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MOSFETs vs. Pills

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MOSFETs vs. Pills

Post by Shaft » Wednesday, 04 February 2009, 14:13 PM

Can any one tell me why amps use the pill type transistors instead of a bank of MOSEFT type? It seems that an amplifier made with MOSFETs would run cooler and could be smaller in size.
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Post by HI-TECH » Wednesday, 04 February 2009, 14:49 PM

because the mosfet's in our neck of the woods (11m RF) is just now starting... there a little more expensive to produce then you would a homemade 2 or 4trasistor amp.... but they deff do run cool and dose not use crap for power. i had a 300W mosfet amp from a former sponsor think only drew maybe 5 amps @ peak on the high side...low side i could run both the radio and my amp off my other 5amp power supply and be doing 130 watts it was awesome :lol:
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Post by Cain64 » Wednesday, 04 February 2009, 14:52 PM

ya, if there were plans to build a MOSFET amp i would jump on that. Probly would be some what simpliar then a regular pill amp.
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Post by treetall » Wednesday, 04 February 2009, 15:41 PM

man i havent even thought about an mosphet amp. but i think ill stick with my pills cause i have a feeling a pill is cheaper and more common to replace than a mosphet but it would be interesting to see one of those boxes like you were talking about Hi tech
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Post by Foxhunter » Thursday, 05 February 2009, 1:32 AM

There are a number of commercial RF amplifier manufacturers already using FET or MOSFET type transistors in their RF amplifier designs. How long until it becomes more widespread in the 11M world here is still a good question. Once again (and sadl)y, foreign makers seem to be leading the way.

While researching and following up on many of the RF amplifier builders and manufacturers in the "Amp Builder History?" thread, to my surprise I found a number of the Amateur Radio manufacturers were producing FET amplifiers. viewtopic.php?p=215858#215858

This German manufacturer Alpin below for instance produces a 1000W all MOSFET linear amplifier.
**ALPIN --by Reimesch Kommunikationssystemme aka SPE-- (ALPIN 100 / ALPIN 100 MKII / ALPIN 200 / EXPERT 1K-FA *three versions)
http://www.reimesch.de/expert_en.html

Also RM Italy, which has many amplifiers available here in the US (they are HUGE overseas) actually have many in their product line that are MOSFET powered. They provide both HF & VHF amplfiers which include Citizens Band.
RM Italy (KL 35 / KL 40 / KL 60 / KL 200 / KL 200 P / KL 203 / KL 300 / KL 300 P / KL 400 / KL 500 / KL 300 24 / KL 500 24 / KLV 350 / KLV 400 / KLV 550 / KLV 1000 / KLV 1000 P / KL 144 / KL 155 / KL 145 / KL 145H / VLA 100 / VLA 100 1 / VLA 200 / VLA 200 1 / VLA 200 2 / V ULA 50 / KLV 250 / K 27M / 27 586 / KL 43 / KL 243 / KLV 250 VIP / KLV 200 VIP / KLV 350 VIP / KLV 400 VIP / KLV 1000 VIP / KLV 1000 P VIP / VLA 200V / VLA 100V / HLA 150 Plus / HLA 150 V Plus / HLA 300 Plus / HLA 300 V Plus / VLA 150 / BLA 300 / KL 501 / KL 501 VIP / KL 800 / V-ULA 50)
http://cbradiomagazine.com/May%202007/K ... lifier.htm
http://www.rmitaly.com/home.asp

There are many more also doing the same. I wonder if it will ever get to the "MOSFET Comp Box" level with "custom-built" CB amps.

Additionally, from reading it seems there possibly is an advantage to be gained in the use of a IR HEXFET transistor over the MOSFET type. Regardless, what Shaft was saying is an absolute as far as the advantage the MOSFET has over the bi-polar Toshiba type transistors we all now seem to be enjoying, using, and sometimes burning up. For any CBRT Forum viewers or visitors, the following excerpt maybe best sum up why.
Secondary Breakdown

An attraction in choosing the MOSFET transistor in preference to the bipolar transistor is the absence of second breakdown. An explanation of this, extracted from the IR handbook, is given in the following paragraph:

One of the outstanding features of IR's power MOSFET is that they do not display the second breakdown phenomenon which is frequently the Achilles heel of bipolar transistors. A simple physical explanation accounts for this superiority. If localized potentially destructive heating occurs within a MOSFET transistor, the carrier mobility in that area decreases. As a result the MOSFET has a positive temperature coefficient and acts in a selfprotective manner by forcing currents to be uniformly distributed through the silicon die. In contrast and particularly under conditions of high collector-emitter voltage, a bipolar transistor displays "current crowding" in the base region. This causes hot spots. Because of the bipolar's negative temperature coefficient, these hot spots tend to further "hog" the current and cause instantaneous catastrophic destruction of the die.

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Post by zman350 » Thursday, 05 February 2009, 4:53 AM

here is something to look at.


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Post by Foxhunter » Friday, 06 February 2009, 19:04 PM

Hmmm.......wathca got there Zman? What type of transistors and what's the expected output. That there is something you don't see everyday!

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Post by North Texas Mudduck » Friday, 06 February 2009, 19:11 PM

hey that one looks almost like this one
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Post by lonesome 500 » Friday, 06 February 2009, 19:24 PM

both transistors........work on ohms law

size and ease of manufacturnig

mosfet will really pop......when someone builds a base box...at 24 or
36v....because there are single mosfets that will disapate 300 plus watts each

they go easily into oscillation
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Post by drdx » Friday, 06 February 2009, 20:04 PM

I believe Tokyo hi-power builds kw plus amps using them, and many newer ham rigs use them as well. I'd guess it is just a matter of time. It will be funny when the 2879 is seen as "old school" like we see tube stuff today. Will that ever happen? Who knows, but stuff changes all the time. Years ago, 100 watts mobile was a big deal, look at it now. That's MAYBE the deadkey on a guy's driver these days. I got into a conversation with 2 other mobiles today. One guy had a 500 watt carrier, 1500 swing, the other a 400/1000 setup, and neither are considered real players. What will it be in 20 years?

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Post by Gummybear » Friday, 06 February 2009, 21:30 PM

All I want to know is where do i send my income tax money???
More power with less power and runs cooler??? sign me up.

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Post by lonesome 500 » Saturday, 07 February 2009, 8:05 AM

More power with less power and runs cooler???
no such thing......no getting more energy out than you put in....on anything

may run cooler.......but go easily into oscillation
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Post by Gummybear » Saturday, 07 February 2009, 8:14 AM

HI-TECH wrote:.... but they deff do run cool and dose not use crap for power. i had a 300W mosfet amp from a former sponsor think only drew maybe 5 amps @ peak on the high side...low side i could run both the radio and my amp off my other 5amp power supply and be doing 130 watts it was awesome :lol:

Then what is he talking about??? sorry Hi-tech for throwing you under the bus... :D I just need the facts...

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Post by lonesome 500 » Saturday, 07 February 2009, 8:57 AM

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Post by Gummybear » Saturday, 07 February 2009, 10:14 AM

I understand says the link chaser. you guys always have the best links. I cant seem to find half the stuff you guys do.

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Post by plutonium233 » Saturday, 07 February 2009, 12:07 PM

Overdriving a BJT to gross levels will, in fact, cause "secondary breakdown" where Vce (collector-emitter voltage) goes to near zero and the Ic (collector current) runs away. This usually leads to what EE's call burning, or more commonly called blowing or popping the transistor, and it is due to exceeding the design limits of the transistor such as TJ(max) and Ic(Max). As Vbe increases from zero the transistor goes from cutoff, to the active mode of operation and then on to saturation(bad) then the secondary breakdown region(really bad). This stems from two factors in how a BJT operates which are time consuming to explain, but it has to do with the biasing of the base region of the transisitor and some complicated phyisical phenomena (such as tunneling) involving the carriers(holes/electrons).

This is not such a problem with MOSFETs since the the n-channel or p-channel can only reach a certain maximum width for any given gate voltage. The transistor operates in cutoff, triode, and saturation where saturation is the desired mode of operation for amplifiers. The drain current does have a design limit that can't be exceeded so you can still burn them; however, there is no gate current to speak of (on the order of pA and fA) whereas the gate current in an overdriven BJT just makes it dissipate more heat and lets thermal runaway occur faster.

The only way the MOSFETS will be better is if Pd (Power dissipated) is lower per watt out of the amp (PL). This just means it is more efficient. This could come from the fact there is no gate current to deal with. The other thing is that it may operate more efficiently with a given biasing (Class C, AB) than a BJT in the same situation. I am not sure since I haven't studied the use of MOSFETs as output stages in Class C settings yet. I'll have to dig out my book and see what I can find out anything else.

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Post by Foxhunter » Saturday, 07 February 2009, 15:12 PM

Wow, finding this thread really interesting.

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Post by plutonium233 » Saturday, 07 February 2009, 17:04 PM

plutonium233 wrote:Overdriving a BJT to gross levels will, in fact, cause "secondary breakdown" where Vce (collector-emitter voltage) goes to near zero and the Ic (collector current) runs away. This usually leads to what EE's call burning, or more commonly called blowing or popping the transistor, and it is due to exceeding the design limits of the transistor such as TJ(max) and Ic(Max). As Vbe increases from zero the transistor goes from cutoff, to the active mode of operation and then on to saturation(bad) then the secondary breakdown region(really bad). This stems from two factors in how a BJT operates which are time consuming to explain, but it has to do with the biasing of the base region of the transisitor and some complicated phyisical phenomena (such as tunneling) involving the carriers(holes/electrons).

This is not such a problem with MOSFETs since the the n-channel or p-channel can only reach a certain maximum width for any given gate voltage. The transistor operates in cutoff, triode, and saturation where saturation is the desired mode of operation for amplifiers. The drain current does have a design limit that can't be exceeded so you can still burn them; however, there is no gate current to speak of (on the order of pA and fA) whereas the gate current in an overdriven BJT (Pill) just makes it dissipate more heat and lets thermal runaway occur faster.

The only way the MOSFETS will be better is if Pd (Power dissipated) is lower per watt out of the amp (PL). This just means it is more efficient. This could come from the fact there is no gate current to deal with. The other thing is that it may operate more efficiently with a given biasing (Class C, AB) than a BJT in the same situation. I am not sure since I haven't studied the use of MOSFETs as output stages in Class C settings yet. I'll have to dig out my book and see what I can find out anything else.
I just made a couple minor corrections/additions. I also want to clarify for those who don't know:

BJT: Base, Emitter, Collector which is analagous to MOSFET: Gate, Source, Drain

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Post by plutonium233 » Saturday, 07 February 2009, 17:06 PM

I forgot to say that gate in red above should be base since it is a BJT.. I wish is could have just edited it instead of posting again..

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Re: MOSFETs vs. Pills

Post by madsage » Thursday, 05 March 2009, 8:57 AM

Also keep in mind, OHMs Law only applies to DC power. Not AC RF
We have to use RMS values which is AC power in DC equivilence.

AC power in watts at RMS value is equal to DC power.

1kw RF PEP is only equiv to about 400watts DC. depending on modulation and other variables.
You typicaly use a 1000hz tone audio signal for reference point.

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Post by 216 » Thursday, 05 March 2009, 10:14 AM

zman350 wrote:here is something to look at.


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Thats one of my experiments that worked out very well,16 IRF520's @60 cents apiece.
3.75 watts in gives 100 watts out,swings 400pep read on a Bird43.Ill sell it eventually and build another using a 4port splitter and combiner instead of 2.
Heres a front pic


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Another precision built amplifier from myself,regulated biased AB with thermal tracking at the driver and finals. 1x4,2290 driving 2879's.All holes drilled and tapped,transistors and heatsink lapped to make everything perfectly flat.No sheetmetal screws in my shop.

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Re: MOSFETs vs. Pills

Post by 127 » Thursday, 05 March 2009, 18:29 PM

It has been posted many times that Mosfets blow a lot easier than standard pills. Overdrive a mosfet and they're done.
Call me old fashioned but if it can't take a licking and keep on ticking, it's not for me.

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Re: MOSFETs vs. Pills

Post by lonesome 500 » Thursday, 05 March 2009, 19:34 PM

madsage wrote:Also keep in mind, OHMs Law only applies to DC power. Not AC RF
We have to use RMS values which is AC power in DC equivilence.

AC power in watts at RMS value is equal to DC power.

1kw RF PEP is only equiv to about 400watts DC. depending on modulation and other variables.
You typicaly use a 1000hz tone audio signal for reference point.

73 from
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huh?

ohms law applies to ac or dc......in current - voltage and output....or resistance

the reference was....you don't get ''more'' out than you put in/consume

and rms output....is the only thing i look at
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Re: MOSFETs vs. Pills

Post by 216 » Friday, 06 March 2009, 6:38 AM

RMS doesnt apply here unless you have an rms wattmeter? lol.every wattmeter i know of including Bird reads average or pep.

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Re: MOSFETs vs. Pills

Post by 127 » Friday, 06 March 2009, 9:49 AM

lonesome 500 wrote:
madsage wrote:Also keep in mind, OHMs Law only applies to DC power. Not AC RF
We have to use RMS values which is AC power in DC equivilence.

AC power in watts at RMS value is equal to DC power.

1kw RF PEP is only equiv to about 400watts DC. depending on modulation and other variables.
You typicaly use a 1000hz tone audio signal for reference point.

73 from
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huh?

ohms law applies to ac or dc......in current - voltage and output....or resistance

the reference was....you don't get ''more'' out than you put in/consume

and rms output....is the only thing i look at

Less power draw but more output? That guy named Ohm sure was wrong. :shock:

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Re: MOSFETs vs. Pills

Post by Foxhunter » Wednesday, 11 March 2009, 13:33 PM

You'd never get more out than what you'd put in-----so there is always a loss and efficiency factor to be considered. 100% or more efficency would be creating power. lol at posts or ideas to the contrary.

Otherwise, you'd have an amplifier capable of "Cold Fusion" style energy production! If you own or have made an amp like this let the world know----you'll be nominated for the Nobel Prize!

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Re: MOSFETs vs. Pills

Post by Crusher » Saturday, 14 March 2009, 13:18 PM

This is probably the most informative post I have seen in a very long time.

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Re: MOSFETs vs. Pills

Post by Gummybear » Monday, 06 April 2009, 9:52 AM

***BUMP***

Anyone learn anything new about these mosfet amps?
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Re: MOSFETs vs. Pills

Post by madsage » Wednesday, 08 April 2009, 7:31 AM

Foxhunter wrote:You'd never get more out than what you'd put in-----so there is always a loss and efficiency factor to be considered. 100% or more efficency would be creating power. lol at posts or ideas to the contrary.

Otherwise, you'd have an amplifier capable of "Cold Fusion" style energy production! If you own or have made an amp like this let the world know----you'll be nominated for the Nobel Prize!

Image

Cold Fusion

Yes most of us know ohms law formula, maybe i didnt articulate well that PEP is AC and does not apply to ohms law accuratly. it has to be converted to RMS for accurate caluclations with ohms law. As ohms law applies to DC power. AC must be converted to RMS for the power formula to be accurate.

Example: if my amplifier is PEP 1000watts at 12volt. I am not drawing 83amps. the RMS wattage would be closer to 400-600watts. (depending on modulation type, voice or 1000hz signal tone) so the amplifier would actually be drawing about 33-50amps not 83 as ohms law would suggest. Does that make better sense? Its been awile since my last electronics class. But i do recall, ohms law applies to DC and we must use RMS values for DC equivalence, not AC PEP.

