[AMRadio] AM Transmitter Advice??


Jim candela jcandela at prodigy.net
Mon Jan 30 20:36:28 EST 2006



   Guys,

I just looked up the MRF150. For the moment look at the characteristics
below with a big AM final amplifier in mind.

With a 300 watt Pd rating, this thing might do well as a efficiency
amplifier (modulate the gate bias) to make 100 watts AM, and do so with room
to spare. So with say 33% efficiency (SWAG), 300 watts DC input, you get 100
watts RF output with 200 watts Pd. With modulation the efficiency improves
(doubles @100% modulation from sine wave tone input).

If you do a conventional Class C high level modulated amplifier with say 150
watts Pd, and 75% efficiency (SWAG), that gives you 600 watts DC input X .75
= 450 watts RF output! There are tons of hi power solid state audio
amplifiers around, so there is your modulator too!

Sure, the application notes dwell on SSB linear usage, but heck if it works
on SSB, you can make it work on AM!

Regards,
Jim

 So how much do these things cost? :-)


MRF150

MAXIMUM RATINGS
Drain–Source Voltage VDSS 125 Vdc
Drain–Gate Voltage VDGO 125 Vdc
Gate–Source Voltage VGS ±40 Vdc
Drain Current — Continuous ID 16 Adc
Total Device Dissipation @ TC = 25°C
Derate above 25°C
PD 300
1.71 W/°C
Storage Temperature Range Tstg –65 to +150 °C
Operating Junction Temperature TJ 200 °C


MOTOROLA RF DEVICE DATA
ELECTRICAL CHARACTERISTICS (TC = 25°C unless otherwise noted.)
Characteristic Symbol Min Typ Max Unit
OFF CHARACTERISTICS
Drain–Source Breakdown Voltage (VGS = 0, ID = 100 mA) V(BR)DSS 125 — — Vdc
Zero Gate Voltage Drain Current (VDS = 50 V, VGS = 0) IDSS — — 5.0 mAdc
Gate–Body Leakage Current (VGS = 20 V, VDS = 0) IGSS — — 1.0 mAdc

ON CHARACTERISTICS
Gate Threshold Voltage (VDS = 10 V, ID = 100 mA) VGS(th) 1.0 3.0 5.0 Vdc
Drain–Source On–Voltage (VGS = 10 V, ID = 10 A) VDS(on) 1.0 3.0 5.0 Vdc
Forward Transconductance (VDS = 10 V, ID = 5.0 A) gfs 4.0 7.0 — mhos

DYNAMIC CHARACTERISTICS
Input Capacitance (VDS = 50 V, VGS = 0, f = 1.0 MHz) Ciss — 400 — pF
Output Capacitance (VDS = 50 V, VGS = 0, f = 1.0 MHz) Coss — 240 — pF
Reverse Transfer Capacitance (VDS = 50 V, VGS = 0, f = 1.0 MHz) Crss — 40 —
pF

FUNCTIONAL TESTS (SSB)
Common Source Amplifier Power Gain f = 30 MHz
(VDD = 50 V, Pout = 150 W (PEP), IDQ = 250 mA) f = 150 MHz


Full data sheet:

http://www.twhrf.net/mkic/mrf150.pdf






-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Jim candela
Sent: Monday, January 30, 2006 6:54 PM
To: Discussion of AM Radio
Subject: RE: [AMRadio] AM Transmitter Advice??




Larry,

   I am interested in what your friend Jim, KR1S has in mind for that 100
watt AM solid state exciter for 75 meters. Is that carrier output, or PEP?
In thinking about it, this could vary from a "scaled up CB" rig with a tuned
class C final with high level modulation, or maybe a small AM exciter with a
solid state push pull linear, or maybe a class E setup. We seemed to get
side tracked on the 813 amp idea, and glossed over the exciter concept you
laid out.

  How about a nice juicy MRF-150 with high level drain modulation, or low
level gate bias modulation? I bet no one has ever tried it either way..

Regards,
Jim
WD5JKO


[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Larry Keith

My friend, KR1S (Jim Kearman) is designing an AM
transceiver, for me.  We plan for this to be a
solid-state unit that will deliver about 100 watts
out. It will be single band (75 meters) and I have
been considering using that to feed a homebrew linear.
I have the makings of a 2 x 813 amp thanks to my and
Jim's hamfesting efforts.
Larry
KQBY

Larry Keith

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