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Questions about MOSFETs

Discussion in 'General Electronics Discussion' started by quantumtangles, Apr 20, 2013.

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  1. quantumtangles

    quantumtangles

    152
    3
    Dec 19, 2012
    I would like to know more about MOSFETs, maybe get hold of some for my parts bin and try and make useful devices on the breadboard (an audio amplifier may be a trifle optimistic at this point).

    Why bother with MOSFETs if you can use operational amplifiers instead? Is it a power handling capability thing?

    If, for example, I got hold of some N-Channel MOSFETs (which I thought could only be used as on/off switches), would I be able to observe their behaviour on an oscilloscope. Should I be able to see when the gate voltage trigger point is reached, and see current flowing from drain to source...what could I hope to see on an oscilloscope and how should I try to set things up? (how should I connect my bench power supply and scope probes and what settings should I use to see what is going on).

    How can an on/off switch manage to become a key component in an audio amplifier?

    What I am really hoping to see is resistive behaviour below the gate voltage threshold, followed by current source behaviour above that threshold (in saturation) as between drain and source, but can I really get to see this happening in real time on an oscilloscope if I set things up properly...and how in the world can I make an audio amplifier from what amounts to an on/off switch?

    If anyone has any tips or simple cool experiments with MOSFETs I would love to hear from you.

    These little devices have got my attention because I find their behaviour counterintuitive, and apparently, in saturation, they can be audio amplifiers.

    Assume savage ignorance and shameless curiosity on my part. Any observations/simple experiments/suggested components...greatly appreciated :D
     
    Last edited: Apr 20, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    As a partial answer to some of your questions...

    Assume a transistor (whether bipolar or mosfet) is like a valve.

    There are 2 ways of using a valve. You can either turn it on and off, or you can use it somewhere between on and off to control the amount of flow.

    When used as a switch, the transistor is turned on or off, but never left in between these values.

    When used in an amplifier, the mosfet is used in the in between region, and rarely (or never) totally turned on or off.

    A discrete mosfet may be capable of switching many tens of amps, an op-amp, maybe only a thousandth of that.
     
  3. quantumtangles

    quantumtangles

    152
    3
    Dec 19, 2012
    Thats really clear. Very helpful thanks Steve.
     
  4. duke37

    duke37

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    Jan 9, 2011
    Mosfets are often used as switches since they can have a very low resistance when turned on fully. If they are not fully turned on, they can be used to amplify signals like other transistors. They need negligible current to turn them on or off, the input is more like a capacitor.

    There are small fets and BIG fets that can take many amps. The fet will pass very little current until the input threshold voltage is reached, then as the voltage is raised, they will pass more current. The threshold voltage for normat fets is about 3V but there are logic level devices with lower thresholds.

    The fets can be used as switches in class D audio amplifiers.

    Why not get a SPICE program, I use 5spice and LTspice, and see how different circuits behave?
     
  5. quantumtangles

    quantumtangles

    152
    3
    Dec 19, 2012
    Thanks Duke. I did not think of using a circuit simulator...got drawn into the idea of using an oscilloscope.

    LTSpice is great but I prefer CircuitWizard Pro. You can export to Gerber (the industry standard if you want someone to make a PCB for you), but with CircuitWizard Pro you can also convert your schematic into a PCB layout instantly. Handy if you are prone to making mistakes when converting schematics into real life circuits, as I am. CircuitWizard Pro is also significantly easier to use than LTSpice. It is almost exactly like using Microsoft Word. Very intuitive drag and drop component menu. (I am reading the sticky on converting schematics to real life devices with great interest).

    Excellent idea Duke. I can have a look at the voltage and current behaviour and logic. It did not even occur to me to use the simulator. Doh.

    Getting back on point re MOSFETs, I think I am beginning to get an understanding of them now...thanks again Duke :D
     
    Last edited: Apr 20, 2013
  6. BobK

    BobK

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    Jan 5, 2010
    If a MOSFET is used in an audio amp output stage, it is probably a class D amplier. A class D amplifier does amplify audio by swithing only fully on or fully off. It does this by using PWM. I.e. the percetange of time the switch is on corresponds to the voltage of the signal being amplified. Filtering takes out the actual on / off transitions leaving an analog voltage as the output.

    Bob
     
  7. quantumtangles

    quantumtangles

    152
    3
    Dec 19, 2012
    Thanks Bob. I just got hold of an author-less book entitled MOS INTEGRATED CIRCUITS AND THEIR APPLICATIONS (Mullard Limited, London, 2nd Ed., 1973) that should also help me get up to speed (at least to 1970s levels of knowledge) :D
     
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