i can't earn rf bjt amplifier gain as same as gain in datasheet!!

Discussion in 'Wireless & RF Design' started by alitex, Mar 23, 2019.

  1. alitex

    Thread Starter AAC Fanatic!

    Mar 5, 2007
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    hi all
    i made class a vhf amplifier using bfg97 but i can't earn gain as noticed in datasheet. i got only (17db) but in datasheet and s2p file i should get 29db @ 100MHz.
    i also make another one using 2sc5085 but also i got only 16db but in datasheet it should 26 db.
    i add impedance matching to make bfg97 stable @ 100MHz using ADS software to input and output and i got approximately good results(32db) using nonlinear model from NXP.
    any one know why?
    thank you so much.
     
  2. alitex

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    Mar 5, 2007
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    may i know if any one can help?
     
  3. AlbertHall

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    Well, I'm no RF expert but have you taken into account how much of your input signal is lost in the base coupling and bias components (it will not be zero), and likewise for the collector load and output matching components.
     
  4. alitex

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    Mar 5, 2007
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    thank you AlbertHall.
    in ADS i got about 30 db gain and after added input and output matching i got 36 db. in s2p file from NXP company written 30db @ 100MHz.
    i got only 18 db in practical circuit with matching. i don't know?
    can you tell me where is the problem?
    thank you in advance
     
  5. AlbertHall

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    I'm afraid you will have to wait until someone who knows what they are talking about comes along.
     
  6. alitex

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    is my expression not understandable?
     
  7. AlbertHall

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    The fault is in my understanding not your expression.
    I am nobody's idea of someone knowledgeable about RF.
     
  8. alitex

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    Mar 5, 2007
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    is it possible there is no one expert in RF of all (all about circuit) forum?
     
  9. cariban

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    Aug 14, 2018
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    What type of capacitors are used for the impedance matching? It is not very clear in the figures. You need to check if the resonance frequency is well above 100MHz.

    Also it is much better to make a serious layout for RF amplifiers. The tracks should be strictly impedance controlled. 100MHz is not low frequency any more. Using fly wires sounds a bad idea. You may observe that the gain may change just by pulling some wires up and down.
    The component size should be reduced as well. For this frequency, EIA-0402 usually is a good choice.

    If your project is in a hurry, you may also order the evaluation board and do test/tuning there.
     
  10. alitex

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    Mar 5, 2007
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    thank you so much Cariban for replying.
    i used smd 1206 capacitor for impedance matching. and i used ceramic capacitor for (dc-block ) and bypass. is there mistakes for this kind of capacitors?
    how i can check resonance frequency? and how to make serious layout please?
    and may i know what is the evaluation board??
    thank you i nadvance!
     
  11. cariban

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    Aug 14, 2018
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    Just had a quick check the NXP homepage. bfg97 is a very old chip and the status is "end of life". It may be even not RoHS compliant. Are you sure you still want to use this chip? There are tons of other amplifiers you can use for 100MHz.

    The datasheet was last revised in 1995, 24 years ago!
    https://www.nxp.com/docs/en/data-sheet/BFG97_CNV.pdf

    But if you really get stuck with this chip, refer to Fig. 3 in the datasheet for proper layout. Once again, I would strongly suggest to use another chip.
     
  12. bertus

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    alitex likes this.
  13. alitex

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    Mar 5, 2007
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    i appreciate your help, thank you so much.
    i'm going to make the proper layout as fig. 3 in the datasheet, and try to measure the gian.
    thank you so much.
     
  14. alitex

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    Mar 5, 2007
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  15. BobaMosfet

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    Jul 1, 2009
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    @alitex

    Just wanted to say thank you for posting images, and _a schematic!_ and other test result info.... really, really nice. Nobody minds a hand-drawn schematic if everything's legible, and yours is. The other schematic on the computer was nice, too.

    Your capacitors and inductors both serve (being field devices) as a means to control current flow, usually in opposition to one another. Having both can make it complicated. Beyond that, the BJT is controlled primarily by the bias on the base in accordance with the q-point so the wave-form isn't clipped on either side. Have you checked your output from the transistor to see if signal is getting clipped?
     
  16. alitex

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    thank you for giving support!
    when i applied -47dbm i got -29dbm only (only 18 dB @ 100MHz), when i increase the power for input about 0dbm i got only 13dbm. i think at this point it happen clipping for output waveform and decrease the gain,
    i used gummel-poon model for bfg97 bjt from NXP in ADS. after simulation i measured S-Parameters and i got so close values to S2P file @ same frequency and Q point and i got 36dB Gain. but in practical circuit i only got 18dB.
    i'll try to do same layout in the datasheet and use 0805 capacitors and try to measure the gain.
    thank you so much BobaMosfet and all friends that try to help!
     
  17. loulou31

    New Member

    Feb 23, 2018
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    Hi the two inductors in the top are quite big and close and coupling between have an effet of negative feedback ( collector to base) that could reduce gain. Chech your capacitor, for RF use use ceramic NPO model . For decoupling reduce length of wire.
    Jean-Louis
     
  18. alitex

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    Mar 5, 2007
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    ok loulou31, i'll try to do another circuit. thank you so much
     
  19. alitex

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    Mar 5, 2007
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    hi all again!
    i have made RF layout for BFR96 amplifier @ VCE=10V,IC=70mA and freq=100MHz.
    i got 21.5 dB power gain. after i add variable capacitor for impedance matching i got 23.5 dB.
    i think it's good result!
    what do you think friends?
    thank you so much
     
  20. cariban

    Member

    Aug 14, 2018
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    Good job! You may measure the returnloss of both input and output ports if you have VNA in hand. This helps to assess if there is room for further improvement.
     
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