20db VHF amplifier help!!

Discussion in 'The Projects Forum' started by EEUTSA2014, Aug 4, 2012.

  1. EEUTSA2014

    Thread Starter New Member

    Aug 4, 2012
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    Im trying to prove that the circuit for a 20db VHF Amplifier does exactly whats its suppose to. Ive been working on this for a week now and i cant seem to get the db vs frequency graph to show there is in a face a 20db gain at the vhf bandwidth. Im thinking it has something to do with my input signal and output load resistance. But im not really sure. Im using Orcad PSPICE and Muiltisim. All and any input is greatly appreciated! Here is a website that shows the exact circuit im trying to simulate and prove.
    http://www.diy-electronic-projects.com/p80-20dB-VHF-Amplifier
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
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    Looking at the circuit I don't see how you can get much gain due to the large amount of negative feedback provided by C3 and C6 at high frequencies. :confused: Try removing those and see what the simulations show.
     
  3. EEUTSA2014

    Thread Starter New Member

    Aug 4, 2012
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    Yea ive getting negative gain actually. My professor swears it works but refuses to help me.lol Ill try removing those. what do you recommend i use for my source? Should i insert the Rload before or after the C7?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    If your professor swears it works, have him show you a working circuit and explain how it can with so much negative feedback in each stage. It's the same as having a very large amount of Miller capacitance. Inquiring minds want to know. :rolleyes:

    Typically real VHF sources have a 50 ohm series resistance.

    The load should be at the output of C7 to ground.
     
  5. EEUTSA2014

    Thread Starter New Member

    Aug 4, 2012
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    Ive already tried. He refuses to give me any hints or examples. Im on my own on this one.lol Ok so a 50ohm source resistance and load resistance. Im using an AC Voltage signal, is that correct? If so what voltage should be applied? Thanks again for your help.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It's possible that parasitic inductance in the feedback path is behaving like a peaking coil. If the layout is poor enough it may be possible at VHF - but I would be surprised.
     
  7. #12

    Expert

    Nov 30, 2010
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    The AC input signal must be at least 20db less than 12V peak to peak...right?
     
  8. EEUTSA2014

    Thread Starter New Member

    Aug 4, 2012
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    Ok so i finally got it to work. Im still having trouble understanding how the circuits works. Can anyone offer a explanation. thanks!
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You got it to work with the capacitors?
     
  10. EEUTSA2014

    Thread Starter New Member

    Aug 4, 2012
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    yep, i dont know if its completely correct though.
     
  11. vk6zgo

    Active Member

    Jul 21, 2012
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    It is not usual practice to quote gain figures in dB without knowing the input & output impedances of the device.

    If they are the same,you can use the voltage formula : 20log v1/v2.
    If not,you need to calculate power in & power out & use10log p1/p2.

    Your simulation program may not like it if you just tell it the input voltage & "20 dB".,

    Of course,if the following device has a very high Z input,w.r.t both the original source & the amplifier output Z,it won't much care what impedance it is bridging,& you can just go with the voltage gain,although it is technically incorrect.
     
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