I thought the:
- clean current would get into the transistor through the collector
- the signal inserted through the base
- the current from the collector would combine itself with the signal
- that would emit from the emitter an amplified signal
but by looking at most circuits that amplify with one transistor they all place the output before the collector. Why?
![](https://backyardbrains.com/experiments/img/CommonEmitter.jpg)
Because that's not how transistors, or FETs, or vacuum tubes (valves) work!
Imagine, if you will, the transistor in your Schematic being replaced by a variable resistor with a knob you can turn to increase or decrease the resistance.
This device, which we can call VR1, along with Rc, forms a voltage divider between your "power source" & ground.
The "output signal" is the voltage between the junction of Rc & VR1, with respect to ground.
If you move the knob controlling VR1 fast enough, you get a varying voltage at the output which looks like a very low frequency audio signal .
,'
Imagine if you had superpowers, you could listen to some music & twiddle the knob in time with it.
If you listened to the signal with an amplifier & speaker, you would hear music!
Unfortunately, nobody has super powers, but early experimenters looked for ways to vary the resistance of part of a voltage divider to do something along those lines.
The first success was the carbon microphone.
The sound of someone speaking into the microphone varied the resistance of the resistive element, & hence, varied the voltage available at the output of the voltage divider made up of that element & its load resistor--- in many cases, the earphone of a telephone somewhere remote from the mic.
This gave them a very sensitive microphone, but it still didn't solve the problem of amplifying a signal which was already an electrical one.
One idea was to use a very sensitive earphone, mechanically coupled to a carbon mic.
This worked, but not too well.
Ultimately, the first workable vacuum tubes were made, revolutionising the field of Electronics.
The "tube" was still a "variable resistor", but one where the input signal controlled the amount of current flow through it, & hence, its effective resistance value in the "voltage divider".
Most amplifying devices, although the mechanism varies significantly, still are "part of a voltage divider".
Some circuit configurations are a bit hard to visualise as a simple "voltage divider", but it works well with the "Common Emitter" & " Common Collector"(Emitter follower) circuits, & their equivalents with other devices.