Hows does a transistor amplify sound?

Hi CD,

  I am working on my physics report and i am doing it on Transistors. I have tried A LOT of websites, including youtube, wiki, howstuffworks, and even the teacher. Even he does not completely understand it. My last and hopefully best resource is CD. Can someone please explain in the simplest form you can,because  i need to present this and my teacher is likely to ask me hard questions on this topic. 

What i understand so far:
There are three parts
(1) Base
==> Which the small current comes in
(2) Emitter
==> Can either be input or output(but depending on what?)
(3) Collector
==> Can only be output.

My question is:
How does it actually amplify? What happens?
How does it turn on or off?
==> Some websites said that if current is put through the BASE and is higher than 5V then collector opens and it is ON
==> Or if current is put through BASE and is lowest than 5V then collector does not open and it is OFF
P.S. Is this accurate>?

If someone can please help me out on this. I am having some serious troubles, I have tried sooo many websites and i do not get it. Please help me my report and presentation is Monday :slight_smile: Thanks in advance.

Guess who just got done with the EE version of that?

I also think that you’ve got your emitter and collector swapped. (Emitter emits; emit means put out; ergo, emitter puts out whatever it’s supposed to be dealing with.) Collector collects whatever it collects. On the diagram I have in front of me, I have B and C going into E through a transistor.

Short answer: The current (and voltage) into the base controls how much current (and voltage) goes out the emitter. V=IR, so if you know your resistances, you’re set.

There is a longer answer, but I’d probably go into my EE301 textbook for that. (EE301: Intro to Circuits at SDSM&T, colloquially referred to as “Circuits for Non-Believers” because EE majors aren’t supposed to take it, but most other majors that might deal with electronics need to.)

Over the weekend, talk to an EE or someone like that. They’ll be able to give a more complete picture.

[EricH] covered the basic version, here’s a little (or a lot :rolleyes:) more detail if you want it:

Note that there are many different types of transistors, based on the different types of semiconductor (different types of doping) that are in the region of the transistor, and what materials are actually used in its construction. The most common type of transistor, especially in intro EE classes, is a NPN Bipolar Junction Transistor. The Wikipedia article on BJTs is a little long, but I think it’s fairly readable if you have a basic knowledge of voltage and current and so forth.

Like [EricH] said, in NPN transistors, current passes from the base and collector to the emitter, and in forward-active mode when using DC, the relationship between these two currents can be approximated as linear, i.e. <Collector-Emmiter current> = B * <Base-Emiter Current>, where B is usually something around 100. AC signals (like audio signals) get more complicated, but if you understand the DC relationship, it’s somewhat similar, so if you have a small amount of current from an audio entering the base, then a larger current will be generated from the collector to the emitter. Again as [EricH] said, if you know the resistance through your circuit (more properly, the impedance since audio is an AC signal), you can determine the voltage generated by Ohm’s law. Voltage is usually fairly equivalent to volume in audio circuits.

A couple of extensions to cover some of your other questions:

In PNP transistors (made with a different set of dopings), the current basically flows in the opposite directions, but the concept of the relationship between emitter-base current and emitter-collector current is essentially the same.

Transistors have different regions of operation. The three basic regions are foward-active, cutoff, and saturation. Which one the transistor is in basically depends on how much voltage and current are applied to the transistor. For audio amplification, you usually run the transistor in forward-active mode, because this allows for the linear amplification of the input signal. Cutoff happens when the base-emitter voltage drops below a certain point, usually about .6V for BJTs, and results in very little current flow through the transistor. This is usually referred to as the transistor being “off.” In either of the other two modes, the transistor is usually considered to be “on.” If the current through the transistor gets too high, the transistor goes into saturation, which is basically like it sounds: the current through the transistor has maxed out.

Unlike in audio circuits, if you’re using a transistor in a digital circuit (like a computer), you try to minimize the amount of time spent in forward-active, because the output voltage can be somewhere between the voltages defined as “0” and “1.” So in a digital circuit, “off” is cutoff, and “on” would be saturation. This is probably what you saw referenced in some of the websites, because 0V is usually chosen for a digital 0, and 5V is a common value for a digital “1.” Again, note that this is a different usage case than what you’re talking about with audio circuits.

Did that cover all your questions? Feel free to post back if you’re still confused.


