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Why is it that when I see single MOSFET amplifiers or even complementary ones (one NMOS and PMOS), they would use what in single BJT amplifiers would be called "Common Emitter Amplifier". The output is on the Drain side of the MOSFET. This always outputs an inverted signal.

I could see the advantage in BJT as that would provide the highest voltage sourcing, though it will be inverted. This inversion, I've always supposed, does not matter in the application it's usually used for, say working with 1.) frequency or 2.) averaged or RMS amplitudes, etc.

But doing the same with MOSFET (output from Drain or Common Emitter), the signal is also inverted. It would be okay while working with (1) and (2) above, but not for anything else. But if we used the Source side for NMOS and PMOS's, we get a non-inverted output and SHOULD BE the same amount of amplification.

I don't get it... Is it because the tutorials/articles are just following the pattern set up by single BJT Amplifier examples?


EDIT: I replaced "Common Emitter" with "Common Base", as I thought I had it wrong. Then it was pointed out that it really was "Common Emitter", so it was editted back.


Just to clarify everything I am providing this response so that we are all on the same page. I dunno, I must be the one who's not...

This is what I know to be a NPN Common Emitter:

schematic

simulate this circuit – Schematic created using CircuitLab

Sources from V_DD, hence higher voltage amplification, but inverted.

This is it's direct equivalent in NMOS (I just checked and this seems to be a Common Source, though my example lacks a resistor on the source side)

schematic

simulate this circuit

This is what meant:

schematic

simulate this circuit

or for complementary (and my original intended question):

schematic

simulate this circuit

(The Opamp above, for simplicity, does not do any feedback for amplification)

With the last figure above was my original intended question, but I just had to ask what I'm asking here before I go with this.

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closed as unclear what you're asking by Dave Tweed Aug 5 '18 at 19:00

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    \$\begingroup\$ ..."SHOULD BE the same amount of amplification". Why do you expect the same amplification factor? Do you know the consequences of negative feedback caused by an emitter resp. source resistor? \$\endgroup\$ – LvW Jul 5 '15 at 8:41
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For the BJT, the "common collector" configuration is called an emitter follower. This has current gain, but no voltage gain. The emitter sits at 0.7V (PN Junction) below the base voltage. The voltage gain of this circuit is just below 1, and the current gain is equal to the beta of the transistor.

The same thing happens with "common drain" - also called "source follower". The source of the FET sits at a voltage which is the turn-on voltage below the gate voltage. Because the turn-on voltage of the FET changes with current level, this circuit has voltage gain slightly less than 1, and which reduces as the current level increases.

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  • \$\begingroup\$ So why "common source" not usually depicted in tutorials and articles? I know it should work, it just gave me the impression I must be wrong. \$\endgroup\$ – Dehbop Jul 5 '15 at 9:11
  • \$\begingroup\$ Of course, it "works" - but with a gain of (slightly below) unity, caused by heavy negative feedback. \$\endgroup\$ – LvW Jul 5 '15 at 9:18
  • \$\begingroup\$ Common Source is equivalent to Common Emitter BJT circuit. From my experience, this is the most basic voltage amplifier using FETs. Wikipedia has a good description of the different topologies. What do they show in your tutorials? \$\endgroup\$ – harry courtice Jul 5 '15 at 9:20
  • \$\begingroup\$ @LvW Negative feedback, from where? \$\endgroup\$ – Golaž Jul 5 '15 at 9:41
  • \$\begingroup\$ Uhh.. you know what? I only bothered to read one or two tutorials on the subject then got by, by Googling images... I didn't even try to find out if was in Wikipedia in a series of small articles that I can read bit by bit... Just found out it is. \$\endgroup\$ – Dehbop Jul 5 '15 at 9:43
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If you connect as a source follower, it will work, and it is often used in H-bridges, because NMOS are preferable over PMOS. The disadvantage is that the Vbe of emitter follower is 0.7-1v and Vgs can be several volts.

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