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mosfets

bob85 said:
ken its been a long time since i was at colledge (25 years) but i remember they taught us that mosfets where high impedance voltage gain devices and when used in an amplifier typically produced 10x the distortion of a bjt stage with higher open loop gain and different gain/bandwidth which resulted in the need for more feedback to reduce gain and distortion plus a revised bias supply,
they also said fets have transconductance droop similar to a valve/tube amplifier which made them less prone to thermal runaway than a typical bjt amplifier, something was said about the need to avoid self oscillation but i dont remember what it was, maybe it was something to do with the capacitance which i guess would also effect bandwidth,

have i remembered what i was taught or do i need to get in my parents loft and find my old schoolbooks??
i have not looked at mosfets since i was 15 years old lol,
i am sure i could learn something if i read my old schoolwork.
Click to expand...

Your analysis is pretty good, especially for a time lapse of 25 years...

The current in the drain/source of a MOSFET is proportional to the square of the gate-to-source voltage. So if the change in gate-to-source voltage is small, the output current will be fairly linear even though a squared input is controlling it. If the change in gate-to-source is large, the output current will be non-linear, or distorted even if the gain is small. This is good for a mixer stage, or tolerable for a voice audio stage, but it is very bad for a linear amplifier RF stage. One other big issue is that the transconductance (gain) is very low near cut-off and is at its highest at saturation, so large signal gain from a single stage is impossible without lots of distortion.
 
So, the bias circuit and coupling circuit will really need to be redesigned to get the correct operating, gain, and high frequency response desired - a MOSFET is not a dirrect drop in replacement for a BJT.[/quote]

for those that understand what that means is surely a good thing Ken . Something like that should be added to those CBTricks instructions for these mosfets. IM not sure I understand where your coming from with your knowledge or school books Bob ? Like the 1969 mod , I do understand there's much more to it then just swapping finals , for me and my knowledge , it took a few different instuctions before I personally got it right as far as I was concerned. I really have no disrespect for anyone here or there knowledge of this stuff but I very often sence a better then though attitude from others that do know. It kind of reminds me of the computer tech I met after I baught my first PC some 10 years ago , I spent upwords to 3000.00 for my first system from the same people , I knew very little at the time and it surely cost me when I didn't know I remember the last time i did business with them , I was asking the tech a question on how to do something and he wouldn't tell me (after all the money I had spent ) I remember him saying something like ......to bad !! I went to school for so many years and it cost me 10.000.00 dollars to do so , that's why you get to pay me . SO BE IT !! That was the end of that. Or another time about 10 years ago or so , when I heard a tech say that he really didn't get paid for the labor as much as he got paid for understanding and knowing how to do it in the first place. The I know and you don't syndrome and you really need to take the same steps that i did to know , just doesn't always set well with me. But if that's the way folks truly feel the way they need to feel ? SO BE IT !! I read the prideful ballon crap everyday of the week. I guess that's just the way the cookie crumbles with others.
 
Switch Kit said:
...I really have no disrespect for anyone here or there knowledge of this stuff but I very often sence a better then though attitude from others that do know...
Click to expand...

My intent is to share my knowledge, and years of experience, not to look down on anyone. I do get frustrated when I hear expert comments about concepts or practices that I know are untrue and are just part of the internet folklore. I am guilty of being terse in those instances...

:oops:
 
hey switch i hate it when people who do know what you want to know wont try and help you understand something but i do understand why they wont tell you when its their livelyhood,
i will never understand why they wont tell you when they have nothing to lose,

you dont understand where im comming from and i dont understand why you would even mention it?

let me clarify my position

what i posted is the little i remember from school not even sure if i remember correctly,
i am here to learn nothing more nothing less,
thats why i posted what i did no other reason,
i am glad ken elaborated on it,
if i had not posted that then chances are ken would not have followed it up and none of us would be any wiser about mosfets used in different types of amplifiers discounting those that do know already, which i dont,
i for one dont see a guy sharing his factual knowledge as been a bigheaded asshole and hope they keep it up so i can learn. something, when it stops i will be away to find another forum,

peoples personal none factual opinions mean nothing to me period, bullshit is 10 cents a gallon anywhere you look for it,
i hope thats makes my position clear.
 
Yes it does Bob , and I have always found you to be very helpful when ever you could be. Ken seems fairly new around here but he also seems to know what he's talking about and also seems very helpful when he can be. As I said ,there was no disrespect meant towards any one person , and I think you do know what IM talking about to a degree Bob , I've done a lot of reading over the years around here as I know you have Bob, and I personally couldn't tell you what I read last year let alone 25 years ago unless it was truly that important to me.(I might have been a little confused on your comment, IM sorry) These folks can make all the money they want to for all I care , they can also float around these Forums and talk about this and that as well, if they feel the need not to spell it out in detail ,that's there business , weather I like it or not or have any other feelings about it. .......I must really have a way about myself , I state how I feel about something concerning mosfets and CBTricks and the next thing I know it turns in to this. My humble apologizes......I honestly don't think I intended that.
 
