UHF (70cm) PA
BV
Got a pair of MRF648 and would like to build a 100W 70cm PA running at
13,8V.
I have been looking for constructions/idéeas on Google without luck.
Anybody able to help me?
If You got an article i can use, please mail it to me: bv(--at..)
mailme.dk
MRF648 data:
DESCRIPTION:
The ASI MRF648 is Designed for
12.5 V UHF large signal amplifier
applications up to 512 MHz.
FEATURES:
. Internal Input Matching Network
. PG = 4.4 dB at 60 W/470 MHz
. OmnigoldT Metalization System
MAXIMUM RATINGS
IC 11 A
VCBO 36 V
VCEO 16 V
VEBO 4.0 V
PDISS 175 W @ TC = 25 °C
TJ -65 °C to +200 °C
TSTG -65 °C to +150 °C
?JC 1.0 °C/W
73 de OZ1BV, Brian
BV
73 de OZ1BV, Brian
K7ITM
Hi Brian,
My Motorola data book says these are designed for class C (e.g. FM)
amplifier service, not linear. I suppose you should expect to get a
little less PEP power out of them in linear service, for good
linearity.
The Moto data sheet has a test circuit; I could scan it, if it would
be any use to you. The microstrip dimensions are for glass-teflon
board, er=2.55, 1/16" (1.6mm) thick. Of course, the test circuit is
for a single transistor, not a pair. It is solidly class C: the base
and emitter are both at DC ground.
Cheers,
Tom
Bob
> P.S. It's a linear amp i want to build
>
> 73 de OZ1BV, Brian
A bit of advice:
The Motorola data book suggests that these devices are designed for Class C
service, and their design guide gives a circuit without DC bias. The Vbe max
is only 4 Volts, which gives a useful hint!
Your DC bias circuit needs to /very/ accurately track temperature to prevent
thermal runaway - the design of the bias circuit /will/ /not/ be trivial.
The gain of the circuit will be disappointing - you'll be lucky to get close
to 10 dB - so the drive requirement will be surprisingly high.
If I was designing a UHF PA, I'd choose FETs these days!
Bob
geek
<snip>
> If I was designing a UHF PA, I'd choose FETs these days!
Alas, if automobiles used 48V instead of 12 ;-)
Cheers!
BV
> Hi Brian,
>
> My Motorola data book says these are designed for class C (e.g. FM)
> amplifier service, not linear. I suppose you should expect to get a
> little less PEP power out of them in linear service, for good
> linearity.
>
> The Moto data sheet has a test circuit; I could scan it, if it would
> be any use to you. The microstrip dimensions are for glass-teflon
> board, er=2.55, 1/16" (1.6mm) thick. Of course, the test circuit is
> for a single transistor, not a pair. It is solidly class C: the base
> and emitter are both at DC ground.
>
> Cheers,
> Tom- Skjul tekst i anførselstegn -
>
> - Vis tekst i anførselstegn -
This is correct, but I found out that Tokyo Hy-Power HL-120U uses
these transistors in this PA. If they are able to make a 100W linear
amp i hope to be able to do the same:-)
(The list where I figured out what transistor type is udes in PA's:
http://www.electronicpool.de/en/funkgeraetetypen.html)
I have not been able to find a schematic for HL-120U. It would be nice
to see how they designed the bias circuit.
Medt 73 de OZ1BV, Brian
Tio Pedro
"BV" <b...@mailme.dk> wrote in message
news:f536e4eb-bf0a-4a19...@p2g2000prn.googlegroups.com...
Hi Tom
You will find that the Mirage linears used the same family of
transistors as well.
pete
JIMMIE
48 volts is not that hard to do in an auto if you can mount a second
alternator. There is plenty of info on this on the web.
Jimmie
Allodoxaphobia
They will.
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
* Killfiling google & XXXXbanter.com: jonz.net/ng.htm
Paul Keinanen
wrote:
>On Mon, 19 Jan 2009 01:17:52 -0800, geek wrote:
>> On Mon, 19 Jan 2009 06:00:45 +0000, Bob wrote:
>>
>><snip>
>>
>>> If I was designing a UHF PA, I'd choose FETs these days!
>>
>> Alas, if automobiles used 48V instead of 12 ;-)
>
>They will.
A few years ago 42 V (3x14 V) was supposed to be the new automobile
voltage, but I have not heard much of it lately :-).
Paul OH3LWR
raypsi
> BV wrote:
> > P.S. It's a linear amp i want to build
>
> > 73 de OZ1BV, Brian
>
> A bit of advice:
>
> is only 4 Volts, which gives a useful hint!
>
I don't thin so. The hint should be the input impedence is like less
than 1 ohm. And although you may state that Vbe max is 4, that may
apply in class C but not say class AB where the all yoiu will be doing
is varying the current thru the base and the voltage will swing next
to nothing, like a base swing current of 10 or more amps into the base
impendence.
So you can't find a good schematic for a linear, I suggest you get a
good spice program, and a really big smith chart.
I wouldn't use teflon board of any kind, when teflon burns it turns to
mustard gas.
73 OM
n8zu
Allodoxaphobia
> On 19 Jan 2009 17:49:47 GMT, Allodoxaphobia wrote:
>>On Mon, 19 Jan 2009 01:17:52 -0800, geek wrote:
>>> On Mon, 19 Jan 2009 06:00:45 +0000, Bob wrote:
>>>
>>><snip>
>>>
>>>> If I was designing a UHF PA, I'd choose FETs these days!
>>>
>>> Alas, if automobiles used 48V instead of 12 ;-)
>>
>>They will.
>
> A few years ago 42 V (3x14 V) was supposed to be the new automobile
> voltage, but I have not heard much of it lately :-).
I read about the engineering and design that was going into the "new
48V automotive systems" in the IEEE Spectrum in the last year or so.
A brief Google search of ieee.org did not locate the article for me.
Jonesy
joeturn
BV
I found this: http://www.emartin.it/it9vky/Risorse/uhfpa/uhfpa.htm
Seems like a very good, even a PCB layout is downloadable...
Best 73 de OZ1BV, Brian
Chris Jones
A lot of stereos have a DC-DC converter to generate a supply voltage like
that for the audio amplifier.
If you do build a DC-DC converter, then at least the output voltage could be
well regulated and free of spikes, unlike an automotive "12V" supply. Of
course quite a bit of shielding and LC filtering of the supply may be
needed to stop the PA supply from having ripple that will cause AM
sidebands on the Tx signal at an offset equal to the switcher frequency.
Chris
highlandham
Switch Mode converter 12V ------> 48 V
Frank KN6WH / GM0CSZ