amplifier supply and ground

Discussion in 'General Electronics Chat' started by alarassi, Feb 20, 2010.

  1. alarassi

    Thread Starter Member

    Feb 6, 2010
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    hello guys​

    im doing a project and i need to amplify a bipolar signal!
    as i know the amplifier shoud be supplied with Vcc+ & Vcc- in order to amplifiy the both side of the bipolar signal!​

    my question here is how can i have a ground and Vcc- at the same time!? im using a battery as my supply voltage and if i assume Vcc- as my ground then the negative signal will be cut!​

    i want to construct the circuit on breadboard but i still couldn't amplify the signal so far!​

    i tried to make a line as my ground insted of Vcc- line but i still couldn't !!

    can anyone tell me the correct way to do it?

    thank you​
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Is this signal AC or DC?

    For AC signals the simplest method is a blocking capacitor.

    A bipolar signal is also called balanced and a unipolar one unbalanced. A Balun transformer is another way to convert.

    For DC signals you need a level shifter, which is just applying a DC bias voltage to your signal to shift the entire signal to one side of the origin.
     
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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    What will be your VCC+ and VCC- voltage?
     
  4. alarassi

    Thread Starter Member

    Feb 6, 2010
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    it's AC signal (output of electret condenser micrphone)!
    can u clarify how can i use this blocking capacitor!?

    my Vcc+ & Vcc- will be something between -,+9 to 15V ! i might use normal battery with -,+9V ! but i still don't know how can i have ground and Vcc- !!!
     
    Last edited: Feb 20, 2010
  5. t06afre

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    May 11, 2009
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    Hi this drawing will explain it I hope. You have to use two 9 volt batteries.
     
  6. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    Try reading this link: http://www.eng.usf.edu/~saddow/eel3302/741.html
    It is a good tutorial about the 741 opamp, and applies to other opamps also.

    Down the page is a method of supplying + and - voltages, with a common reference point between them (ground), by using 2 batteries. This will allow you to amplify your sine wave above and below ground, and you won't have to bias the signal. If you were to use a single 9V battery, you would have to use a resistive voltage divider to get +4.5V and -4.5V, which wouldn't give you much head room for your input signal...

    From the electret mic, you probably want to place a capacitor in series at the opamp input to be sure your input signal is AC only. Any value from about .1uF and up will work.
     
  7. t06afre

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    Which type of opamp do plan use?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    This is my schematic of an opamp preamp for an electret microphone using a low noise opamp and a single supply voltage. The opamp is biased to the middle of the supply voltage and the microphone is capacitor-coupled to it.
     
  9. alarassi

    Thread Starter Member

    Feb 6, 2010
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    thank you guys for your help but im really confused here!

    i want to amplify the output signal of an electret condenser microphone (heart sound signal) and then this amplified signal will be fed into AD converter to be used with PIC! so that means the amplified signal must be in positive range in order for the ADC to read it!

    here, should i use two batteries to amplify the bipolar signal and then use level shifter to shift the signal to positive range ( not sure if this way is correct)!?
    or are there any simpler and direct ways to have the desired output!?
    im really confused here so pls clarify it to me..

    thank you



    i might use lm234 or tl074 because it's low noise and i also want to use an active filters! if you think other types of amplifiers might be better for me to use pls tell me
    :)
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The output of my preamp for an electret mic has a positive DC voltage at the output pin of the opamp that averages half the supply voltage. It goes up to 1V less than the supply voltage and down to +1V.

    The LM234 is a current source, not a low noise opamp. An LM324 quad opamp has very high noise, high crossover distortion and has poor high audio frequencies response.
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    If you do this with an opamp you need a high input (FET input) impedance one such as the RL071 suggested by AudioGuru. The 324 /74i series have too low input impedance. The 324 series are not opamps.
     
  12. alarassi

    Thread Starter Member

    Feb 6, 2010
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    thank you for your replay!
    your circuit seems suitable but i need to understand it first and learn!
    how could u be able to have a DC voltage! or in other word what was the method u used in ur circuit to have a positive only!?

    how if i want to have an output from 1 to 5v , can i have your circuit with a supply voltage of 6V or how ?
    i really need to learn and understand the circuit before i can use it!



    i think i'll be using tl074 if it's suitable. thank you :)
     
  13. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    The TL074 should do the trick, as long as you neutralize the unused sections (or maybe you will be using them?). You can bias an opamp to operate anywhere you want, by adding a DC voltage to the input. You'll then have an AC signal riding on a DC level. That DC level can be used at the output as your control voltage.
    The biasing is done using a simple resistive divider.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    I made a reply, not a replay.

    r2 and R3 bias the input at half the supply voltage and the input, output and feedback have capacitor coupling.

    No.
    The minimum supply voltage for a TL07x opamp is 7V, not 6V.
    An OPA134, OPA2134 or OPA4134 opamp works fine from a 6V supply in my circuit. They have very low noise and are wideband.
     
  15. alarassi

    Thread Starter Member

    Feb 6, 2010
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    i tried a circuit with resistive divider , it worked nicely with a function generator and gave me the correct output! but when i used it with a mic i got no output at all..!

    someone commented to me that it might be impedance mismatch problem! my mic output impedance is 2.2k and im using tl074 as my preamplifier with has high input impedance!
    how can i solve this problem!?
     
  16. t06afre

    AAC Fanatic!

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  17. alarassi

    Thread Starter Member

    Feb 6, 2010
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    i tested the microphone and connected it directly to an oscilloscope, it gave me good signal ! and i also tested the circuit with a fucntion generator and also gave me signal! but when i connect the mic it gave me no signal!!
     
  18. alarassi

    Thread Starter Member

    Feb 6, 2010
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    is there something wrong related to the 2.2k impedance of the mic and high impedance of the amplifier!? or it's probably something about connection? in other word, can low output impedance work with high input impedance!?
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    Why are you using a voltage divider with a microphone? Its output level is low and you do not want to reduce it.

    A high input impedance is needed on the amplifier so that the output level from the mic is not loaded down which reduces its level.

    My mic preamp has the pin numbers for a TL071 single opamp. Since you are using a TL074 quad opamp then its pins have different numbers.
     
  20. alarassi

    Thread Starter Member

    Feb 6, 2010
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    NO, i'm just using two resistor to bias the signal to half the supply as you told me guys here! but for the function generator it works nicely, while with the mic it gave me no signal!
    as i can understand here if the output impedance of the mic is 2.2k that means it matches with the high input impedance of the amplifier right?
     
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