger current from a smaller current. It merely allows a low current to control the flow of a much larger current. The large current is provided by a power supply of sort connected to the transistor circuit.
This is the simple answer to the question. . . Just to elaborate on the "merely allows a low current to CONTROL. . ." part, this is to say that the larger current (being controlled) basically mirrors the characteristic curve of the smaller current (controlling current).

*The characteristic curve just refers to the current-voltage relationship represented as a graph.

I hope this answer gives you the picture.
 

BR-549

Joined Sep 22, 2013
4,938
AdityaS......No, they do not amplify a current or a voltage. They amplify a signal or a pattern.

An amplifier takes a small pattern or wiggle........and impresses/copies/replicates that wiggle on a higher current or voltage, to make a larger pattern or wiggle.

The pattern/wiggle gets amplified.....not current or voltage.
 

MrAl

Joined Jun 17, 2014
8,146
Hello there,

Here are some definitions of "amplify" and related...

  1. transitive verb
  2. to expand (something, such as a statement) by the use of detail or illustration or by closer analysis

  3. to make larger or greater (as in amount, importance, or intensity) : increase : to increase the strength or amount of; especially : to make louder, to cause (a gene or DNA sequence) to undergo amplification.

  4. intransitive verb
  5. : to expand one's remarks or idea.
From the above we see that to amplify something is and act that does something to something that increases it in some way. This does not specify how that happens, and that means that it has to be specified on a case by case basis. So saying "amplify" should not imply anything other than that the signal has increased in some way. This does not mean that it had to actually create something out of nothing.

When we look at transistor operation, we see that it is really a control device. It controls current flow, and by doing that we can make it increase the signal in some way. The catch here though is that to get the signal to increase to some level, we must already have a power source that can provide the power for this increase. The power source is the power supply. The power supply must be able to handle the proposed increase.

So for example if we wish to amplify 1 volt up to 10 volts, we must have at least a 10 volt power supply available in addition to the original signal input. If we wish to amplify 1 amp up to 10 amps, then we must have a power supply available that can handle 10 amps in addition to the original input signal, unless we use a common collector circuit and then we need a power supply that can handle at least 9 amps (the emiter current is the sum of 1+9 amps approximately).

It's best to look at some specific simple transistor amplifier circuits and then ask how they work. The three simple types are common emitter, common collector, and common base.

It is worthwhile to note that a switching transistor power supply does not have to meet the requirements outlined above. Those circuits however give up one benefit for another by actually converting energy from one form to another. They can put out more amps than the original power supply can produce, or more volts then the original can produce, but they give up the other in favor of the more desirable. These circuits should be studied later.
 
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