Current Mirror shows Current Spike!

Discussion in 'The Projects Forum' started by sailmike, Feb 10, 2014.

  1. sailmike

    Thread Starter Member

    Nov 11, 2013
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    3
    Using PSpice I simulated the current mirror shown in the photo without the LED's. The LED's in the photo are there just to show what I intend to use this circuit for, a current steady current source. The output graph (see photo) shows a current spike up to about 7 A in the first few nanoseconds then it settles at the intended 100 mA output. How do I get rid of that current spike?

    Thanks,
    Mike
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Use an RC filter on the power supply, e.g. 1 Ohm, 1uF. The spike is due to inherent circuit capacitance.
     
  3. sailmike

    Thread Starter Member

    Nov 11, 2013
    143
    3
    Thanks for your quick reply. I'll add that to my simulation and run it again.

    I'd like to understand the cause of the current spike if you don't mind explaining.

    Thanks,
    Mike
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The gates of the FETS have capacitance. The LEDs have capacitance. This all takes a finite (albeit small) time to charge when the power is applied to the circuit; and if there is no resistance to limit the charging current it can be surprisingly high.

    Edit: Btw, using the current mirror to control LED current is not ideal. The current is very dependent on the particular gate turn-on thresholds of the individual FETs (so the mirrored current is accurate only if the FETs are identical internally and operated under identical conditions), and the thresholds will be temperature dependent.
     
    Last edited: Feb 10, 2014
  5. crutschow

    Expert

    Mar 14, 2008
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    Using NPN transistors, such as a 2N2222, instead of MOSFETs will likely give a more accurate constant current since the base-emitter junction voltages tend be fairly similar between two different bipolar transistors of the same component number, particularly at low currents.

    Edit: The short current spike you see is unlikely to have any adverse affect on the LEDs since it is such a short duration and carries very little energy. Real circuit impedances will also likely reduce its value from what the simulation shows.
     
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  6. #12

    Expert

    Nov 30, 2010
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    I figure 14 nano-watt-seconds. Not enough to burn out a mosquito.

    Capacitance from drain to gate seems to be a likely cause. Maybe a few dozen picofarads, gate to ground would quench that behavior. You're only trying to slow it down by about 2 nanoseconds.
     
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