Hearing Aid Design

Discussion in 'The Projects Forum' started by nahomyaja, Mar 24, 2012.

  1. nahomyaja

    Thread Starter New Member

    Feb 28, 2012
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    Hi,

    What is the minimum input (mic output) that can be amplified by OPA134 IC?

    It didn't work for 0.2 Vpp input for me :(

    Kindly, help in this regard. Or can you post a circuit diagram for OPA134 amplifier? I used an ordinary (741 Op-amp) non- inverting amplifier design for a gain of 10 max (variable gain)
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    It has a typical voltage gain of one million from DC to 8Hz without negative feedback. You did not show your schematic so we don't know how much negative feedback it has which reduces the gain and distortion and widens its bandwidth.
    Its maximum output swing is +0.5V above its negative supply voltage to 1.2V less than its positive supply voltage when it has a high resistance load connected to half the total supply voltage.

    A microphone might produce 200mV p-p if you SCREAM closely to it.

    If it works from your low supply voltage (it is designed for a 30V supply) and if it has a high resistance load then its max p-p output is about 2.4V less than the total supply voltage.
    Then with an input of 200mV p-p its output will be undistorted at 2.2V p-p and a supply of at least plus and minus 3.4V must be used.

    Please attach your schematic.
     
  3. nahomyaja

    Thread Starter New Member

    Feb 28, 2012
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    0
    Thanks Audioguru :)

    The gain designed is almost 15 (3 (bjt amp)*5(opa134)).. But the headphone output is nearly similar to the mic input..

    What can I do in this regard? When I changed the gains, there was presence of noise.

    (This project was developed from this thread only from all your posts. Thanks to all)
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    I can barely read your parts values.

    If your mic is an electret type then R1 should be 10k ohms.
    The BC549 has a very high current of 8mA. Its datasheet shows it producing low noise when its current is only 200uA.
    The OPA134 has its (+) input biased at 0V so must use a negative supply.
    Is V3 (V-) a negative supply?
    Q2 and/or the battery will burn out instantly unless Q2 has a series base current-limiting resistor.
     
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  5. nahomyaja

    Thread Starter New Member

    Feb 28, 2012
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    Thanks audioguru :)

    Currently,

    R1 = 2.2K
    R2 = 82K
    R3 = 33K
    R4 = 680 OHM
    R5 = 220 OHM
    R6 = 1K
    R7 = 10K
    R8 = 47K
    R9 = 4.7K
    R10= 2.2K
    R11= 10 K
    Q1 - BC549
    Q2 - BC558
    U1 - LM324
    U2 - NE555
    U3 - OPA134PA
    C1 = 0.1 uf
    C2 = 100uF
    C3 = 0.01 uF
    C4 = 0.1 uF

    Please check the transistor design. It worked in simulator, but getting a saturated output in bread board :-(

    Also, if I provide positive supply to opamp from the output of the PNP transistor switch and using a negative supply V3, will the opamp work?

    If we use split supply (don't know the design), will OPA134 amplify the signal faithfully? (by placing a capacitor in the output will we get a signal without dc offset)
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Please make the corrections I said in my post #19.

    The OPA134 does not need a split supply. Most audio circuits use a single supply, bias the opamp at half the supply voltage (yours is biased at 0V) and use input and output coupling capacitors. The OPA134 should be non-inverting so it can have a high input resistance.
     
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  7. nahomyaja

    Thread Starter New Member

    Feb 28, 2012
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    Sure Audioguru :)

    But, I have failed in designing a single supply OPA134 amplifier. Also, a lecturer told me that, it won't work for input voltages as low as 200mV.
    Is that true?

    In the design schematic, the positive bias of op-amp is given from the output of a transistor switch. Only when the switch if off (PNP transistor), the bias is given to the op-amp. That is how, I want the circuit to work. Hope it works :)

    Can you please provide me with design of single supply OPA134 non-inverting amplifier? I've to complete the project by Monday :-(
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    It is simple to use a single supply with an opamp. Make a voltage divider with two series resistors from the supply voltage to 0V. Their junction connects to the (+) non-inverting input as its DC reference voltage.

    No.
    Any opamp with a single supply and biased properly will work perfectly.

    The PNP transistor has no series base resistor so it and/or the 555 and/or the battery will burn out.
    It does not "bias" the opamp, it applies a positive power supply to the opamp.

    It is your project, not mine. It is simple so you should learn how to do it.
    Ask your teacher to teach it to you.
     
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  9. nahomyaja

    Thread Starter New Member

    Feb 28, 2012
    8
    0
    Thanks a lot Audioguru :)

    Will get back to you, if any difficulty occurs :)
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Welcome to AAC!

    A thread belongs to the OP (original poster). Trying to take over someone elses thread is called hijacking, which is not allowed at All About Circuits. I have therefore given you a thread of your very own.

    This was split from Hearing aid design
     
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