Transistor Amplifiers

Thread Starter


Joined Jun 9, 2007
How exactly do transistors amplify voltage? I read on a tutorial that

The difference amplifier has two inputs and one output.
It amplifies the difference between the voltages at the two inputs.
If the voltage on one input is 10 mV and 15 mV on the other then the difference is 5 mV.
If the amplifier amplifies by ten times then the output voltage will 5 mV times 10 which equals 50 mV.
In the case of the transistor which is the output? The collector or the emitter? Lets say I have 10V going into the emitter and 5V into the base electrode which gives a difference of 5V at the output. How would the transistor amplify that 5V at the output?


Joined Jan 28, 2005
It would assist us in helping you with your inquiry if you could post an example of the amplifier circuit you are studying. Otherwise each member is left to their own interpretation of your question.

Some posters with similar questions have posted a link to an example from the Internet of the circuit they are studying along with their question. This seems to work well.


Thread Starter


Joined Jun 9, 2007
It was the difference amplifier I was looking at.

I thought at the time that was a transistor in the diagram but I don't think it is.

So a transistor initially amplifies current? With a regular NPN transistor how could I amplify current with it? Would it work if I was to split an input wire of 50V (100Amps). I hook the 50V up to the emitter terminal then I hook the other part up to the base terminal with a resistor so its only inputting 20V(40Amps). How would I amplify the 40Amps at the output?


Joined Apr 20, 2004
Our Ebook has a section that explains transistor operation - Take a look and see if this answers some of your questions.

The amplifier aspect comes from the ability of a small current fed into the base being able to predictibly control a larger current through the collector. In the common emitter configuration, the emitter is tied to circuit ground. The load, through which the current flows, is between the voltage source and the transistor's collector. The current goes through the load and the transistor, through the collector to ground by way of the emitter.


Joined May 19, 2004
the differential amlifier is actually two transistors tied in a circuit where the emitters are tied common and the two outputs are taken off of the collectors - one is in phase with the input the other is out of phase. the input is placed on one base, the other, or both. the diff amp is typcally utilized as a voltage amplifier.

you need to investigate the three transistor amplifier application circuits - common emitter, common collector, and common base which refers to which lead is tied common with the input signal. each have particular operational characteristics as far as having voltage or current gain possibilities, and input/output impedances, and more.
horsebox, the differential amplifier is a whole circuit, not just a transistor. It consists of transistors and passive components.

And there is something else you might have got wrong. A transistor obeys the conservation of energy. That is, it doesn't output more than its input. Instead it allows you to control the flow of a larger current with a smaller current.

For example, the small current is a small audio signal. The transistor works like a tap, making the large current (which is coming from a power supply) adopt the same waveform as the input signal: Output(Time) = Input(Time) x Gain

But the transistor is not that easy to describe accurately in a few words. Try looking through resources that have been suggested to you.

Thread Starter


Joined Jun 9, 2007
The whole transistor ideas still confusing me. Lets take this circuit as an example

How does the current flow in this circuit? My guess is it flows from where it says current source to point 1, then flows into the base terminal which "throttles" the collector terminal which then flows to point 2 past the ammeter to point 3 and all the way back to the emitter terminal.

Is that correct? Why are there 2 power sources on each side of point 3?
Uh... the ammeter symbol is wrong. It's not a battery or voltage source.

There are two loops in the circuit. You should imagine that current from \(I_1\) flows through the left one only, while current from \(V_1\) flows through the right one only.