I do not know exactly what you are asking but I have attached some powerpoints from my electronics class at my community college. These are about what transistors are and how transistor amplifiers work.

Transitor and Amplifier PowerPoint (580 KB)

Transitor and Amplifier PowerPoint (580 KB)

To picture amplification in the simplest form, you only need to see one transistor connected to a voltage source and a signal. If I have a .7 Volt on the BASE of this transistor, the transistor turns on and current flows from the collector to the emitter(current flows from 5V to 0V.) The “Signal” is 0V due to the low resistance in the transistor. If I have 0V on the Base, then the transistor in is OFF(No current flows from 5V to 0V). The Signal is now 5V.

Note * In this example the transistor “amplifies” and inverts the signal. Another Transistor in series would invert the “signal” again. *

If you have a few hours to burn, please read

It is easily the best intro to amplifiers that I’ve ever found. It assumes no prior knowledge of electronics and runs all the way up through the full history of amplifiers. You will understand amplifiers in a way that will put you years ahead of your classmates when you are finished. I can not praise this enough.

OR, the short answer:

A small amount of current into the base allows a larger amount of current into the collector. The ratio is called Beta, and is typically on the order of 100. That is, if you put a 1mA signal in, you can get a 100mA signal out.

Take a look at this figure:

Write appropriate scales on the axes, and you are good to roll.

This is where i am confused. Isnt the only one source connected to the base? When we apply just a little signal, how can a bigger flow throw collector to emitter, when we only applied it to the base? Or does the collector have something connected to it as well?

The collector has power connected to it. The signal flowing from the base to the emitter controls the larger signal flowing from collecter to emitter.

Think of the transistor as a variable resistor. The input signal (from base to emitter) varies the resistance (from collector to emitter). This varying resistance controls the output signal according to Ohm’s law.

Read the wikipedia article that Ryan linked to. It has lots of graphics and schematics.


So basically the BASE is the input such as a sound coming in from a microphone, and the COLLECTOR is connected to like a power supply or outlet in a wall, or battery or something, and the EMITTER is the output, the amplified sound?

If this is correct then can a EMITTER only be a output? always? some websites were saying collector can also be? :S


and the COLLECTOR is connected to like a power supply or outlet in a wall, or battery or something,

depends on whether the transistor is NPN or PNP

and the EMITTER is the output, the amplified sound?

The entire emitter-collector circuit is where the output current flows. Different circuit designs place the load in different locations in this circuit.

If you want better understanding, do yourself a favor and read the links the nice people here have provided. There are lots of graphics and schematics to help you.


I have went to all the links provided ^ but as i have NO knowledge of circuits or current because i am 15 and in grade 11 we have yet not started current and etc. I Attempted to understand the links provided but it got confusing real fast. Just verifying my understanding and if it is accurate i will go and start finishing my report.

There are three parts to the transistor:
(1) BASE
==> It is where some kind of signal, or small current is entered(microphone for example)
==> It is connected to a power source/supply, like a battery or power outlet
==> It is always the output, the amplified current

So if i make a small sound in the base, and i have connected the collector to a outlet, then that sound will get amplified and be outputted in the emmiter.

For it to act as a switch it is basically, putting forth more than 5V current will turn the collector gate ON, and less than 5V would get it into saturation mode or OFF.

Just need someone to confirm this so i dont tell the class and teacher something utterly wrong :slight_smile:

OK, I think I see where you’re going wrong.

Think of the transistor as a water faucet. The transistor’s base is the faucet knob. Just a small effort can control a huge amount of ‘water’. The valve has pressurized water on one side and air on the other. As you turn the knob, water flows, and you can vary that.

Ether had it almost perfect: Think of a transistor as a voltage-controlled resistor. (In an electric circuit, as resistance drops, more ‘current’ flows). As you change the voltage on the base, the amount of resistance the transistor collector-emitter terminals have also changes, allowing more (or less) electric current to flow.

So what a transistor is doing is taking the power from the power source and controlling how much power gets to the output. The controlling thingus is the base voltage.

The name Transistor came from a contraction of the term “transfer resistor”. IN the early days, variable resistor was the best model they had to describe what these odd things did, because they were so very different from the existing technology, the vacuum tube.

