The transistors may also be related to the MRF428 or the SRF3513 which
were the original
transistors in the Motorola 300W gain-block that was used almost
unchanged in the HF-80.
There are lots of designs around for class-AB bias networks.
I have two of them as scanned schematics in my home computer,
one is the Philips recommended circuit from the early 70's, that was
used
in the ITT-Standard Radio SSA-family of HF solid-state amps,
and other is the Rockwell-Collins circuit from the HF-80 and Spectrum-
2000 families.
I can mail you both scans.
Both should have the same performance, but you generally have to look
out for two things
when biasing a bipolar transistor stage into idle current:
1. The gain on all frequencies increases, sometimes dramatically, so
all bypasses
and decouplings have to be checked carefully to avoid parasitic
oscillations.
Sometimes RF negative feedback circuits have to be changed to insure
unconditional stability;
2. The operating points of the RF transistorns themselves are very
temperature sensitive,
so to avoid thermal runaway you will have to provide a temperature
compensation circuit.
This is usually done with a transistor or diode that is held in close
thermal contact with the RF transistors.
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från: dh5hv@dh5hv.de
Datum: May 31, 2006 9:55:42 AM
Till: Reflektor-Amps <amps@contesting.com>
Ärende: [Amps] Looking for shematic to set MRF448 into Class AB
Hello.
I got a commercial RF-generator for ENI.
It has 4 power modules with two ENI21 transistors per module.
The ENI21 for sure is a Motorola device and due to the fact that each
module should bring up more than 300W with two of these ENI21 at a
voltage of aprox. 50V up to max 30MHz I would say it should be the
MRF448.
Am I correct ? Otherwise correct me !
Now my problem:
Looking into the shematic I see the MRFs in zero bias mode.
The base of each transistor is connected to ground and the modules
just get the 50V. Nothing more.
I would like to cut the connection of the base to ground and spend
some voltage there to get a current flowing through the MRFs.
I found also an application in the Motorola application book, but
maybe someone has a nice design on his PC including shematic and maybe
layout to make some prints ?!?
I just have the 50V with looooooots of amperes in the generator to
build the bias voltage..... ;-)
Maybe I can also place a very small second trafo inside, which should
be the better solution maybe....
Jochen
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|