LF357 help reqd

Discussion in 'The Projects Forum' started by kahafeez, Feb 15, 2009.

  1. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
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    Hi all, i need to know that which chip can i use to amplify a signal of 16.6MHz???? i used this IC called LF357 which has a bandwidth of 20MHz.... its supposed to amplify a signal of 16MHz....

    it did amplify the signal bt reduced the signals freq to almost like 7MHz which i'm unable to understand why..... i used the schematic attached..... the input was a 1.4V, 16MHz sine wave which was clamped to 1V.... meaning the lower peak was at 1V and the upper peak was at 2.4V

    so i wz wondering where the problem lies.... like is there any problem with the design or is the IC unable to deal with the freq??? plz suggest something....

    incase the IC is incapable then do suggest me a few part numbers that are most likely to be found in the local market.... as i live in Pakistan.... the part numbers that are used in VCRs, Cassette Players are easily available and the stuff that isnt used in these items is hard to find....
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The LF357 has a to low bandwidth.
    Try a EL2125 or EL8102.
    See datasheets.

    Greetings,
    Bertus
     
  3. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    Thanks Bertus, now i'll call the shops and ask if this is available or not.....

    this is the biggest problem.... if i get it i'll try it and post the results....
     
  4. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    Hey Bertus, couldnt find any of the ICz u suggested :(

    can u plz suggest a few more that are likely to be found......

    question: actually bandwidth is the difference between the lower critical freq and higher critical freq bt i never find them on the datasheet..... does that mean that the lower critical freq is 0Hz ???? plz clarify...
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    You want to amplify a signal of 16 Mhz with an opamp.
    The gain set in your diagram is 10 times.
    You will need at least a bandwith of 10 X 16 Mhz = 160 Mhz.

    Here is a XLS sheet with high speed opamps from national.
    (remove the .txt)

    Greetings,
    Bertus
     
  6. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    thanks Bertus....
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A bandwidth of 20MHz means that the open loop gain of the amplifier drops to 1 at 20MHz. Use the frequency response of the op amp to see what will be the gain at 16.6 MHz (not much thought).
     
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