single supply op amp problem

Discussion in 'The Projects Forum' started by volvo340, Feb 1, 2011.

  1. volvo340

    Thread Starter New Member

    Feb 1, 2011
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    0
    Hello, first time on the forum, any electronics forum in fact so thanks to any one who can make any suggestions.

    I have the op amp circuit below. It's a simple non-inverting LM358 circuit.
    Vin is supplied by the output of a bluetooth receiver which outputs about 0.6 volts when the bluetooth receives a signal. The current output seems to be very low, maybe 20 microamps (too small to read on my meter). I took the BT receiver from another product which has further circuitry to drive some LEDs.

    I want to use the output of the receiver to switch a transistor used as an electronic switch. So I want to saturate the transistor so that it can pass 200mA (which is the current drawn by the load I want to turn on and off using BT). So it will need some amplification (current and voltage) between the BT receiver and the input to that transistor switch and also some buffering. I hope to acheive these things with this op amp circuit.

    SO the problem.With the circuit values as shown I get a Vout of 1.2V, which is what I expect since gain = 1+R2/R1. When I increase R2 to 33k though I only get an output of 1.5v. in fact 1.5v is the maximum output whatever values of R I choose.
    (Rin in included because if it's not there then the opamp output goes to around 4.5v).
    (The 6V is supplied by 4 AA batteries)

    Thanks for your time
     
    Last edited: Feb 1, 2011
  2. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    I think you are going to need a rail to rail op amp for this project.
    Look them up....
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    I think your input impedance is too high with that Rin load. Try a 1 MΩ or even higher.

    Are you saying it CAN go to 4.5v (by removing Rin), but cannot go above 1.5v with it attached? That's good, that the amp can output that voltage when it wants to. Otherwise that might be a concern, that your amp is toast.
     
  4. volvo340

    Thread Starter New Member

    Feb 1, 2011
    4
    0

    Sorry I don't understand that. If you say 100K too high already, then wouldn't a 1M make it even higher?

    Yes, the output goes to 4.5 volts unless the input (output of the BT device) is grounded through a resistor.
     
  5. volvo340

    Thread Starter New Member

    Feb 1, 2011
    4
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    Why do you think that?
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    I meant the load is too high, resistance too low. I consistently get those turned around, so do as I do, not as I say.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    Even when the BT output is off? Are the two circuits well grounded to each other? You may be looking at an output that goes open (and up) when off, and opens a path to ground when the LED turns on. The 0.6v would be the voltage across the output transistor.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Try wiring it like This Circuit

    If you are looking for voltage gain.
     
  9. volvo340

    Thread Starter New Member

    Feb 1, 2011
    4
    0

    I'll give that a try.

    Yes, the output of the BT Rx is connected to the ground of the op amp supply and everything works exactly as expected up to a gain of two.

    I'll try supplying the circuit from another source to and see if that makes a difference to isolate the cause of the problem to the BT output. I already know that out drops when connected to the base of a transistor directly which is why I was hoping to buffer it.

    Grateful for the suggestions everyone, thanks a lot.
     
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