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RF Amplifers vs. Op Amps

  1. Jul 10, 2013 #1
    Sorry if this seems like a simple question, but I am new to RF systems.

    Is there a difference between an RF amplifer and a single rail operational amplifer? Or can you boost the power of an RF signal with a generic op amp?

    Also, when dealing with an RF signal, are coax cables and SMA connectors more ideal (esp with higher frequencies)?

    Thanks,
     
  2. jcsd
  3. Jul 10, 2013 #2
    Op amps are typically way too slow to amplify an RF signal. A properly compensated op-amp amplifier has a constant Gain-Bandwidth product. Since these op amps have very high gain, they have low bandwidth.

    For RF you either have to buy an RF amp or make one yourself using an RF transistor.

    And yes, properly terminated coax and SMA connectors are best for RF. Be careful that you know the characteristic impedance of the coax you want to use. If it is intended for cable TV it may be 75 Ohms and you would need SMB connectors designed for 75 Ohms (I think SMAs only come in 50 Ohms).

    Good luck!

    Carl
     
  4. Jul 10, 2013 #3
    RF amps are specifically designed to deal with voice modulation levels an op amp could be used however would not work nearly as well particularly with the higher frequencies.

    For cabling you want 50 ohm cable do not use RJ-6 which is 75 ohm. You will damage equipment through impedance mismatch. RJ-58 is a good cable for short runs LMR400 and superflex cable are larger diameter cables for larger runs. Lower loss preferable, of which their are numerous 50 ohm cables to choose from. Yes its all coax.
     
  5. Jul 10, 2013 #4
    Carl,

    Thanks for the reply. That makes much more sense now. Could you recommend any RF amps or RF transistors you have used in the past? There are so many lol. My application has a frequency of around 2000 MHz and I am looking for a gain around 15-20 dbm.

    Any suggestions would be appericated.
    Thanks for the help.
     
  6. Jul 10, 2013 #5
    Another quick question regarding the cables. I have the 50 ohm coax (RJ-58). What is the best way to connect a wire to a coax cable.

    In other words, the signal I recieved out of an IC chip (mixer) is just on a standard IC pin or wire. I want to get that into a coax cable. What is the best way to make that a solid connection. Or is that even possible?

    Thanks again for the help.
     
  7. Jul 10, 2013 #6
    The Analog Devices RF differential amplifiers are works of art. They are as easy to use as op amps. I highly recommend them.

    http://www.analog.com/en/specialty-...al-amplifiers/ada4960-1/products/product.html
     
  8. Jul 10, 2013 #7
    The best way is to use a small PCB with the chip and a trace going right to a BNC connector as close to the pin as possible. If it MUST be free space just solder the wire to the BNC connector's conductor. Be aware that you will be picking up a TON of noise and interference.
     
  9. Jul 10, 2013 #8
    Great! Thanks for the recommendation. Okay I was going to try something like that but I was afraid there was going to be a ton of noise. Is it possible that Ferritte beads would help that at all, or an I am doomed to have the noise? lol
     
  10. Jul 10, 2013 #9
    If you can do a PCB you might be OK. You just don't want a lot of unshielded wire. If you make it short enough twisted pair could work, too.
     
  11. Jul 10, 2013 #10
    Great, I will give it a try. Thanks for the help!
     
  12. Jul 10, 2013 #11
    ferrite beads do help but not by much your better off making a bandpass filter for the desired frequency range and filter out the unwanted noise. If say your filtering for 150 mhz as the carrier you will need a 25 khrz range plus or minus for voice modulation and privacy tones if applicable. Their are duplexers designed for RF uses that are adjustable if you have the right test equipment if not get the manufacturer to tune the duplexer.

    edit just saw the above post which is also good advice on the sheilding

    http://en.wikipedia.org/wiki/Duplexer
     
    Last edited: Jul 10, 2013
  13. Jul 10, 2013 #12

    davenn

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    Science Advisor
    Gold Member

    Hi amanno

    just something for your learning :)
    power gains/losses are measured in dB, actual power produced is measured in dBm
    0dBm = 1mW, 10dBm = 10mW, 20dBm = 100mW, 30dBm = 1W etc

    for 2GHz that you are interested in there's lots of excellent MMIC's from places like Minicircuits
    These can easily be cascaded ... there is a large variety with various gains and frequency responses and
    noise figures.
    MAR1 - MAR8; ERA1 to ERA5 just to name 2 of the series

    here's a pretty conventional way to mount SMA connectors on a PCB.....

    attachment.php?attachmentid=60193&stc=1&d=1373514494.jpg

    here's a pretty conventional way connect coax to a board ....
    this is actually a 2GHz synth that I deal with

    attachment.php?attachmentid=60194&stc=1&d=1373514882.jpg

    Teflon dielectric coax is the preferred type as you can solder all the way around the braid to the ground plane

    cheers
    Dave
     

    Attached Files:

  14. Jul 10, 2013 #13

    davenn

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    Science Advisor
    Gold Member

    here's a really nice 2GHz directional coupler using both N type and SMA type connectors

    attachment.php?attachmentid=60195&stc=1&d=1373515448.jpg

    big secret when soldering centre pin/flat tab of connector to the PCB track is to have as little
    solder as possible... a process called "sweating". This is where you tin the connector pin and the track bring them together and heat then to the tiny amount of solder there melts and flows
    there is absolutely NO EXCESS solder ... that is, no big blob/mound of solder

    This becomes more critical the higher in frequency you go.

    Dave
     

    Attached Files:

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