Replace BJTs with MOSFETs

Discussion in 'General Electronics Chat' started by chboizz1, Apr 28, 2016.

  1. chboizz1

    Thread Starter New Member

    Aug 10, 2015
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    Is there anyway to replace this Darlington BJT amplifier pair in this signal detector with MOSFETs instead? Perhaps using MOSFETs to amplify the current instead.

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  2. crutschow

    Expert

    Mar 14, 2008
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    3,235
    Any such circuit will be rather insensitive, whether you are using BJTs or MOSFETs.
    What is the purpose of the circuit?
    What signal are you trying to detect?
     
  3. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Seeing as the antenna and diode are shorted to ground, this circuit cannot work. A coil is needed between the junction of Ant1 and D1 replacing the wire to ground.

    And even then, you are going to need quite an RF field to get to the 1.2V needed to activate the darlington pair.

    Bob
     
  4. chboizz1

    Thread Starter New Member

    Aug 10, 2015
    11
    0
    Apologies, my schematic was incorrect. The antenna in the schematic was grounded, I was using a function generator in CircuitLab to mimic the behavior of an antenna. The purpose of this circuit is to simply light up the LED if a cell phone is used near it, (typically 500MHz?)

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  5. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    You'll need a circuit will a lot more sensitivity than the circuit you show to detect a nearby cell phone.
    It will need amplification of the RF signal before the detector.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    A typical power MOSFET needs at least 6V on the gate to conduct fully, generally TO92 style parts make do with about 2V.

    The easy route is to order logic level MOSFETS.

    A good source of parts is old PC motherboards, some have the next size down from the SMD version of TO220. Those MOSFETS are typically only 30V - some even as low as 20V. The gate threshold voltage is also correspondingly low. You could find parts with VGSthr as low as 1.6V - which is only slightly more than the Darlington.

    With the Darlington; you have to overcome 2xVbe before any base current flows - A MOSFET with 1.6V VGSthr will probably light your LED at somewhat less gate voltage.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    You can improve the sensitivity significantly by changing Q2 to a PNP, pulling the LED up to 9 V rather than down to GND.

    ak
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,127
    3,048
    A comparator would make a lot more sense than either a darlington or a MOSFET. Far more sensitive and much higher impedance.
     
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