Mosfet voltage follower help needed

Discussion in 'The Projects Forum' started by Piggins, Oct 27, 2014.

  1. Piggins

    Thread Starter New Member

    May 5, 2014
    26
    0
    Hi,
    I am trying to learn to make a voltage follower using a mosfet. It is supposed to be following the voltage signal given by an op amp that gets its signal from PC sound.

    [​IMG]

    Oscilloscope shows 2V voltage swing (is this -1 to 1V swing?) from PC and 0-5V swing over the 1M resistor before the mosfet.
    The mosfets I tried to use for this are 50N06 and IRF740. The problem I have is that the mosfet is not opening. My oscilloscope doesnt show any voltage over the load. I have tried using a 4W DC motor and various resistors as a load.

    Could someone tell me what in the plan is wrong?

    images taken from
    http://blog.oscarliang.net/wp-content/uploads/2013/10/electronics_mosfet_schematic.png
    http://www.rason.org/Projects/opamps/design1.gif
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi P,
    Check the datasheet for Vgs for those MOSFET's [ Voltage Gate to Source ]

    E
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    A MOSFET voltage (source) follower is a common-drain configuration, not a common-source as you show.
    Note that the follower has a voltage drop between the gate and source equal to the MOSFET threshold (Vgs) so the gate needs to be biased at a DC level above that to allow for the audio signal output going plus and minus. For that you can add a bias resistor from the drain to the gate of the appropriate value to get the voltage needed.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,175
    397
    Looks something like a under drived class B ckt. Might try adding a pot, 10k, wiper to bottom of 1M R, ends to + & -.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,804
    1,105
    Try a resistor in the 200k-1meg range for the bias suggested by Crutschow in post #3. If you plan on replacing the 1k load with something more power-hungry, be aware that the FET will get hot and probably need a heatsink.
     
  6. Piggins

    Thread Starter New Member

    May 5, 2014
    26
    0
    Ahh, I had the load resistor in the wrong side of the mosfet, I changed that. Also, I added a 100k resistor between the gate and the drain and the mosfet started to follow the pulses from the op amp. So the drain-gate bias resistor forms a voltage divider with the 1M resistor between the gate and the ground? And that biasing is then added to the signal coming from the op amp?
    Thanks all for help.

    Edit: Also I dont get why the original circuit didnt have any voltage over that load. It had 0-5V between the gate and the source so that was well over 2V threshold voltage at times.
     
    Last edited: Oct 27, 2014
  7. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    It has 0V DC on the gate due to the 1MEG resistor to ground, and you originally stated it was a 2V swing (+/- 1V at the gate), which is not enough to turn it ON (note that "open" or "closed" is not a term used to describe whether a transistor is OFF or ON).
     
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