Wednesday, December 9, 2015

Linear Amplifier Problem- 3

A friend experienced a problem and solved it. first image depeicts the problem and second image depicts the solutions.

Question :---
My friend Vadivel requested in the Indian Homebrewer forum ::  " Help Needed on MOSFET Linear amplifier. I'm using Linear amp used with Minima TRX. Its working very fine. But the low pass filter is getting too hot / smoke after TX of 10~20 secs. I used 2x TV balun glue together and 3 turns of 28 SWG. SWR is corrected with the use of Pi network ATU. Any suggestions welcome. "

 The problem:---

The RF theory :---

The low pass filter was made with the pig nose baluns. But, IMHO opinion it can not handle the passing of 15watt rf.  If  core gets burned or becoming hot, its probably got damaged due to the heat. Please do not use it further. discard that piece and never use this baluns as filter eliment. they can handle just 1watt.

In my experiments I have found that, You need atleast minimum 4 big pignose baluns stacked for linear amplifier transformer.

The solutions :---

The coil is made over the felt pen body. The sketch pen body is the beast material to make the coil for low pass filter. Its diameter is uniform, widely available and most of the time they are in standard size.

A friend suggested  to use air core. And our great elmer shri  Dev Rama Prabhu say's, let me reproduce in his own word ::
Dear sir, if any inductor core is becoming hot it means it is very lossy! Just replace with one more core as these cores are not repeatable! Otherwise as Mr Rajesh has suggested use aircore and I always use aircore with Teflon formers. 73

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