transistor circuit

Discussion in 'General Electronics Chat' started by m121212, Jul 18, 2012.

  1. m121212

    Thread Starter Active Member

    Jul 24, 2011
    71
    0
    I'd like to come up with a circuit that has 0 or +5 volts as an input, and -5 or 0 volts as the output.

    I have access to 0, +5, and -5 rails. Currently I implemented this using a SPDT reed relay, but I have some switching noise issues. Instead of putting in a debouncer, I thought I could come up with a solution using a transistor or two.

    My first thought was to use an NMOS inverter, with output at the drain. The drain resistor would connect to ground, and the source to -5. This doesn't work because the gate can't be at a higher potential than the drain.

    If I can avoid it, I'd like to not use op-amps (eg. adder circuit).
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    How much current do you need to switch? Resistive load?
     
  3. m121212

    Thread Starter Active Member

    Jul 24, 2011
    71
    0
    It's only about 20 mA when the output is 0V, and almost no current when the output is -5V. The load is the gate resistor and subsequent gate of a npn BJT.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Do you mean base?
     
  5. m121212

    Thread Starter Active Member

    Jul 24, 2011
    71
    0
    oops, I meant the base. Thanks for the correction.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    It may be easier to recommend something if you show us the circuit you want to drive with that inverter.

    Why don't you want to use an Opamp?
     
  7. m121212

    Thread Starter Active Member

    Jul 24, 2011
    71
    0
    I wanted to simplify the problem to the core issue, but perhaps it would be more helpful to show what I'm trying to do.

    This is a high-side drive for a MOSFET based on a description shown here:
    https://engineering.purdue.edu/Courses/ECE433/exp5_5th~6thweek_.pdf

    The drive MOSFET is fully on when Vgs is plus 20, and fully off when Vgs is -5. This is accomplished by having FET Q2 charge up a capacitor to +20V, which is then applied to Vgs when Q2 and the BJT are turned off. When Q2 and the BJT are on, the cap charges, and VGS is pulled down to -5.

    The correct drive for this circuit has Vin transitioning 0->5V or 5V->0 while the BJT base is driven from -5V->0 or 0->5V respectively. The BJT and Q2 are either off simultaneously, or on simultaneously.

    My original solution had Q2 driven by the buffered output of a 555 circuit. That also actuated a SPDT relay to apply a -5 or 0 voltage to the BJT base. Because that was noisy (switch bounce), I thought to replace the relay with more transistors. That would also help with the cost, since the switch is a bit expensive.

    The opamp would work too. The one drawn just adds -5V to the input, so 0 in yields -5 out, and +5 in yields 0 out. I was reluctant to uses this at first because I was concerned about footprint, and I thought it would be more elegant and simple with bulk transistors. I gave the opamp a chance anyhow and wired up an OPA244 (not a 171 as drawn). The opamp died shortly after initially working, even though R2 was 470 ohms. I'm not sure what happened. Perhaps it was unhappy with the noise on the power rails?

    I guess my questions are the following:
    1) Is there a basic transistor solution that would replace the relay or the opamp?
    2) If not, do you have any ideas on where I may have gone wrong with the opamp?
     
  8. m121212

    Thread Starter Active Member

    Jul 24, 2011
    71
    0
    Perhaps I should buffer the opamp output. The base resistor should limit the current to 10 mA though, which the OPA244 should be able to drive.
     
Loading...