Odd behavior of a NPN Inverting Amp (2N2222A)

Discussion in 'The Projects Forum' started by botagar, May 14, 2014.

  1. botagar

    Thread Starter New Member

    Jun 30, 2013
    Hi there.

    I'm trying to build a simple transistor voltage amplifier to boost a 0-5V signal to 0-12V to drive a PNP Mosfet acting as a highside switch.
    (attached is the spice circuit)

    In the sim, the circuit works perfectly fine but when i breadboard it up, there is a 4V drop across the 22kR (R2) resistor when the input at the base of the 2N2222A is 0V. This leads to my output signal being 8V instead of 12V, meaning the MOSFET is not switched off.

    Could someone point out to me whats going on here and how to correct it?

  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    I'd suspect one of the transistors is heading south. Try disconnecting the fet gate and see what happens.
  3. DickCappels


    Aug 21, 2008
    The circuit, as drawn should work, but the results you are getting is clearly not what is expected of this circuit.

    The Alpha & Omega Semiconductor AO6407 datasheet shows the maximum gate-to-source voltage to be 8 volts. You might be working with a damaged MOSFET -to diagnose, reduce the power supply to 8 volts and try a fresh MOSFET.

    I would like to add a couple of notes.

    If the problem is in the NPN transistor, it might be that you do not have the transistor connected in the circuit correctly. Please check the transistor basing diagram and make sure you've got wired it right.

    As drawn, you are shoving over 20 milliamps into the base of the transistor, yet you only ask the collector to sink less than 1 milliamp. If you stay with a 22K load, then you should be able to drop the base current to a more reasonable few hundred microamps and still have plenty of margin.

    From the datasheet and the values on your schematic, the gate drive turn-off time constant is 24 microseconds. If your part has a low threshold, there could be a delay of several tens of microseconds until the MOSFET begins to slowly turn itself off.

    If you want the circuit to switch faster (what did the simulator tell you?) then the 22k resistor would need to be lowered and the base resistor adjusted accordingly.

    Hope this helps get you to where you want to go.
  4. botagar

    Thread Starter New Member

    Jun 30, 2013
    Thanks for the replies. I should have clarified that i was testing the amplifier without the MOSFET attached.
    I have tried several 2N2222A (I bought 50 about a year ago) and so far, 4 of them display the same behavior. Either I've gotten really unlucky with a batch of these transistors or I might have gotten the pinout wrong.

    I've been using this datasheet for reference: http://www.fairchildsemi.com/ds/PN/PN2222A.pdf
    Though I'm not sure if Fairchild semi was the manufacturer of the ones I bought. Can pinouts be different between manufactures even though its the same TO-92 package?


    (Also thanks for the note about the base current)
    Last edited: May 14, 2014
  5. botagar

    Thread Starter New Member

    Jun 30, 2013
    I've discovered that the 2N2222A transistors I have are infact reversed from the fairchild ones.
    This is the more appropriate datatsheet: http://www.farnell.com/datasheets/1693304.pdf

    It works now when disconnected from the MOSFET.
  6. DickCappels


    Aug 21, 2008
    Good job!

    Maybe what you have are not "exactly" 2N2222's. Happened to me once, buying from a small dealer.
  7. MrChips


    Oct 2, 2009
    This should be a sticky note somewhere.
    Always check the pinout for 2N2222.
  8. crutschow


    Mar 14, 2008
    The 2N2222, which I think is usually in a TO-18 metal can, has only one pinout. But the plastic package versions, such as the PN2222, apparently can have different pinouts from different manufacturers for the same package, as botagar discovered (unless one of the data sheets is in error).
  9. RichardO

    Well-Known Member

    May 4, 2013
    It does happen!
    Just recently, I found a pinout for an SOT-23 transistor with the pin numbers for the TO-92 part. :-( Years ago I did a PCB with this wrong pinout and failed to realize it was not my mistake at the time.