RF Amplifier for AM Radio

Discussion in 'The Projects Forum' started by BzkingUber, Apr 16, 2014.

  1. BzkingUber

    Thread Starter New Member

    Apr 16, 2014
    3
    0
    Hi All,

    Attached is my design for an RF Amplifier for an AM Radio using an LT1222 OP AMP.


    This RF design is calculated to give a 40dB gain (100V/V) output @ 1.4MHz, however with this configuration and providing 1.4MHz via function generator the output waveform is attached and does not give the desired output.


    I would appreciate any help on this matter, also the unused pins are not grounded, not sure if that would be the issue. I am also posting a link to the LT1222 datasheet: http://cds.linear.com/docs/en/datasheet/1222fc.pdf

    Again I very much appreciate any help.
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,395
    497
    From datasheet, page 6:
    "Feedback resistors greater than 5k are not recommended because a
    pole is formed with the input capacitance which can cause
    peaking or oscillations."
     
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  3. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    It's not "an RF design's bootlace!"

    That aside,it has close to the gain you wanted,but the wave form seems to be covered with higher frequency crud.

    Check what the 1.4MHz signal looks like out of your FG.

    Don't try to look at so many cycles of the signal -----one or two will suffice.

    If the input is clean,have another look at the output,the "noise" may be an aliasing effect on your 'scope.

    What is your application?

    If it is meant to be an RF preamplifier for a Receiver,the problem is that it will amplify any bit of RF or other noise within its passband,to the detriment of your wanted signal.

    The required signal will also be a lot lower in level than 100mV,so the gain will not improve your signal to noise ratio.

    If you are trying to use it as part of a Transmitter,the problem of its wide bandwidth remains.

    If you have a look at real RF Amplifiers,you will notice that general purpose Op Amps are not usually used.
     
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  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    Yes make Ra 5k & Rb 51 ohms and add 10pf to pin 5 to ground. Add some good high frequency decoupling to the power supply while your at it. It would help to know the application, but this might stop the oscillations at least.
     
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  5. BzkingUber

    Thread Starter New Member

    Apr 16, 2014
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    Guys thanks for the feedback, I will try these changes and post the results
     
  6. BzkingUber

    Thread Starter New Member

    Apr 16, 2014
    3
    0
    Also in response to application, the RF amp will be fed an antenna instead of function generator signal. And the output of the RF is going to a bandpass filter. Also if you dont mind me asking, what does the capacitor to pin 5 help with?

    Thanks
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,395
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    Read section titled Compensation on page 7 of your datasheet.
     
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