You arnt geting more for less, as the PEP of voice modulation is in milliseconds, and then the next millisecond the signal maybe 0 and then the next millisecond it maybe 75% of the first highest peak... its all over the place! for DC equivalence or RMS you take the peaks and apply the root mean square formula. now you have RMS and ohms law will apply accuratly.

In mathematics, the root mean square (abbreviated RMS or rms), is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids. The RMS over all time of a periodic function is equal to the RMS of one period of the function. The RMS value of a continuous function or signal can be approximated by taking the RMS of a series of equally spaced samples.

Ohms law only applies to DC. Yes you can apply them to AC but it will not be accurate unless you use RMS values.

feedback welcome,

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Re: MOSFETs vs. Pills

Post by 'Doc » Wednesday, 08 April 2009, 15:26 PM

AC, DC and Ohm's law. Ohm's law applies to both. The 'trick' is in how it applies, and applying it correctly. That can get to be quite some 'trick'! There ain't no quick and dirty answer, unless you know what you're talking about. When you do know what you're talking about is the only time a 'short cut' won't put you in the toilet.
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Re: MOSFETs vs. Pills

Post by sparky17 » Thursday, 09 April 2009, 0:17 AM

Mosfets aren't always more expensive.

The RM Italy's use 70 cent mosfets. 4 of them equal one 1446, so $3 in mosfets is equivalent in power output to a $20 1446.

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Re: MOSFETs vs. Pills

Post by madsage » Thursday, 09 April 2009, 6:29 AM

'Doc wrote:
This $#;+ sure is fun, ain't it?
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yeah and you know were just scratching the surface, its gets much deeper. with inductive and capacitive reactances and impedances.
not to mention all the theory and math behind what the semicondictors (transistors/diodes) are doing for us... :alien:
I started school tryign to obtain a degree in electronics, i thought it would be cool to work in a lab with boards and components and test equiptment,
until i found out its mostly all math! hehe, i ended up majoring in beer drinking and college girls. (not a bad study really!) hehe, after school i ended up on an assembly line. :icon_e_surprised: then ended up in the computer IT industry later in life.

i still love it all, learning and remembering and applying this stuff when possible.
lets keep it going.

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Re: MOSFETs vs. Pills

Post by rubicon » Thursday, 09 April 2009, 7:00 AM

Ok here we go, the new guy here is opening his mouth.
Up here in the NWC two of us are experimenting with highvoltage mosfets. 300volts to be exact. Each transistor can produce 600 watts Bird Rms with a maximum of 20 watts rms driven swing input.
No transformer needed for these boxes they run off of 3phase 220-240 volts out of your electrical outlet, then internally bridged and rectified for dc.
There is currently a 2kw bird rms 4 pill in testing at a close friends house up here. Youll here him in DX land if you dont already.
The issue at hand with mosfets are the protection of the transistors during high reflect issues, fuseing is not fast enough to protect the trans. as of yet. When these mosfet transistors pop at 200 bucks each...... ouch is right
Just a tid bit of info as whats to come.
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Re: MOSFETs vs. Pills

Post by madsage » Thursday, 09 April 2009, 11:03 AM

very very cool rubicon.

looks like metal oxide fets will be the future as the materials for doping semiconductors are becoming more and more regulated. damn enviro-whacks!
Already, i've heard the 2sc's we used to get are not the same with substitute materials for the transistor junctions. And still they are getting more and more expensive. keep up the good work and tell everybody involved thanks for thier efforts. looking forward to see/read more on these new mosfet amps.

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Re: MOSFETs vs. Pills

Post by rubicon » Thursday, 09 April 2009, 15:21 PM

madsage wrote:very very cool rubicon.

looks like metal oxide fets will be the future as the materials for doping semiconductors are becoming more and more regulated. damn enviro-whacks!
Already, i've heard the 2sc's we used to get are not the same with substitute materials for the transistor junctions. And still they are getting more and more expensive. keep up the good work and tell everybody involved thanks for thier efforts. looking forward to see/read more on these new mosfet amps.

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Well I will pass that along to Prime, and thanks for the thumbs up on our efforts. The goal is to have the first 11 meter reliable 100kw solid state amplifier, fully protected from reflect issues and plug in the wall.
The last part perhapes might be the most challenging due to current draw and feeder service sizes limited to a 200 amp service to most residences.
Mr. Old school meets new school- mosfets and tetrodes united.
From up in Audio county and a member of the Broadcast Click in the NWC,
Droppin this Skullcracka Tetrode like an RF Frieght train.
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Re: MOSFETs vs. Pills

Post by executioner » Wednesday, 14 October 2009, 13:24 PM

rubicon wrote:
madsage wrote:very very cool rubicon.

looks like metal oxide fets will be the future as the materials for doping semiconductors are becoming more and more regulated. damn enviro-whacks!
Already, i've heard the 2sc's we used to get are not the same with substitute materials for the transistor junctions. And still they are getting more and more expensive. keep up the good work and tell everybody involved thanks for thier efforts. looking forward to see/read more on these new mosfet amps.

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Well I will pass that along to Prime, and thanks for the thumbs up on our efforts. The goal is to have the first 11 meter reliable 100kw solid state amplifier, fully protected from reflect issues and plug in the wall.
The last part perhapes might be the most challenging due to current draw and feeder service sizes limited to a 200 amp service to most residences.


P=I*V, 200amps rms X 220 v ac rms = 44000 watts rms this is the input power !

44Kw input and mosfet magic is 100kw output RF power ? Please say it anit so !

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Re: MOSFETs vs. Pills

Post by madsage » Monday, 09 November 2009, 21:24 PM

hey does anybody have any mosfet amp designs they could share?
i would really like to see a basic pushpull stage utilzing 4 cheap fets.

thanks in advance,

933 Arizona

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Re: MOSFETs vs. Pills

Post by Sheriff Bart » Tuesday, 10 November 2009, 8:10 AM

I'll be keeping an eye on this thread. Mosfets are definitely the way for the future..Is there enough for the Mods to make this a sticky ???

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Re: MOSFETs vs. Pills

Post by drdx » Tuesday, 10 November 2009, 8:59 AM

This does bring about interesting possibilities in regard to the evolution of amplifiers. I wonder if some day we'll have modfet "modules" that are easily replaced, on the cheap, when they get blown, like a tube. Everything else in the world seems modular. I guess it would be a matter of popularity and demand for development that would drive its progress, but it would be nice to have a high count mosfet amp with easy to replace components or better yet do it yourself modular amps that would reduce price and make it easy to sell on the open market.

One wildcard I keep seeing is the draw these amps have. Folks are claiming output amounts that exceed ohm's law. I'm wondering if this is the result of dirty signals on a cheap wattmeter creating illusions or what not. We need some really good non biased results performed by real life labs. Isn't Tokyo hi-power or somone already putting out mosfet amps? I haven't looked around enough but plan to. I'm patient enough to know that this will be a slow process to success but considering the speed of technology in this hobby in the past, this is moving at light speed in comparison.


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Re: MOSFETs vs. Pills

Post by drdx » Tuesday, 10 November 2009, 9:09 AM

I just did a quick look at some of the kw+ tokyo hy power amps and they are listed to have amperage draws that are similar to that of the current non mosfet amps on the market. That tells me that these claims of just a few amps for a few hundred watts is probably someone seeing garbage. That same operator will then claim "man, when I switched it on, my signal went up 2 s-units on the other end". I don't doubt it. Going from barefoot to a SMALL amp may do that. If you've got a 2 watt carrier, going to 75 watts will give you that little 2 pound jump.

