+ Post New Thread
Results 1 to 12 of 12
  1. #1
    Newbie level 4
    Points: 1,261, Level: 8

    Join Date
    May 2006
    Posts
    7
    Helped
    0 / 0
    Points
    1,261
    Level
    8

    opamp (3v power use)

    Hi,

    I have a 3V power supply. I am using a op (IC 741) with 3v and ground...
    Is there an easy way to use it with 3v and -3v...?
    I cannot change the power supply. I get a signal from an electret microphone. I use a capacity to get rid of AC signal, but after using the opamp as preamplifier with (factor 22) I get a signal which has a dc value of about 1.9V and the 0.5AC Signal on top of it.
    And the negative values could be suppressed by amplification.

    Does anybody know how I can use it correct and easy?


    Best Regards,


    Patrick

  2. #2
    Advanced Member level 5
    Points: 74,061, Level: 66

    Join Date
    Oct 2004
    Location
    West Coast
    Posts
    7,942
    Helped
    2329 / 2329
    Points
    74,061
    Level
    66

    Re: opamp (3v power use)

    3V single supply for the LM741 opamp is to low; in fact, it shouldn't be used below +/-5V (9-10V single supply) ..
    Try to repalce it with the LM358 (or similar), which is rated for ≥3V single supply ..

    Regards,
    IanP



    •   AltAdvertisement

        
       

  3. #3
    Newbie level 4
    Points: 1,261, Level: 8

    Join Date
    May 2006
    Posts
    7
    Helped
    0 / 0
    Points
    1,261
    Level
    8

    Re: opamp (3v power use)

    Hi thanks for the answer. Would you be so kind to tell me where I can see which vcc power is needed? I looked in the icc 741 data sheet and found nothing.


    Best Regards,


    Patrick



  4. #4
    Advanced Member level 5
    Points: 74,061, Level: 66

    Join Date
    Oct 2004
    Location
    West Coast
    Posts
    7,942
    Helped
    2329 / 2329
    Points
    74,061
    Level
    66

    Re: opamp (3v power use)

    Information on supply voltage ranges can be found in every datasheet ..
    Maybe it is not given as numbers, but if you look through attached charts you will find what you are after ..
    For example, take a look at the pictures below: they show that the usable supply voltage range is from +/-5V to +/-20V ..
    I am pretty sure you will find similar charts for any selected opamp, including the LM741 ..

    Regards,
    IanP



    •   AltAdvertisement

        
       

  5. #5
    Newbie level 4
    Points: 1,261, Level: 8

    Join Date
    May 2006
    Posts
    7
    Helped
    0 / 0
    Points
    1,261
    Level
    8

    Re: opamp (3v power use)

    Thanks a lot,

    I have one more question. If I use a op with ground I need to get the absolute value of the signal, so that I do not loose the negative parts...

    Does anybody know a easy circuit for that?

    Best Regards,


    Patrick



    •   AltAdvertisement

        
       

  6. #6
    Advanced Member level 5
    Points: 74,061, Level: 66

    Join Date
    Oct 2004
    Location
    West Coast
    Posts
    7,942
    Helped
    2329 / 2329
    Points
    74,061
    Level
    66

    Re: opamp (3v power use)

    Connect input and output of an opamp amplifier through coupling capacitors ..
    This will allow the opamp to have its dc offset and make signals before input capacitor and after output capacitor referenced to 0V .. see picture below ..

    Regards,
    IanP



  7. #7
    Newbie level 4
    Points: 1,261, Level: 8

    Join Date
    May 2006
    Posts
    7
    Helped
    0 / 0
    Points
    1,261
    Level
    8

    Re: opamp (3v power use)

    Ah,

    so this means that if I power the opamp with 3V and ground and use a capacity on input and output, the Signal will be amplified and after it dc is set to zero. But I do not loose the negative part of the signal, because the input capacity puts the signal on the dc value (1.5v) of the opamp.
    The dc signal is not amplified by doing this?

    patrick



  8. #8
    Advanced Member level 5
    Points: 74,061, Level: 66

    Join Date
    Oct 2004
    Location
    West Coast
    Posts
    7,942
    Helped
    2329 / 2329
    Points
    74,061
    Level
    66

    Re: opamp (3v power use)

    Take a look at the example above, in particular at the function of the 10µF capacitor in the feedback loop: it cuts-off the path for dc setting the dc gain to 1 (output = (+) input), whereas the AC gain is set by the voltage divider 33kΩ - (1.5kΩ+27kΩ) ..

    Regards,
    IanP



  9. #9
    Newbie level 4
    Points: 1,261, Level: 8

    Join Date
    May 2006
    Posts
    7
    Helped
    0 / 0
    Points
    1,261
    Level
    8

    Re: opamp (3v power use)

    Circuit:
    is shown in attachment

    Hi,

    i have made a preamplifier with the Opamp LM324. I am using a non inverting op.
    I use a virtual ground of 1.5V. (this is working). It is used, because the op is used with 3V and ground power supply.

    Input of the microphone is 0.2V (max). However I do get only a constant signal at the output. i do not see the ac signal.

    What is wrong? Normally I should get 1.5v at the output and 0.4v additional cause of the input signal an V = 2 (R1=R1)

    p.s. I tried another op (which should workwith 3V- 0 supply) with the same results...

    I use the virtual mass in hope that the 0.2v signal is big enough for amplification. With 1.5 it is shifted in the middle, so it should work. The bandwidth of the op should be high enough to see something

    Best Regards,

    Patrick



  10. #10
    Advanced Member level 5
    Points: 74,061, Level: 66

    Join Date
    Oct 2004
    Location
    West Coast
    Posts
    7,942
    Helped
    2329 / 2329
    Points
    74,061
    Level
    66

    Re: opamp (3v power use)

    Try to connect 10kΩ - 100kΩ resistor bewtween 1.5V virual ground and opamps (+) input .. you need dc bias there .. and connect ≈100µF capacitor between 1.5V virual gound and 0V ..

    Regards,
    IanP



  11. #11
    Newbie level 4
    Points: 1,261, Level: 8

    Join Date
    May 2006
    Posts
    7
    Helped
    0 / 0
    Points
    1,261
    Level
    8

    Re: opamp (3v power use)

    Hi,

    the ac part is 0.2 volt and i want to amplify it by factor 11. So I get 2.2v at the output.
    What happens if i connect the virtual ground to input? E.g. the signal is not around 0v, it is at 1.5 +- 0.2...
    Is the dc part amplified,too? I only have 3v, so there should be a large cut of. I Or amplifieng with factor 10 is not possible with dc 1.5v.

    Why do I need the capacity between virtual ground and ground, and why 100uF? Please tell me,

    Patrick



    •   AltAdvertisement

        
       

  12. #12
    Advanced Member level 5
    Points: 74,061, Level: 66

    Join Date
    Oct 2004
    Location
    West Coast
    Posts
    7,942
    Helped
    2329 / 2329
    Points
    74,061
    Level
    66

    Re: opamp (3v power use)

    We are going in the wrong direction .. you don't need to create separate virtual ground .. biasing the (+) input of the opamp with 1/2Vcc will be enough ..
    Just copy the circuit that I posted on the 3rd of May (with the switch closed the gain is 22) ..
    If you use the LM324 you can supply that circuit of 3V ..

    Btw. in all these circuits the gain for dc =1 .. this means that the dc is not amplified and you will have 1.5V on the (+), (-) and output pins of the opamp ..

    Regards,
    IanP



--[[ ]]--