(Please note that this explanation simplifies several important points of transistor operation, such as: The ‘output’ can sometimes be on the power-source side, the input signal is turned upside-down by some circuits, there is a narrow range of base voltages that produce a linear output, the rest have the transistor acting like a switch, and more. Folks above have covered it well)

Are you trying to say that your age affects your intelligence? I hope not.

Almost but not exactly.
Base is essentially right. It can be a small signal (like a microphone) or a large signal (like the power source voltage or ground). Note that with a small signal (an AC signal like a microphone), you need to change the DC voltage associated with it to be sure the transistor is in its “linear” region, otherwise it wants to act like a switch. This is called “Biasing” the transistor.

Collector and Emitter change roles depending on what type of transistor (PNP or NPN) and what you need to accomplish with the circuit.

You’d be correct in writing or saying:
==> It is usually connected to a power source, like a battery or power outlet
==> It is usually the output, the ‘amplified’ current flowing through the transistor. It can also be connected directly to ground in some circuits, the signal being tapped off at the collector.

The rest seems accurate.


==> It is connected to a power source/supply, like a battery or power outlet

If it’s an NPN, yes. If it’s a PNP, + is connected to the emitter.

==> It is always the output, the amplified current

In an emitter-follower circuit, the output load is in the emitter leg.

In a collector-follower circuit, the output load is in the collector leg.

An emitter follower has low voltage gain.

A collector-follower has both voltage and current gain.

If you’re trying to drive a speaker (output load) with a microphone (input signal), you’re probably gonna need both voltage and current gain.

can a EMITTER only be a output? always?

No. The output load can be in the emitter leg (emitter follower, aka common collector) OR the collector leg (collector follower, aka common emitter).

some websites were saying collector can also be?



There are really two questions here… what does a transistor do… and HOW does it do it.

To get a look at the what does it do, try using my favorite circuit simulator and take a look under the “circuits” menu item for transistors and then choose PNP or NPN transistors as appropriate. Play with the sliders on the bottom right, and watch the current flow.

You’ll find other, more complex, circuits in there to check out and… when you get the hang of it, can modify them or even build and simulate your own circuits. So even those who already know what transistors do… it is worth your time to check the software out. Easily one of the coolest java applets that I’ve come across.

Now, as for the “how does it do it”… well, if you just said “it uses magic smoke and I’ll worry about the rest of it when I have to”, it would be completely forgivable. The first thing you need to know is how a diode works. My favorite diode explanation (skip the first minute of it… it is really good after the first minute…) is on youtube at

Once you’ve got the idea of a PN junction diode and doping then you can understand what a PNP or NPN junction might be like. But most diagrams of transistors aren’t really clear about the fact that the two outer layers are doped slightly differently from each other and that the middle layer (the base) is really, really thin relative to the other two layers. A simulation is online at

Hopefully that helps your understanding.


This is kind of hard subject to explain when one does not understand the electronic basics of voltage and current in resistance. When a voltage is connected across a resistance current flows. Also if something causes a current to flow through a resistance, a voltage is developed. In your simple microphone transistor amplifier model, the microphone generates a voltage when sound occurs near the microphone element. But the transistor is a current controlled device so the microphone voltage is connected across a resistance to generate a small current. This current, is in turn, connected to the transistor base lead. However, it still will do nothing until another current is generated in the collector to emitter path. To make that current, a power supply voltage (i.e. a battery) is then connected across another resistor and the collector/emitter circuit. The small current coming into the base then can control the current in the collector/emitter circuit causing it to increase and decrease relative to the incoming signal from the microphone. This varying current then generates a varying voltage across the resistor that was used to generate the collector/emitter current. Please remember that there are two separate currents at play here. The current flowing into the transistor base and the current flowing in the collector/emitter circuit. This is an amplifier in it’s simplest form, a base resistor, a collector resistor, the transistor, a battery and a input capable of generating base current.
To confuse things, if you have started to understand schematics, you might find an audio amplifier that has many resistors, and transistors. These additional elements allow the signal to be amplified several hundred times while making the circuit stable under all conditions.
The ARRL Radio Amateurs Handbook is an excellent place to begin your quest of understanding. Every public library has one on the shelf and they are available from the ARRL directly at For $49.95 it could make a nice Christmas gift and it comes with a searchable CD.

Search Youtube for “how transistor works”. You’ll find some helpful stuff, including the basics: