William Shockley, one of the inventors of the transistor, once explained transistor-amplifiers this way:
>*”If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification.”*
which seems to describe the concept but not the process.
In: 6
The base is the middle junction of the transistor and acts as a valve between the end junctions, collector and emitter. A small current from base-to-emitter opens the valve and allows a much larger current to flow collector-to-emitter. Think of a valve at the bottom of a damn. The force to open the valve can be very small in relation to the force released.
That is indeed a terrible analogy.
There are a couple different ways, but I like this analogy:
You have two pipes carrying water, one large and one small. The large one has a valve, and the flow after the valve is dependent on how open the valve is.
The second, smaller pipe ends at a spring loaded piston. The more pressure it exerts, the further the piston extends. The piston physically connects to the valve handle.
As the smaller pipe experiences more or less pressure, it controls the output pressure of the larger pipe via the valve, which happens to have a greater maximum pressure.
Transistors can be used this way, just using voltage, which is electrical pressure
How it fundamentally work require an explanation of semiconductors that are really not something you can put in a post here I would look at
https://www.youtube.com/watch?v=33vbFFFn04k and https://www.youtube.com/watch?v=DXvAlwMAxiA for a detailed explanation.
A simple transistor has three terminals. You control the current between two with a third. It is the current or voltage depending on the transistor model that controls the flow between the two other connectors.
So the transistor does not create a larger current it regulates the current from another source by the input. You can compare it to if you have a vale in a water pipe. If you have a vale that can be partially open you can control the flow rate as you can do with the taps of a sink.
You could design a valve that opens itself to different degrees depending on the water flow through another pipe. If you did you have something that functions as a transistor but with water.
Shockley worked on the Junction Transistor, these days generally called a BJT or Bipolar Junction Transistor, which is also the easiest to understand amplifiers with
BJTs act like little valves, they have a collector, and emitter, and a base. The main voltage will be applied from the collector to the emitter and a small signal will be provided into the base
On a BJT, when a current flows into the base and out the emitter it opens up the valve and lets a much larger current flow from the collector to the emitter, but this isn’t an all or nothing deal there is a gain factor. If the gain factor is 20 then pushing 1 mA into the base will let 20 mA flow into the collector, 2 mA will let 40 flow, and thus your small current into the base is amplified by 20x.
MOSFETs behave similarly in their linear region, but they’re controlled by a voltage on the gate not a current.
A transistor is an electric valve, controlled by a voltage. This valve is normally closed, the more voltage you send to the base the more it opens.
In your question: The transistor is the core of an amplifier but it doesn’t actually amplify anything. A circuit makes a lot of power, then the transistor takes this power and reduce it to what you want and give it the shape you want.
You send to the transistor infinite power one side, you connect the exit to speakers.
Then on the base of the transistor you send a voltage, more voltage, more the transistor open the path for the power.
Let’s say, before the transistor we put a source of 500watt of power. On the base of the transistor we send a weak music signal, just a voltage that goes up and down the same as the music sound wave. At the exit of the transistor you have 500watt going out with exactly the same ups and down as the music you sent to the base.
Voilà, you have transformed a weak music signal into 500watt of music signal.
It doesn’t amplify anything. It uses a tiny amount of electricity to operate a gate to let a large amount of electricity flow through.
So … “CMOS” field-effect transistors — the ones which make up 99.99% of modern computers — don’t *on their own* amplify current. They behave more like tiny on/off switches (I.e., transimpedance amplifiers) they do current-to-current or voltage-to-voltage amplifiers. They can, however, be *combined* in many different ways (aka, “circuits”) which can amplify voltage and/or current.
However … Another type of common transistor is a “bipolar junction transistor” which can, on its own, amplify current. It is a three terminal device, and a small current injected into the Base of a BJT will result in a much larger current flowing into the Collector, through the transistor, and out the Emitter.
The underlying semiconductor physics about why Base current is “multiplied” by a BJT in this way is complicated. But i like to think of it that Base current — which can be though if as a stream of tiny charge particles — get absorbed into a place in the semiconductor that wants to be empty of these types of charges. The semiconductor then “overreacts” to the presence of these charges, and works to “flush them out” in a way that we perceive as current multiplication.
I often call a transistor a “voltage-controlled switch”, but that’s not really true. Transistors are voltage-controlled *resistor*. MOSFET transistors have 3 terminals: gate, source and drain. The amount of voltage at the gate terminal determines the amount of resistance between the source and drain terminal. If you connect the source terminal to a supply with a higher voltage, you can get more voltage out of the drain terminal that you supply at the gate. You generally use a additional resistor between the transistor and the supply to bias the amplifier to the range of voltages you want.
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