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FET Mixer

Discussion in 'Electronic Design' started by redhat, Jun 21, 2005.

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  1. redhat

    redhat Guest

  2. Jim Thompson

    Jim Thompson Guest

    Calculate the gate voltage resulting from the voltages at IP1 and IP2
    and then you'll understand. (There is no current into the gate.)

    ...Jim Thompson
  3. Joel Kolstad

    Joel Kolstad Guest

    ....and then pickup a copy of the Art of Electronics if you want to calculate
    what the gain is going to be. (Hint: It's approximately R4/R3)
  4. Joerg

    Joerg Guest

    Hello Joel,
    ....and don't forget that the FET needs a DC bias. Capacitively coupling
    both audio sources into the resistors would not be a good thing here...

    Regards, Joerg
  5. Hello "redhat",

    There is a big misunderstanding about the word "mixer".
    When people coming from audio talk about a mixer, then
    they only want an "adder".
    This means the circuit adds the signals of two sources.
    It can be done in the most simple way with just two
    equal resistors as it's done in this circuit.

    When we can neglect the source resistance of the audio
    sources then the result is:

    The FET in this schematics just acts as a buffer to get a low
    output resistance. It could be replaced by any kind
    of transistor or an opamp if some small circuit modifications
    will be done.

    Best regards,
  6. Tim Wescott

    Tim Wescott Guest

    I'd be quite inclined to capacitively couple into those resistors, then
    add a third one to a bias source of _my_ choosing, rather than depending
    on some unknown audio input to have something suitable.
  7. Joerg

    Joerg Guest

    Hello Tim,
    That is how I would do it as well. Unless the inputs are XLR transformer
    coupled and the xfmrs are on the circuit.

    Regards, Joerg
  8. Pooh Bear

    Pooh Bear Guest

    Quite badly I suspect ! Ppl who are clueless realy shouldn't post their
    ignorance on the net.

  9. Pooh Bear

    Pooh Bear Guest

    Please *do* explain how R4 and R3 affect the gain !

    Note it's an n-channel fet. R4 is simply a waste of space.

    It's basically a source follower Gain =~ 1 with an input attenuation of 6dB (
    presume R1 = R2 ) caused by the passive summing.

    The output will need to be ac coupled too. I won't even start on the
    indeterminate input biasing.

  10. Jim Thompson

    Jim Thompson Guest

    Graham, That's rather rude. Didn't get any last night ?:)

    ...Jim Thompson
  11. doesnt stop me......

  12. Tam/WB2TT

    Tam/WB2TT Guest

    He is right, though. Bad mixer, but maybe OK combiner if he puts some bias
    on the FET. I am somewhat concerned about one source back driving the other
    one. Op amp would be better, unless he is going to build 8 million of these.

  13. Jim Thompson

    Jim Thompson Guest

    I wasn't commenting on the mixer, just the "Ppl who are clueless realy
    (sic) shouldn't post their ignorance on the net".

    I have no problem with students who are asking a non-homework
    question. There are no stupid questions, just stupid answers.

    I DO have a lot of problems with rude, pushy, IGNORANT punks, or is it
    pukes, like that TokaMundo.

    ...Jim Thompson
  14. Pooh Bear

    Pooh Bear Guest

    Pfftttt... ! Silly boy - lol . ;-)

    Keep them coming.

  15. Pooh Bear

    Pooh Bear Guest

    It was mean to sound humourous actually, on account of how the question was
    phrased. Sorry if it missed the spot. We have this rather 'dry' sense of
    humour in the UK that you guys sometimes find trouble tuning in to it seems.
    Different culture I suppose.

    It *is* a truly awful example of an incomplete and bad circuit though !


    p.s. and no I didn't. :-( Peggy was meant to ring back but got 'diverted'
    it seems.
  16. Tam/WB2TT

    Tam/WB2TT Guest

    You haven't run across Doctor Slick, have you ! .

  17. Tam/WB2TT

    Tam/WB2TT Guest

    I gather you are trying to add/combine the two signals, not mix the
    frequencies. Your schematic implies the inputs come from current sources; in
    that case you want to have a buffer with a very low input impedance, like
    the (-) input of an opamp, not a high input FET.

  18. Jim Thompson

    Jim Thompson Guest

    I probably already have him permanently kill-filed, just like

    ...Jim Thompson
  19. hehe, just having a bad day. Been trying to understand Avid Xpress
    colour correction. I hadnt realised how soft some of my shots are, but
    what a klunky counterintuitive program. And I thought some PCB design
    programs were bad.......

    At least "her next door" cooked me a nice curry

  20. Pooh Bear

    Pooh Bear Guest

    Mea culpa. ( re: the sic )

    I've always preferred the idea that the best spell checker is yourself. You may
    understand this better if you've seen what Microsoft Word ( hissss - spit - spit
    our the damn talking paperclip ) is capable of. I recall one instance where Word
    had a hissy fit over something one of my colleagues had typed and 3 of us looked
    at it for around 5 mins and couldn't for the life of us work out why it had
    highlighted it - even including considering context !

    I do however type ( should the be typo ? ) quite fast and inaccurately sometimes
    and simply review the result and correct. I find this to be an effective way of
    generating text over the one-fingered approach.

    I regret that one slipped past the sub-ed's eye. ;-)

    As for ppl posting shit on the net. I stand by my comment. Some ppl may read the
    shit and actually believe it !

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