Inverted output at collector, why?

Hi guys,
With very little experience and training on transistors, can someone explain why the output of collector is inverted in a common emitter amplifier? I suppose the same reason goes for fets too?

Thanks in advance!

Learner said:

Hi guys,
With very little experience and training on transistors, can someone explain why the output of collector is inverted in a common emitter amplifier? I suppose the same reason goes for fets too?

Click to expand...

A transistor is current driven, increasing the base current (by the voltage going higher) causes the collector current to increase (in line with the gain of the device). Because the collector current increases, the voltage dropped across it's collector load increases, so it's collector voltage drops. This gives an inverted signal.

A FET is voltage driven, so doesn't take gate current - but the effect is pretty well the same. The gate voltage going positive will increase the drain current, causing a larger voltage drop across the drain load - just as a transistor does.

Incidently, a valve (tube) gives the same effect, for the same reason.

Say an NPN is connected with collector directly to + and emitter directly to ground (with current limiting resistors of course). When the transistor turns on it basically shorts + to ground, which brings the voltage/current output down.

When the transistor turns off, that path to ground is gone, so the voltage/current rises because it goes through the output instead of through the transistor to ground.

Thanks a lot guys!!!!!

Learner said:

Hi guys,
With very little experience and training on transistors, can someone explain why the output of collector is inverted in a common emitter amplifier? I suppose the same reason goes for fets too?

Thanks in advance!

Click to expand...

Logically speaking, a BJT transistor comes in 2 flavours, a PNP and an NPN. the middle letter (as shown in every manual) represents the base.
The outer letters represents both the collector and emitter. Because the base is different from collector by its polarity, that can give you big hints that the output is in the opposite polarity (inverted).

mstechca said:

Learner said:

Hi guys,
With very little experience and training on transistors, can someone explain why the output of collector is inverted in a common emitter amplifier? I suppose the same reason goes for fets too?

Thanks in advance!

Click to expand...

Logically speaking, a BJT transistor comes in 2 flavours, a PNP and an NPN. the middle letter (as shown in every manual) represents the base.
The outer letters represents both the collector and emitter. Because the base is different from collector by its polarity, that can give you big hints that the output is in the opposite polarity (inverted).

Click to expand...

That makes no sense. What about an emitter follower (common collector)? There is no phase inversion, but the base is still doped differently from the emitter or the collector.

His question is related to a common emitter amplifier. I wanted to make it simple for that. I didnt think about any other amplifier configuration.

mstechca said:

His question is related to a common emitter amplifier. I wanted to make it simple for that. I didnt think about any other amplifier configuration.

Click to expand...

Well, my point is that the terms NPN and PNP don't lead to the conclusion that the circuit will invert. It is useful to remember that common emitter is the only bipolar configuration that inverts.