Hi

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

P.S. It's a linear amp i want to build

73 de OZ1BV, Brian

BV wrote:

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

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 ;-)

Cheers!

On Jan 19, 4:17*am, 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 ;-)

Cheers!

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

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.

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
* Killfiling google & XXXXbanter.com: jonz.net/ng.htm

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 :-).

Paul OH3LWR

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 ;-)

Cheers!

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

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 ;-)
================
Switch Mode converter 12V ------ 48 V

Frank KN6WH / GM0CSZ

On Jan 19, 1:00*am, Bob wrote:
BV wrote:
P.S. It's a linear amp i want to build

73 de OZ1BV, Brian

A bit of advice:

..........a circuit without DC bias. *The Vbe max
is only 4 Volts, which gives a useful hint!

Hey OM,

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