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Re: MOSFETs vs. Pills

Post by Lost Ram » Tuesday, 10 November 2009, 15:07 PM

drdx wrote:I just did a quick look at some of the kw+ tokyo hy power amps and they are listed to have amperage draws that are similar to that of the current non mosfet amps on the market. That tells me that these claims of just a few amps for a few hundred watts is probably someone seeing garbage. That same operator will then claim "man, when I switched it on, my signal went up 2 s-units on the other end". I don't doubt it. Going from barefoot to a SMALL amp may do that. If you've got a 2 watt carrier, going to 75 watts will give you that little 2 pound jump.

-drdx
I have been slowly doing tests on the skywalker 300 I got a month or so ago and the test results are inline with ohms law!!! I have posted in the mosfet first look post and need to get my results recorded and posted with my test equipment. So far the testing done in my mobile with a galaxy dx55v (5-15 watts pep) show it to be 100% related to ohms law, it does not mythically draw less amps then my Fatboy amp does. Nor does it hit the 250-300 watts its rated for, about 125-150 peak and 75 watts RMS. I still need to get it on the bench and post some real controlled numbers.
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Re: MOSFETs vs. Pills

Post by Shaft » Tuesday, 10 November 2009, 16:42 PM

Sheriff Bart wrote:I'll be keeping an eye on this thread. Mosfets are definitely the way for the future..Is there enough for the Mods to make this a sticky ???

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Re: MOSFETs vs. Pills

Post by madsage » Tuesday, 10 November 2009, 20:35 PM

I found a project built by a ham for multi band use, using an IRF510. 30watts average out from a $1 fet @ 24v.
love his heat sink, remember those? cool little project.

http://users.belgacom.net/hamradio/sche ... lifier.htm" onclick="window.open(this.href);return false;

i'll try find some more in a push pull configuration.

933 Arizona

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Re: MOSFETs vs. Pills

Post by darkseid » Monday, 07 December 2009, 2:40 AM

drdx wrote: One wildcard I keep seeing is the draw these amps have. Folks are claiming output amounts that exceed ohm's law. I'm wondering if this is the result of dirty signals on a cheap wattmeter creating illusions or what not. We need some really good non biased results performed by real life labs. Isn't Tokyo hi-power or somone already putting out mosfet amps? I haven't looked around enough but plan to. I'm patient enough to know that this will be a slow process to success but considering the speed of technology in this hobby in the past, this is moving at light speed in comparison.


-drdx
I think your mixing up rf power with total power output from the transistors, for example one 2SC2879 keys down about 60 watts, 60 watts / 12.6 volts = 4.7 amps ...... There is noway 4 amps would work, however they are rated at Pout Watts 120W Pin Watts 7W = 127 watts total.... thus more amp consumption.

I haven't looked at any mosfet ratings, but I would bet they're no different.. to be honest a 100w mosfet amp shouldn't be hard to build.... I hope to see some DIY mosfet amp threads soon!!!
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Re: MOSFETs vs. Pills

Post by drdx » Monday, 07 December 2009, 18:10 PM

Maybe I phrased it badly. It is just that we're seeing a few reports of extremely low amperage draw for the wattage seen on a wattmeter, as in 5 amps for 100 watts or more, much less than the typical 10 amps per 100 watts ballpark typically scene on a traditional amp with 2879's, 1446's, or whatever. Still, I'm hopeful.

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Re: MOSFETs vs. Pills

Post by Slowdraw » Tuesday, 15 December 2009, 17:48 PM

Guess I'll jump in on this one. I experimented with mosfets (IRF 520's and IRF 530's etc) back in the day and even wrote and article for 73 magazine. It was a 40w amp project for ham bands. But I did design and build a couple for CB (don't tell 'em :-)

My experience:

In circuit and run well within operating parameters, MOSFETs can be bullet proof.

High voltage spikes of only microamps of current can nail them and let all the smoke out of them. This can occur when they are pushed to their limits with applied voltage or combination of high power, high swr or parasitic oscillations.

