Help! Maximum gain of LM741 op amp

Discussion in 'General Electronics Chat' started by momotee, Apr 9, 2012.

  1. momotee

    Thread Starter New Member

    Jan 18, 2012
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    Hi, I have to implement an inverting gain of 10000.
    I configured a LM741CN op amp as an inverting amplifier.
    Using +/- 15Vcc.
    Assuming no saturation of the signal, can the gain of 10000 be realized?

    Or do I have to split the gain into different parts,
    Eg. 100 and 100 (using to op amps) to get 10000.
     
  2. momotee

    Thread Starter New Member

    Jan 18, 2012
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    The signal input is sinusoidal 50 Hz.
     
  3. ssnyde

    New Member

    Apr 4, 2012
    19
    5
    Hi,

    I think more important than the gain is the amplitude of your input signal. To not saturate an inverting amp with gain 10k and +/-15V, your input signal has an absolute value of < 1.5mV. Input offset voltage is typical 2mV, so your output will saturate with your input grounded. So you will need to use the offset null terminals to trim that out. Also keep the resistors under 1meg or at least under 10 meg or input bias current will cause an offset.

    I think you're better off using two op-amps.
     
  4. justtrying

    Active Member

    Mar 9, 2011
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    the other way to deal with offset is to split it into two parts and use a high pass filter in between two gain stages to remove the DC offset. This could help as sometimes it is not possible to fully null the op-amp (I've had that problem before).
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The lousy old 741 opamp is 44 years old and is too noisy (hiss) for high gain.
    Its gain at 50Hz is not much more than 10000 so there will not be much negative feedback so the output will be distorted.
    Use a low noise audio opamp instead.
     
  6. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    You are definitely going to have to use a nulling circuit to get rid of the DC offset at the input. Theoretically, the 741 can do 50,000 gain unless I read that wrong and its 50,000 gain-bandwidth product, at which point you are asking it to go ten times as fast as it can go. That wiil require two stages. Try it with one stage and find out!

    If it's incredibly picky and distorted, use two stages.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I agree with Audioguru on the choice of op amps.
    Regarding offset voltage, the OP said that the input is a 50Hz sine wave. Justtrying posted the solution to the offset problem, except I would AC-couple both stages.
     
  8. #12

    Expert

    Nov 30, 2010
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    AC coupling as suggested in post #4?
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Here is what I wrote:
    Justtrying did post that in post #4.
    I suppose a gain of 100 in each stage would allow a single cap to be used, as suggested in post #4. My bad.:(
     
  10. momotee

    Thread Starter New Member

    Jan 18, 2012
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    0
    Assuming a two stage gain of 100 and 100,
    if the output of the first stage already saturates, it means that the second stage also saturates right?

    That means for a gain of 10000, and a Vcc of +/-15V,
    my input signal cannot be more than 1.5mV,
    regardless of how many stages of gain I use?
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    A lousy old 741 opamp with a supply that is plus and minus 15V has its output saturate at about plus and minus 13V peak. With a gain of 10,000 its input can be no more than 13V/10,000= 1.3mV peak which is almost nothing. The noise level might be more.
     
  12. #12

    Expert

    Nov 30, 2010
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    Yes, 15 divided by 10,000 is .0015, no matter how many stages you use.

    or, as Guru said, 13 divided by 10,000 is .0013

    no matter how many stages you use.
     
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