Problem with output power-linear amplifier

Discussion in 'Wireless & RF Design' started by tzitzikas, May 20, 2010.

  1. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
    41
    0
    hi, i have construct this linear amplifier for mw band: http://tzitzikas.webs.com/linear500w.jpg with 12 mosfets IRFP360.
    some photos from my project here:
    http://tzitzikas.webs.com/linear_photo_1.jpg
    http://tzitzikas.webs.com/linear20.JPG
    http://tzitzikas.webs.com/linear20a.JPG
    http://tzitzikas.webs.com/linear21.JPG
    http://tzitzikas.webs.com/fet21.JPG
    http://tzitzikas.webs.com/pompos20.JPG
    http://tzitzikas.webs.com/T3.JPG (photo of /t3 broadband transformer)

    for ferrite beads i am using FB-43-7351 from https://www.amidoncorp.com/items/44 but i have also orderedFB-43-7352 and the FB-43-1020 ferrite beads.
    The manufacturer claims that with only 2w input r.f power it gives 500w at the output at 110vdc (13A current). But with 5,5w input power at 106vdc my linear gives only 220w output r.f power (7A current) at dummy load 50ohm.
    (here you can see a photo from prototype: http://tzitzikas.webs.com/linear_first.jpg )
    Also, the other problem is that the T3 transformer and the resistors R9,R10,R11,R8 and the r.f choke VK200 are very warm when the linear operates.
    But the basic problem is to increase the output power.
    The mosfets are from Siliconix but i have ordered and i will try to use International Rectifier's IRFP360 mosfets.
    Do you have any idea what is the problem for the low output power? Do you have to propose any change to broadband transformers to increase the output power? I tried to change the value of C4=3,9nf capacitor but the output power was more low.
    Thank you.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Are the mentioned resistors induction free?
    The VK200 inductor is rated for 3 A continuos and 5 A peak.

    Bertus
     
  3. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
    41
    0
    hi. all the resistors are carbon.
    Could i increase the output transformer T1 number of turns, but keep the ratio 1:2 constant, to increase the output power? It will help? (4 turns primary and 8 turns secondary or 5 turns primary and 10 turns secondary)
    Or to increase the secondary number of turns of T3 (with 25ohm coaxial cable)?
    If i use bigger ferrite beads?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Wich type of carbon resistors are they?
    There are carbon resistors that have a spiral cut in to get the correct value.
    That type has some induction.
    The resistors in the attached datasheet are induction free.

    Bertus
     
  5. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
    41
    0
    ok

    ifixit :

    Could i increase the output transformer T1 number of turns, but keep the ratio 1:2 constant, to increase the output power? It will help? (4 turns primary and 8 turns secondary or 5 turns primary and 10 turns secondary)
    Or to increase the secondary number of turns of T3 (with 25ohm coaxial cable)?
    If i use bigger ferrite beads?

    about choke vk200: i am thinking to change the vk200 with a ferrite bead FB-43-7351 (i have many of them) . how many turns i must use to have the same inductance with the vk200?

    If i use 4 vk200 (in parallel ) i will use the same inductance or 1/4 inductance, It will work?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you know wich type of VK200 choke you are using?
    In the PDF there are 8 different types.

    Bertus
     
  7. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
    41
    0
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The picture does not tell me anything about the core material.
    In the table you can see there are different types of W core material used.

    Bertus
     
  9. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    If the problem of low power is with the T1 transformer not having enough inductance, then adding more turns could help get more power. If the problem is core saturating, then using a larger core could help.

    It is difficult for me to trouble shoot from here.

    Please Note: I will be off-line until June 6th. Vacation time.

    Regards,
    Ifixit.
     
    tzitzikas likes this.
  10. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
    41
    0
    thank you. good vacations my friend
     
  11. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
    41
    0
    i am thinking to change the ferrite beads with bigger to reduce the losses and to increase the number of turns at T1 and T3 transformers but i will keep the ratio constant.
    I am thinking at T3, to put 2 turns primary and 10 turns secondary (coil with 25ohm coaxial cable) and at T1 to put 4 turns primary and 8 turns secondary or 5 turns primary and 10 turns secondary. What do you think? These changes will help to increase the output power?
     
    Last edited: May 23, 2010
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Take a look at the attached PDF.
    In there is also a section about RF chokes.

    Bertus
     
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