Tuned circuits worked the best for me. The tended to be stable and 1 single 520 was good for 40 or 50 watts and a 530 could hit 80 watts, and liked an 18v power supply. The IRF 530 worked much better on 80,40 and 20 meters and needed extra drive on 10 meters.

Broadband circuits tended to be problematic and often exhibited parasitic and spurious outputs. Never got one to run "clean" They would work, but anyone within a mile or so, could give up watching channel 5. :-) Mosfets designed for RF do not have this problem as they are optimized for their purpose. I played with some MRF 150s for a while and they were easy to match in a circuit. They just wouldn't out perform 2sc2879's, even at 24 volts, so I saw no purpose in using them except for the novelty.

The IRF 510 is probably more stable, but would produce less power out. I never experimented with them.

The most important thing I found was getting the source properly grounded. RF likes a good low impedance path to ground for good gain in a mosfet amp. I've seen some amps since then that found better ways than I did. There is more than one way to cook a goose!

These Mosfets run more efficiently on higher voltages. 18 volts or so seemed to be comfortable, but you have to monitor and adjust the bias voltage carefully and make sure you don't over drive them too much.

They are cheap to play with. Maybe someday I'll get the bug again.

Slowdraw

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Re: MOSFETs vs. Pills

Post by Lost Ram » Wednesday, 16 December 2009, 8:30 AM

I was playing with my mosfet amp (skywalker 300) last weekend. I came up with strange readings???!!! I used two different radios to drive it. A galaxy 55V doing about 20ish watts max and a dual final (mosfet conversion) Superstar doing about 50-60 watts max. The superstar only provided a few more watts difference, so not really a factor unless you want to split hairs. I first tested the skywalker 300 at 14v, I got 175 watts max using the Galaxy 55V. Just as I did in my car. Also note these mosfets do not defy ohms law!!! At 27.6v I got about 390-400 watts. I expected about 350 watts. Does anyone else have any test results from a mosfet???
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Re: MOSFETs vs. Pills

Post by mlandicini » Sunday, 28 February 2010, 14:57 PM

We need to find a set of plans for a small amp. ANYONE????

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Re: MOSFETs vs. Pills

Post by TheCBDoctor » Sunday, 28 February 2010, 17:50 PM

Mosfets Versus Bi-polar transistors. The biggest reason I would stick with Bi-polar transistors is because I am an old fart and do not take to new newfangled devices. :icon_e_smile:

Mosfets are voltage controlled devices and were never intended for high current draw. Bi-polar transistors are current controlled devices. Mosfets act like tubes and have a high input impedance. As was mentioned they are more subject to oscillating.

The biggest reason they are being used in amps today is because they have overcome the current draw problem and are cheaper to manufacture. I would use a bi-polar transistor over a Mosfet any day of the week. Mosfets and Jfets have their advantages, but handling high current is not one of them.

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1200 watt MOSFET Power LDMOS

Post by Hypo » Sunday, 07 March 2010, 21:12 PM

Check out this data sheet:

http://www.nxp.com/acrobat_download/dat ... F578_1.pdf" onclick="window.open(this.href);return false;


1.1General description
A 1200 W LDMOS power transistor for broadcast applications and industrial applications in the HF to 500 MHz band.

This is a nice aspect as well:

6.1Ruggedness in class-AB operation
The BLF578 is capable of withstanding a load mismatch corresponding to VSWR = 13 : 1 through all phases under the following conditions: VDS = 50 V; IDq = 40 mA; PL = 1200W pulsed; f = 225 MHz.

So much for MOSFETs being sensitive to SWR.

Section 8 has a suggested circuit layout and parts list.
Who's going to be the first to build one?
Umm it won't be a cheap part, AVNET $300 each with a 14 week lead time, but considering the output and simplified parts count and layout it might have a pretty good watts per dollar.

I saw this in the Homebrew section of another site and just had to pass it along.


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