Need help with Non-Inverting amplifier using single supply

Discussion in 'The Projects Forum' started by Yuvraj Abhimanyu, Jun 29, 2015.

  1. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
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    Hey Guys,

    I'm building an Non-invering amplifier using a single 9v supply, and the opamp i'm using in NE5532 from TI.
    Now, My question is, in the circuit diagram below, i'm unable to get the output!
    I dont understand why!

    Plz Help,
    Thank you.
    circuit.png
    Output.png

    P.S- The power supply is 9v, sorry for the wrong value in the circuit diagram.
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    With +4.5V on in the inverting input, what would you expect the OPA's Vout is going to be.?

    Post your LTS asc file.
    E

    BTW: The NE5532 is specified as a dual power supply OPA
     
  3. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
    15
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    I Expected the output voltage centered at 4.5v.

    I donno how to attach a asc file here!

    http://www.ti.com/lit/ds/symlink/ne5532.pdf

    In Page No.7, article 8.4 it says NE5532 can be used as single power supply.! Does it mean something else?
     
    Last edited: Jun 29, 2015
  4. ericgibbs

    Senior Member

    Jan 29, 2010
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    As the OPA is set for an Inverting gain of 1, the Vout of the OPA would try to go to -4.5V, but as its powered by a single +9V, Vout will tend towards zero.!

    Post your asc file and I will post a possible fix.:)
     
  5. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
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    How fail to see how will this be an inverting amplifier?! As far as i no its an non-inverting amplifier because the signal is given to the non inverting side of OPA.

    The circuit i've constructed is from the Page No.5, Section 2.1 (plz refer to the link below)
    https://courses.cit.cornell.edu/bionb440/datasheets/SingleSupply.pdf
     
  6. ericgibbs

    Senior Member

    Jan 29, 2010
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    The page #5 circuit is not the same as the circuit you have posted.??

    Please post your simulation circuit.

    EDIT:
    you appear to have the + and - inputs to the OPA crossed over, the +4.5V, should be on the NON inverting and the AC signal should be Capacitor coupled to the Inverting input.???
    Also the Gain of the NON inverting input is *2, so the 4.5V should be +2.25V
     
  7. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
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    Untitled.png
    Please search for Page 5, figure 3 in section 2.1
     
  8. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    I did post that image in post #6. edit.

    Look at the attached circuit, is this what you are trying to do.?
     
  9. Bordodynov

    Active Member

    May 20, 2015
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    In this particular case, everything can be made easier.

    Ampl.png
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No, it isn't. Your circuit does not have an input coupling capacitor as shown in the TI document. Your input sinewave is centered about ground. This means that the opamp has 4.5VDC between its two inputs. What you want is 4.5 VDC offset on one input, and zero VDC difference between the two inputs. The coupling cap achieves this.

    But your circuit will work only at unity gain. For a more generic AC amplifier that will work at other gains, see:
    http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php

    The last schematic before the comments is this: Non-inverting amplifier using a single voltage rail

    C2 ensures that the voltage gain at DC is 1 no matter what the AC gains is, and that there is 0VDC between the inputs. It forms a high-pass filter with R1, so you have to pay attention to that. R3 and R4 set the DC bias at the output, but also load the input, so you have to pay attention to that.

    ak
     
  11. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
    15
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    Hi Eric,
    I'm trying to build an 'Non-Inverting' Amplifier with variable gain
     
  12. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
    15
    0
    Hi AnalogKid,
    The way i presumed it would work is that the 4.5v DC at the inverting terminal will be the new virtual ground. Is this assumption wrong?

    And can u explain me the "This means that the opamp has 4.5VDC between its two inputs" part plz?
     
  13. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
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    I've two questions regarding this circuit diagram.
    1. what are the purpose of R4 and R5?
    2. without the decoupling capacitors, wont the ac signal go to the DC source?
     
  14. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Your assumption is not wrong, but it is incomplete. Only one of your two inputs is referenced to your virtual ground. The other is referenced to your real ground.
    Your sine source has a DC value of 0V, and R1 is connected to 4.5V. 4.5V - 0V = 4.5V DC between the two inputs.

    ak
     
  15. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
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    Oh i see... Intresting,
    Now how is this solved in the circuit tat u've provided ( the circuit u were referring to in the link tat u provided).
    now wont the inverting terminal has DC value of 0 and non inverting terminal has DC value of 4.5?
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Both the 9V DC source and the sinewave source have zero ohm output impedances. So the 9V source sees 200K to "ground" and the center point is 4.5 V. The sinewave source sees 200K to "ground", and the center point is 0.5Vrms. So the opamp + input sees 0.5Vrns riding on 4.5Vdc. The - input sees 4.5Vdc at an impedance of 100K. The DC difference between the two inputs is 0Vdc, and the non-inverting gain is 2 which makes up for the 2:1 attenuator at the + input.

    ak
     
  17. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Nope. Because of the capacitor in the shunt leg, the inverting terminal will float up to whatever voltage it takes to reduce the differential input voltage to zero. That always is the function of negative feedback. So the cap will charge up until the DC voltages at both inputs are the same.

    ak
     
  18. Bordodynov

    Active Member

    May 20, 2015
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    inverting amplifier:

    inv.png
     
  19. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
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    hey AK,
    I simulated the circuit u were talking about, this is what i got!
    ver2.png
     
  20. Yuvraj Abhimanyu

    Thread Starter New Member

    Jun 10, 2015
    15
    0
    Hey Bordodynov,
    Instead of dividing the supply voltage at inverting and non inverting terminal, why cant i use the concept of 'virtual short/virtual ground' here?
     
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