Well, I am guilty of being short sometimes, ...and it is hard to capture someones intent when reading a typed statement, so I apologize Switch Kit if I misread your comments.

:)
 
it's all good Ken , I think I can be misread quite often. But IM usually pretty firm on what I have to say and I think I might be some times rude in what I have to say ,IM known for my spelling and punctuation problems in my posts along with some times having a hard time explaining myself the way I would like to at times. I've had many complaints from others in the past and IM sure I'll get more in the future. Up to this point , I do the best I can. I'd like to think for the most part , that I do get my point across ,one way or the other.
 
ken white said:
Well, I am guilty of being short sometimes, ...and it is hard to capture someones intent when reading a typed statement, so I apologize Switch Kit if I misread your comments.

:)
Click to expand...

Boy... I haven't stopped over at this part of the forum in a while... this thread sure did take off in a different direction than I thought it was going.

But your right Ken..,.thats the problem with reading posts... they don't capture someones emotions. Most of the time a post can be read either way...in a good manner or personal attack/sarcasticly.
Its hard to tell somtimes which way the poster meant his words to be read.

BTW, I still haven't had time to play around with these fets. I got a bunch in my drawer but just don't have the time. No one asks for these fets at the shop either.. no one wants radios peaked up with them. So I haven't really had the chance to play around with them. The good old 1969 (or one of its copies)is still the way to go up here.
I sure wish I had a bunch of new 1307's insted of these fets.
 
hey switch i hate it when people who do know what you want to know wont try and help you understand something but i do understand why they wont tell you when its their livelyhood,
i will never understand why they wont tell you when they have nothing to lose

that looks like a history saying, and needs to be placed in the books of life :LOL:

As for this mod, it is done and many different ways ,on top of it!!!
it also cuts out a section of the output that i think is firmly needed for regulation and filtering for long term talking and stableness!!!
i seen about 4 versions of this mod and none say updated!
I am in no way back talking anyone or this mod..
I do thank whom ever for this update..but dabbing into electronics it is up to the user to get the best wattage
out of this mod with a well balanced bias..
hell i am happy with just making the swing-kits and installing them a modding the output section a bit more for a safe level
without clipping anything..as for that 1 watt d8jump well theres a bit more to that ..by adding more then a jumper

i would like to see one of these units after ,lets say 6 months
of just straight talking from mobile,and then placed in home on a power supply..
just to see if it can hold up on all the regulated stress..
as you know this is just my opinion..so send all your rebuttals to
channel 32 :roll:
 
DTB Radio said:
The thought is ok, but woukd entail some major redesign, as well as major new design. In the FET mods that I've done (cobra 29's primarily), I see carrier levels of around 5 to 6 watts, with my Wavenode system regularly recording 40 to 45 watt peaks. I've had customers come back after having the EFT mod done and tell me they walked on guys running amps. The only things I do differently from the cbtricks procedure is use higher cap values, change the driver base input resistor to 4.7 or 3.9 ohms, and add a 33pf cap to the spot where the 1k resistor is removed (i forget the number right now, its near the antenna socket by the SWR bridge circuit). Oh, I usually replace that 1k with about 15k so there's still bleed-off for any static electricity that might be building up.
Click to expand...
i dont quite understand? are you adding a 15k & 33pf cap where r55 was??
thanks
 
linearone said:
has anybody simply used a voltage doubler to up the volts on one lets say from 8 or whatever it is running on to 16 or 24?
Click to expand...

I would think a boost supply would be much better than a doubler. Doublers don't work very well when loaded...
 
I have read all the posts here and there is slightly conflicting info but that is alright, different people have different ways of doing things. I just want one set of directions that will let me complete this mod.
O.K., the CB tricks mod says to:
remove R55, R56 and C61.
install the 2030.
pigyback a 68pf cap on C59.
install the EN 1230 where R56 was.
In order to get the most power out of my cobra 29 without hurting it, what do I do next?
By the way, I have been on many different radio forums and this is by far the best I have found.
 
Put a 4.7k 1/2 watt resistor on the gate side of the 2030 (this is the left-most leg when looking at the face of the ERF-2030).

the other end to positive side of c116. that will bring your dk to about 6 watts oterwise your dk will be about 3watts..........theres a way to volt the pre-driver but i only know the cobra 25 mod.....no one seems to want to give the info for the 29
 

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