Single Supply inverting Op Amp ?Problem?

Discussion in 'General Electronics Chat' started by kaning, Aug 25, 2014.

  1. kaning

    Thread Starter Member

    Jun 2, 2014
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    Hello guys, I've been reading about op amps. To start i want to invert a dc signal from 5v to -5v or close to that with single supply. (i want to drive P channel mosfet).

    Im using TL071 op amp, i saw some videos and tl071 datasheet and i designed this circuit for my purpose.

    [​IMG]

    From everything i've read, this is a single supply inverting circuit with a 2,5v Vref on + input and a gain of 2, right?. So, i should be getting - signal on the Vout with this, but im not! no matter what Vin i use, i keep getting 2,49v wich is understandable if i use a Vin of 1,25v or greater but even with less voltage values this is happening. why?

    with this circuit i know i cant get -5v, anyway my main goal here is why im not getting -Vout..

    what am i missing?? how i can get -Vout ? many thanks in advance
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    To get Pin6 to go below 0V (gnd), you would have to power pin 4 with a negative supply voltage, like -10V.

    Note that unless an opamp is advertised by the manufacturer as being rail-to-rail output, the output pin (6, in this case) cannot pull higher than (+supply-~1.5V) or 3.5V if +supply is +5V, and pull lower than (-supply+~1V), or 1V if your -supply pin is grounded.

    Best advice: Get and read the datasheet for the specific opamp you are thinking about using for a project. Read all of parameters on the data sheet, and get to point where you understand what they mean...
     
    Last edited: Aug 25, 2014
  3. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi kanning,
    An inverting OPA means it inverts the 'sense' of the input signal, not the polarity.

    eg:
    Hi in , Low out
    Lo in Hi out
    E
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    An op amp does not make up voltages, in a real sense it redirected the voltage from the power pins to the output.

    So to get -5V out you need to put -5V in. Actually a little bit more.

    What you really want to use here is a "Switched-Capacitor Voltage Inverter" such as the LT1054
     
    Last edited: Aug 25, 2014
  5. #12

    Expert

    Nov 30, 2010
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    The TL-071 is not built for 5 volt supplies. It's built for audio frequencies at +/- 12 to 15 volts.
    Wrong chip, or wrong power supply.
     
  6. kaning

    Thread Starter Member

    Jun 2, 2014
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    all the purpose of this is to drive P channel mosfet on my portable project powered by a lipo battery... i will search about that "Switched-Capacitor Voltage Inverter".

    All i want is a at -4v signal to drivethe mosfet. many thanks for the replies
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What does the control signal come from?
    Do you have a u-controller?
    What supply voltages do you have?
    Tell us more about what the PFET is supposed to switch?
     
  8. kaning

    Thread Starter Member

    Jun 2, 2014
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    Since my goal is a portable device, i dont have negative supply voltage. thats exactly my goal, if i would have negative voltage supply i wouldnt need this circuit :b So, doesnt virtual ground solve this negative supply issue?

    I tried this time with 12v Vcc and on +Input 6v. So its the same as Vcc+=6v and Vcc-=-6v and + Input=0v, right?

    As i said i tried now with 12Vcc and 6v +Input but anyway no matter what Vin (i tried up to 1,6v) and i keep getting always 6v output :/
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Forget the opamp approach. Answer my questions independent of your proposed solution...
     
  10. kaning

    Thread Starter Member

    Jun 2, 2014
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    sorry i said signal and i shouldnt, i want to use the voltage from the portable device power supply (lipo battery) to drive the P Mosfet.

    on this project only one (lipo battery)

    PFET will act as a normal switch. probably i will not use as this neither its important anymore. by now i only want to get over this problem xb and learn with it.

    edited: solar panel will directly charge phone; battery will directly charge phone; solar panel/USB will charge battery. So this PFET is a security precaution for when battery is charging phone, the battery charger IC will not get current. maybe a schematic would be better?
     
    Last edited: Aug 25, 2014
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Ok, I give up.
     
  12. kaning

    Thread Starter Member

    Jun 2, 2014
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    i've edited last answer
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    One schematic is worth 1000s of words...
     
  14. kaning

    Thread Starter Member

    Jun 2, 2014
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    http://imgur.com/S8KGSIr

    this is a diagram i made while i was thinking about it. so the main goal is when a positive voltage is detected on PFET gate, it means phone is charging so no voltage will be going to MCP73831 (battery charger IC)
     
  15. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Now if you'd have started with that last post and explained what you were trying to do the attempts to help might have progressed much better.

    So much less confusion and guessing.
     
  16. kaning

    Thread Starter Member

    Jun 2, 2014
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    what im trying to do doesnt matter anyway because the point of this thread was only how to make the op amp circuit work, whatever the purpose is. On the first post i just mentioned the PFET and i didnt mention it as a pirority of this thread. i see no confusion on what my goal is on this thread
     
  17. wmodavis

    Well-Known Member

    Oct 23, 2010
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    I understand your position but how many responses did it take before you got a useful answer?
    Clear and complete communication is a key factor especially in a technical field.

    If people have to guess what you really want or ask many questions for clarification before understanding, what does that suggest about the communication process?
     
  18. BobTPH

    Active Member

    Jun 5, 2013
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    Do you think a negative voltage is needed on the gate of a P-channel MOSFET?

    It is not.

    The voltage must only be more negative than the voltage on the source.

    So if your P-channel MOSET has the source connected to, say 5V, and you put 0V on the gate you can turn the MOSFET on. However, you will need a "logic-level" MOSFET that turns on with whatever battery voltage you are using.

    Bob
     
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  19. kaning

    Thread Starter Member

    Jun 2, 2014
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    i know that, but isnt the very first post clear enough? :/ nevermind. anyway i understand your point
     
  20. #12

    Expert

    Nov 30, 2010
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    Maybe you broke the chip? Still, you need a different chip.
    Here's a nice one. The important words being, "R-R I-O" Rail to Rail input and output.
    No promise that it's the best chip. It's just an example of one that will work.
     
    Last edited: Aug 25, 2014
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