Choice of opamp for analog RF filter

Discussion in 'General Electronics Chat' started by Peter Pan, May 11, 2009.

  1. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Hello All :)I need to design a band-pass filter amplifiyng signals whose frequencies are between 4 and 5 MHz and rejecting the rest, especially at frequencies below 4MHz. Gain for the band should be about 20 dB.Please, could you advise me an active unit (opamp preferably) for this filter keeping in mind that there is only +12V power supply and ground available.Thank you.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    I'm not sure there is an appropriate choice of an opamp to operate at those frequencies. Operating single supply definitely limits your choices. I could be wrong though, time and technology do march on.
     
    Last edited: May 11, 2009
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Here is a list attached for high bandwidth opamps.
    (remove the .txt to have the XLS file)
    As you know each filter will also introduce a phase shift.
    The bandwidth you require is rather high versus the required frequency.

    Greetings,
    Bertus
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    You'll be sure to let us know how they work out for you. I'm kinda curious.
     
  6. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Thank you, t n k - that opamp is great I sense but a bit out of my budget, unfortunately ...Thanks to Bertus - you've given a very thorough list of opamps and most of these seem suitable. So, I'm overwhelmed :) Would you suggest any specific opamp? (from your experience, perhaps). I mean to use it in low field NMR amplifier (designed for signals with Larmor frequency about 4.333 MHz). The spectrum of signals (after 3-stage amplifier) is extremely noisy, with plenty of broad unwanted signals between 2 and 4 MHz and below 2 MHz as well. So it's necessary to get rid of them in order to have a cleaner spectrum with strongest peak at Larmor frequency (4.333 MHz in my case).
     
  7. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Sure. Although I am a newbie to filters but hope to manage and learn a lot of interesting things on the way! :)
     
  8. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    I have attached a picture of input (blue trace) and output (red trace) of my amplifier. Output signal has to be filtered out so that a single spectral component at about 4.33 MHz (emphasized by dashed line over it) would dominate in a spectrum and the rest (which is unwanted) has to be significantly attenuated or removed (ideally).
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
  10. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Thank you. I'll try LC bandpass first just to see how well it works for my task.
     
  11. Darren Holdstock

    Active Member

    Feb 10, 2009
    262
    11
    It's certainly possible to implement a 4.3 MHz active bandpass filter using some of the excellent fast op-amps available today, but it's not without its problems, and one of the circuits Bertus suggests would be a safer bet. Be judicious in the choice of inductor type, as the core needs to operate at the maximum frequency of interest - this may be 100s of MHz for a 4 MHz centre frequency. The caps will be less critical, but they should still be chosen for minimum equivalent series inductance (ESL).

    If you want to go down the active filter route, then the figure of merit for choosing the op-amp (all other things being considered) is the gain-bandwidth product (GBW) of the device. A specific active filter architecture will have a minimum GBW requirement. These requirement will vary between different architectures, and finding out exactly what is needed is difficult as it's a topic that's conveniently brushed over in most filter textbooks.
     
  12. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Thank you.I have tried to use online LC filter calculator at http://www-users.cs.york.ac.uk/~fisher/lcfilter/ but do not know one of input parameters which it asks for. This is the filter characteristic impedance (which is equal to the filter input (source) impedance). How I could found out what its value should be?
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
  14. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
  15. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    When you take a look at the PDF I attached before it says with some types DC - 2 GHz.
    At lower frequencies the noise will even be lower.
    I think they will work fine.

    I have used them in amplifiers for 40 - 860 MHz for cable TV.

    Greetings,
    Bertus
     
  16. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Yes, I have looked at the file (thank you!) and have not found so far a MMIC from the list which could amplify for 1-10 MHz band. Them seem to be normally used for amplification at higher frequencies - about 0.1 GHz and higher, according to their electrical specifications. At lower frequencies manufacturer does not specifies any gain ...
     
  17. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Here is the datasheet of the MAR6 that I used for that amplifier.
    It also shows how to use it.

    Greetings,
    Bertus
     
  18. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Thank you, Bertus. It's likely that they will do their job at lower frequencies as well!I just have another question (a bit silly perhaps) - MAR6 is an amplifier ... so how I could use it for filtering?!
     
  19. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    You can place the filter before the MAR.
    So filter -> MAR -> (second filter -> second MAR , if needed).
    The Mar will compensate the filter losses and give some gain.

    Greetings,
    Bertus
     
  20. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    I see. Thank you. :)
     
Loading...