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Looking for info on Palomar MOSFET Amp

Mosfets

The 7530 mosfet in question runs very hot. Mosfets should be run at about 50% of rated output or else they will cook everything around them. That means less than 50w a piece if you want it to last.
Don't skimp on the fans...
 
radioman2010 said:
Comparing the ERF7530 to the 2SC2879 is like comparing apples to oranges.

The FET is much better for RF use because they are much faster with less loss, thus less heat generated. The FET does not have the poor grounding and oscillations associated with bi-polar transistors which is another plus.

The Palomar 250FET uses 2 of the ERF7530 and produces peak power of 200 watts.

The Palomar 450FET uses 4 of the ERF7530 and produces peak power of 400 watts.

Intial feedback reports are all excellent.

So I don't know what you're talking about when you say the ERF7530 is good for only 20 watts ? Get a clue.

As for the IRF7530, that transistor has nothing to do with the products being discussed.
Click to expand...

When one tells another to 'get a clue', one must ensure they have a clue as well.

It was stated 20W RMS was the level to keep the single device at. A pair would then be 40 watts.

YOU stated you get 200 watts PEAK.

I'm going to make the assumption he meant carrier power at 20 watts (which generally would mean 20 watts of AVERAGE Pdiss on the device under AM 100 percent modulation conditions, give or take.. You COULD squeeze 30 out MAYBE).... BUT, that's 160 to 180 watts PEAK.

So, there's your clue for the evening.

--Toll_Free
 
I imagine with such a small die it's imperative to have perfect finished heatsink plus fan cooling to keep things under control.

You can push RF fet's as I'm sure you know.

We have seen a genuine 1Kw out of a pair of these at work:
http://www.st.com/stonline/products/literature/ds/15501.pdf

However for reliability and the general public we use four in our 1Kw FM PA.

Is this new ERF fet the absolute highest output 12V fet out there at the moment?

Phil
Unit 148
 
Toll_Free said:
It's the class of service people operate them at.

As you increase efficiency, gain goes down.....

--Toll_Free
Click to expand...

The real issue with the gain is that these are cheap MOSFET's. As such they have higher internal capacitance then you would find on a more expensive RF MOSFET. When you drive a cheap MOSFET at it's extreme frequency limitations, you have to overcome this gate capacitance that appears to shunt some of the drive power. This lowers the gain of the device in use. Quality RF power MOSFET's can have gains around 22 db at frequencies into UHF.

The IRF-520 series transistors have been around longer then a decade. They were designed for the "new" high frequency switching power supplies of the day. Some of the spin-offs like the ERF7530 actually claim to be inexpensively designed for HF RF amplification to 30 MHz. There is the possibility they are nothing more then higher power versions of transistors designed for switching supplies that have been marketed for RF because they happen to function up to 30 MHz. with lower then typical gain.

There is more connection between class of service and efficiency then there is with either of the two effecting the gain of the active device itself. As you approach 360 degrees of conduction, the efficiency will drop due to the power dissipated as DC bias. Interestingly, this does not have a major impact on the overall gain of an RF power stage unless we are talking about very low signal levels that would have to overcome biasing the transistor on. As drive levels go from milliwatts to watts, the difference in gain becomes negligible with respect to bias class. It is true that input or output circuits having a poor match will waste some of the gain the active device can provide due to losses in the mismatch.
 
Last edited:
Shockwave said:
The real issue with the gain is that these are cheap MOSFET's.
Click to expand...

You speak the truth.

But there is more: its also about the drain-gate capacitance, also known as the miller effect. It has been said this capacitance varies over the RF cycle because it depends on drain-source voltage.

This probably has something also to do with packaging. You really cant expect good results when the RF input pin is right next to the RF output pin, which is the case with TO-220 type of package. The gate needs to be on the opposite end of the device from the drain.

All this results in poor efficiency and poor IMD.
Would you trust a vendor without a street address?
 
I was getting my hopes up on these plastic fet's but you're painting a poor picture re their capabilities.

I'm very familiar with the even cheaper IRF's as we used to sell a 40w FM PA with a pair of 510's in a broadband design covering the FM broadcast band 88-108MHz.

I am no designer and gave up diagnosing faults at component level many years ago moving into marketing of RF and DSP based broadcast products (we exhibit at NAB each year).

I think I may give up on these fet's and go back to my 16 x 2879's or the Metron in my avatar :confused:

Btw, are the IMD figures any better with these plastics that the bi-polars?

Phil
Unit 148
 
Just realised my avatar is in another forum :redface:

metron_ma_1000b.jpg


Phil
Unit 148
 
:D

I wish it were just the driver, and it's more than 15 years.

I think many of the Metron's did not have the 10m switch position as your rules as you know do not allow operation above 25MHz.

There was a simple mod to get these to work on 10m in the 15m switch position.

I own two of these pretty looking amp's with one still brand new in it's box which if you think of it makes it pretty rare as they ceased production in the 80's.

I have decided that if nothing comes of these new ERF fet amps in the kilowatt + class then I am considering a professional re-design of the Metron from it's current 8 devices to 16 x 2879's plus the switched Chebychev filters to make it suited to the ham community crazies like myself.

Maybe there's one or two others out there that would prefer to run the full 1500w pep in the mobile.

BTW here is our FM 1Kw unit for the broadcast community, super efficient in the early 80% with four gemini plastic mosfets and a extremely compact 1600W Murata SMPS:

http://www.fm-transmitter.com/images/tx1000-front-large.jpg

Phil
Unit 148
 
Unit 148 said:
:D

I wish it were just the driver, and it's more than 15 years.

I think many of the Metron's did not have the 10m switch position as your rules as you know do not allow operation above 25MHz.

There was a simple mod to get these to work on 10m in the 15m switch position.

I own two of these pretty looking amp's with one still brand new in it's box which if you think of it makes it pretty rare as they ceased production in the 80's.

I have decided that if nothing comes of these new ERF fet amps in the kilowatt + class then I am considering a professional re-design of the Metron from it's current 8 devices to 16 x 2879's plus the switched Chebychev filters to make it suited to the ham community crazies like myself.

Maybe there's one or two others out there that would prefer to run the full 1500w pep in the mobile.

Phil
Unit 148
Click to expand...


Didn't you get a 12 pill that Dennis reworked for you? I figured your ultra clean 8 would make a nice driver for the 12 pill battle box.... Maybe I'm mistaken.... Yeah, it's been awhile :).... Still have that 4CX1000 and socket you sent, although the socket's being modded to run a 3CX1000..... Which is going in my Harris RF-103.. 3-1000Zs are SPENSIVE, and I got the 3CX already.

I use the Messenger 16 for a legal limit amp. It has 10 meter filtering in it already. Messenger also is working on an external bandpass filter. Mighty Fine Junk has one as well.

Gave up on the tube in the 'burban?

--Toll_Free
 

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