[Amps] 160m mosfet linear amplifier problem with ferrite cores

[sasas asasas]

for 1-2mhz operation, what kind of material do you propose? 31, 43 or 77 material???

--- On Mon, 4/19/10, alexeban <alexeban at gmail.com> wrote:


From: alexeban <alexeban at gmail.com>
Subject: RE: [Amps] 160m mosfet linear amplifier problem with ferrite cores
To: "'David Cutter'" <d.cutter at ntlworld.com>, "'sasas asasas'" <tzitzikas_ee at yahoo.com>, amps at contesting.com
Date: Monday, April 19, 2010, 12:15 AM








Guys, the problem is more pernicious than that!
The problem is in the way the transformers are WOUND!
Efficient high power transformers are wound using transmission line coupling, not flux link coupling; they use coaxial line windings, with a very specific Zo. This technique ensures very tight coupling between primary and secondary, thus relieving the core from carrying high magnetic flux. The ferrite only serves to increase the reactance of the windings at the lower frequency end. Actually ferrites with a Ur of about 125 are sufficient in the HF range.
You can find a very good paper on this in Jerry Sevick's , W2FMI - sadly, an SK by now- which should quench a lot of the nonsense prevalent on the matter.
As a rule the optimal Ur for the different bands is approximately:
2-30 MHz    ≈125
10-100MHz  ≈25
100-250 MHz ≈10
Size of the core is dependent more on the peak amplitude of the applied signal than on the power; that is, if the transmission line winding technique is employed.
Alex     4Z5KS
 
-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On Behalf Of David Cutter
Sent: Sunday, April 18, 2010 9:26 PM
To: sasas asasas; amps at contesting.com
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite cores
 
Sasas
 
I looked at the ARRL site for a similar power amplifier to compare notes. 
http://www.w6kan.com/ssamp.html
The devices are completely different and so is the supply voltage, but the 
push pull arrangement is comparable and in this the transformer we are 
talking about is made using type 43 material, though it uses a 2-hole balun 
type Fair-Rite part # 2643540001 beads. .  It makes interesting reading, but 
I don't feel at liberty to quote verbatim.  It could be that another 
material, perhaps the newer 31 would perform even better, but I see no 
reason to replace the type 43 or the transistors until you have found the 
real problem.  Perhaps a binocular core might work better, or 2 of your type 
side-by-side to provide more reactance at the lowest frequency.
 
 
Sasas said:
I am using 12 Siliconix's IRFP360 mosfets, but i have order and soon i will 
replace them with International Rectifier IRFP360 mosfets. Propably this is 
the reason, that the amplifier don't give many watts at outupt.
The other reason, is that the material of ferrite cores (43 material) is not 
the proper for 1-2mhz frequences. What do you think?
 
 
David
G3UNA 
 
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