Design amplifier with gain for 1000

Discussion in 'The Projects Forum' started by xw0927, Jan 22, 2014.

  1. xw0927

    Thread Starter Member

    Dec 19, 2010
    114
    0
    HI all,

    I am doing a DDS function generator by using ARDUINO microcontroller and AD 9850 chip module,the output waveform according to data sheet of DDS chip module is 1 V which is too low,therefore I need to design an amplifier for it which can generate output sinewave within 1-20 V.And the problem I am facing right now is when frequency up to MHZ,the output from DDS chip module begin to decrease,when frequency is 38.5 MHz, the output amplitude equal to 250mV.

    I am planning to use LM741 opamp to implement it, so do it possible to implement by using LM 741 ?or have any suggestion?


    THanks
     
  2. nedix

    New Member

    Jan 21, 2014
    18
    4
    Are you planning to use 741 because you have some around or because the project requires it? I don't think 741 can actually handle that frequency, you may want to have a look at MAX477 or AD711.
     
  3. xw0927

    Thread Starter Member

    Dec 19, 2010
    114
    0
    Hi nedix,

    ya,because I have LM741 opamp around. May you briefly describe why 741 is not suitable? and I am not familiar with opamp, so what specification do I need to consider when choosing an opamp?

    Thanks
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi xw,
    Are you really asking for a OPA type that will output a 38.5MHz sinewave at 20V, with a Gain of 80?

    The 741 will be no use at all for your project.

    A 741 has a unity gain bandwidth of approx 1.5MHz.

    E
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    The 741 is a classic op-amp used as a textbook teaching model. It was introduced in 1968 over 40 years ago and is now past its best due date.

    It will not operate at the frequency and gain you are requesting.

    As Audioguru would say, "it is a lousy op-amp".
     
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  6. xw0927

    Thread Starter Member

    Dec 19, 2010
    114
    0

    Hi eric,

    sorry, I just now recalculate again,ya,I am asking an OPA type which can have a gain of 100,do you have any recommendation?
    what does gain bandwidth mean?

    Thanks.
     
  7. nedix

    New Member

    Jan 21, 2014
    18
    4
    Yeah, 741 is the work-horse of op-amps, it should be used only as demonstrating how OA work. You have the specs here: http://www.ti.com/lit/ds/symlink/lm741.pdf

    You definitely want to look for another option here.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    If you are going to design using op-amps I suggest you look up gain-bandwidth product.
     
    xw0927 likes this.
  9. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi
    Gain bandwidth is basically is the highest input frequency at which the gain of the amplifier drops to 1 [unity].
    ie: simply stated, it will not amplify frequencies above the GBW, which for the 741 is ~1.5MHz.
     
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  10. xw0927

    Thread Starter Member

    Dec 19, 2010
    114
    0
    Hi nedix,

    I have take a look of the data sheet of LM741,but I didnt see that it have mention when frequency up to 1.5 MHz, the gain become unity, may you direct me which page does mention the unity gain?

    Thanks.
     
  11. xw0927

    Thread Starter Member

    Dec 19, 2010
    114
    0
    Hi eric,

    thanks for your precious information, market have a lot of opamp ,beside use wide gain to filter it out, what other thing should I consider when choosing an opamp? and the main problem will be the how to maintain that every frequency will have a constant amplitude? because higher frequency, lower amplitude. Example,for frequency 50 kHz and 30MHz output amplitude should amplify to 20 V.

    thanks.
     
  12. nedix

    New Member

    Jan 21, 2014
    18
    4
    hi, page 3, at the bottom, Bandwidth section.

    There are quite a few 100MHz Op-Amps on the market but you will probably have to nest them to get that gain.
     
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  13. xw0927

    Thread Starter Member

    Dec 19, 2010
    114
    0
    Hi nedix,

    http://www.analog.com/static/imported-files/data_sheets/AD711.pdf
    by referring to the data sheet of AD 711, for 3 MHz min Unity Gain Bandwidth (AD711J) it means that when frequency up to 3 MHZ,the gain equal to unity? and below 3 Mhz the gain is equal to 400?

    Thanks
     
  14. nedix

    New Member

    Jan 21, 2014
    18
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    xw0927 likes this.
  15. nedix

    New Member

    Jan 21, 2014
    18
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  16. xw0927

    Thread Starter Member

    Dec 19, 2010
    114
    0
    hi,

    http://datasheets.maximintegrated.com/en/ds/MAX477.pdf
    for this chip,the supply is +-5V,then it can provide full swing 20 V?
    for the page 8 ,table 1 ,the -3db bandwidth column for various combination of resistor, does it mean that when Rg=500 ohm and Rf =500 ohm ,the gain will become 1 when it up to 120 MHz ,and then if by using calculation of Rf/Rg,isnt the gain should be equal to -1 instead of 2?and firgure 2 is inverting or non-inverting?

    Thanks
     
  17. nedix

    New Member

    Jan 21, 2014
    18
    4
    Well, the positive values should be for non-inverting and the negatives for inverting. The way I see it, for 120Mhz you will have a +2 Gain in the shown configuration (Rg=Rf=500). Even if you have the same Rf=Rg ratio, the gain still varies depending on the actual values of the resistors; this means max 1% tolerance.
    Figure 2 is inverting.
     
  18. LvW

    Active Member

    Jun 13, 2013
    674
    100
    xw0927,

    there is another opamp parameter that deserves attention: SLEW RATE (SR).

    This parameter SR gives you the maximum possible sinusoidal output amplitude v,max for a given frequency - indpendent on the small-signal gain. This leads to the definition of a large-signal bandwidth

    BW=SR/(2*Pi*v,max).

    As an example, the 741 type opamp has a value SR=0.5 V/µs.
    For v,max=10V this gives a maximum allowable frequency of app. 10kHz.
    For larger frequencies the sinusoidal form changes more and more to a triangel.

    Modern opamps have slwe rates up to several hundreds volts/µs.
     
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  19. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    too bad about the 20 volt rquirement, MMIC amps have the gain and bandwidth to do what you want. and are cascadeable, a 20 db gain folloowed by a 10 db gain equals 30 db or gain of 1000x.
     
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  20. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    No, you need a supply voltage greater (by a couple of volts typically) than the peak output voltage. By the way are you wanting 20 volts RMS, 20 volts peak or 20 volts peak-to-peak?

    Here is an amplifier that has the best high frequency, output voltage swing and drive that I have found:

    http://www.ti.com/product/ths6022

    Be warned that this is a surface mount part and _very_ demanding on circuit board layout and power supply bypassing.
     
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