I'm trying to create an amplifier circuit using a n channel MOSFET and I can't seem to get the results I'm looking for. I'm trying to amplify an AC signal up to the +/- Vcc but I keep getting strange outputs. Here's my latest trial:

MOSFET circuit

I've gotten similar results no matter what configuration I try. It's always a signal that is greatly positive or greatly negative when it should not be exceeding the Vcc

  • \$\begingroup\$ Have you tried simulating a current mirror instead of a current source? \$\endgroup\$ Jul 27 '14 at 22:37
  • \$\begingroup\$ Based on what I just read about current mirrors, it doesn't seem like that is what i'm looking for. I'm basically trying to create an op amp using mosfets. \$\endgroup\$ Jul 27 '14 at 23:04
  • \$\begingroup\$ A current source isn't real, which is why you aren't getting real results. \$\endgroup\$ Jul 27 '14 at 23:07
  • \$\begingroup\$ What do you suggest then? Because it doesn't seem like a current mirror will do what I want either. \$\endgroup\$ Jul 27 '14 at 23:20
  • \$\begingroup\$ To clarify, I'm just trying to take a low AC voltage and amplify it. Somewhere along the lines of +/- 5V to +/- 40V. I'm doing this without an op amp because the best one I could find wouldn't go past +/- 35V for its power supply \$\endgroup\$ Jul 27 '14 at 23:36

This isn't a strange result. The ideal constant current source in the source circuit can have any voltage across and produce the specified current. In fact, V2 is essentially irrelevant.

It appears that you've specified that the current source produces a 0.5A constant current. Since the current source is ideal, the voltage across it will be whatever it needs to be such that there is 0.5A through the attached circuit. It 'doesn't care' what your supply voltages are - not one bit.

But this isn't remotely physical so, the first thing you should is get rid of the ideal current source. Also, if you want voltage gain, you shouldn't be using a common-drain configuration, as you have drawn, since the voltage gain is less than 1.

Honestly, I think it's likely that you're in over your head on this. For voltage gain, you probably want a common-source configuration and, for sure, forget that ideal current source.

  • \$\begingroup\$ I figured the current source was the problem. But I was originally trying a common source configuration and was running into similar problems. I am definitely over my head on this. Analog circuits isn't my field of study and transistors are beyond me \$\endgroup\$ Jul 28 '14 at 8:54
  • \$\begingroup\$ mosfet2 Here's my common source set up. The problem is the wave has a very small peak to peak. I want it to be +/- 40V \$\endgroup\$ Jul 28 '14 at 9:08
  • \$\begingroup\$ @user3427168, you will almost certainly need a multi-transistor design with negative feedback to produce a linear, large-signal voltage amplifier capable of +/-40V output. \$\endgroup\$ Jul 28 '14 at 15:32
  • \$\begingroup\$ Is there a particular name for such a configuration? Can't seem to find anything helpful through google \$\endgroup\$ Jul 28 '14 at 16:43

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