Monday, December 8, 2014

RF-AMP designing problems


On the Indian Homebrewers facebook groups posted this Indrajit Sinh
I want to try this ckt, as it does not need toroids. hope simple balun cores will work. i have 36volts power supply ready. dsb,cw tx produces about 2-4 watts. From your view points am i missing anything ? or suggestions please. ckt is on workbench.....thanks.
I want to try this ckt, as it does not need toroids. hope simple balun cores will work. i have 36volts power supply ready. dsb,cw tx produces about 2-4 watts. From your view points am i missing anything ? or suggestions please. ckt is on workbench.... I NEED MORE THAN 20watt.thanks.



    PLEASE NOTE THAT ABOVE CKT DOES NOT WORK.

  • I also added that how much power should i expect ? i have big heatsink with fan. what will be tune up proceedure of gang capacitor. this is rough sketch, yellow lines shows my intended modifications. i am soldering it. Not finished yet,...

    Ashhar Farhan the output circuit is a series trap connected to the ground! thats a terrible thing to do (it passes the rf to ground at the operating frequency! i would suggest that you could use a tv balun or even an tap washer. this will also need a low pass filter in the output. it not only clears the output harmonics, it absorbs the stray capacitances of the bifilar output transformer.

    Indrajit Sinh Thanks Ashhar Farhan May be i will try put PI or LC filter. Yes, sir tap washer i have tried in 5watt amplifier. It works. due to 30 swg wire it becomes very hot. This time i wanted the more than 20watt and did not know whether tap washer will work at these power level !

    Dev Rama Prabhu Rathorejee, there is one QRP boots circuit in your Handbook which also does not need Toroid's! By replacing his RF transistor with IRF510 we can easily get up to 35 watts with 24 volts supply, except that it is only 40 MTR band! I have tried that circuit and shall post a foto tomorrow! Need to take care of capacitors, higher voltage better!

    Indrajit Sinh I am amazed at the whealth of much needed information available with this group. Thanks VU2DEV sir, looking for it. 40 meter is also okey. I need something consistent which when fired give me an assured contact with friends.


    Nidhi Rajeeve Buying some good toroids and following standard proven circuits will avoid wastage of time and money. Moreover proven circuits will work like a charm. Its high time,that we should junk PVC gang caps and go for reproducible home-brewing.

    Aravind Balasubramanian This is where kits make life easy Try joining the Siars Chennai VUSDR Kit group assembly which also has seperate PA board with Lowpass filters covering all the HF bands

    Indrajit Sinh Aravind Balasubramanian digital is out of reach for me. This is my desperate attempt to get a few cw/ssb/dsb contacts before i once again go QRT. It seems there are very few peoples, who build make RF PA.
    3 hrs · Like · 1

  • Indrajit Sinh Nidhi Rajeeve, i dont have any fobia or toroid fobia or dislike for digital, as simple brewer i want something which can be reproduced by anyone. I make and if it does not work then tell the crital report in a clear impartial words.... may be what i do is a re-produceable one . is it not so ? TAHIR we have see the path of signal , from where RF is coming and where RF is going. FARHANS comment is right. At the series LC the impedance will be minimum and rf will follow the minimum losss path i.e ground instead of going to hi-z the antenna connected at the tap. In the filter rf is experiencing the impedance and only way out easily is the antenna. So , i doubt there is little drawing mistake or 2vkc might have modified but not cared publish it. To put end to our discussion only the 2vks or 3nsh can correct our doubts. Thanks to all. PY2OHH, ararinha, first version has the solutions. It used the simple coil for impedance transformations. Once again thanks all. de vu3inj.

    Ron Mallik This will not work: the output is a grounded series resonant network! RFC from drain to Vdd and then a l or gamma network, followed by a 5 pole lpf, can be managed with air core coils on discarded white board marker pens as former. In your case: load impedance is higher than drain output impedance..so use netowrk 3 or 5...or whatever: here: http://home.sandiego.edu/.../e194.../jwmatcher/matcher2.html


  • Ron Mallik Use Drain impedance as Z0 = Vdd square / 2 Po....ignore drain capacitance for now....
  •  Finally this ckt was found to work for me....
  •  
    
    RF Linear
     
    here it is ...... writeup is on next blogpost.

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