[ICOM] AG-25 Pre-amp

Wed Apr 20 20:00:40 EDT 2005

Hi Gary,

Thanks for checking in on this. I always wondered
about that, as ICOM never said anything but the
typical "High gain, low noise, designed for
high-performance microwave applications, blah blah . .
." marketing spiel about the device, but never gave
any figures. 

I never saw a datasheet for the device either, and
assumed that the NF would be "average" for the time,
meaning not real great, but not too shabby either.

Heck, in the mid-80's there were MOSFET's (and
bipolars!) that could beat that NF easily.

I am pretty sure the same device was used in the AG-1
and AG-35 at 432 MHz, and don't want to hazard a guess
at the NF there. I don't EVEN want to know if they
used the same device in the AG-1200, it might do
irreparable harm to my very high opinion of ICOM, hi!

73, Frank W3UHF

--- Gary Fiber <gfiber at comcast.net> wrote:

> Frank,
> 
> When I was in tech support at Icom we asked the
> factory for the noise 
> figure and I remember it being something like 3db.
> We were amazed as the 
> AG-25 did a good job and had lots of people using
> them on Yaesu and Kenwood 
> products. A popular modification was to remove the
> DC from the coax and add 
> external power wires. I have no idea if Icom still
> has access to the GaAs 
> FET that was utilized in the AG-25 preamp.
> 
> Gary K8IZ
> 
> At 04:44 PM 4/20/2005, you wrote:
> >Hi Fred,
> >
> >Good luck in finding one. This is a very old design
> >(the AG-20, same circuit as the AG-25, was an
> optional
> >internal preamp unit for the IC-271A, being
> produced
> >starting in 1984) and the FET is an early GaAsFET
> >device.
> >
> >The first gate is not part of the RF path and is
> used
> >only for device bias. It is self-biased identically
> to
> >the source lead, with a resistor to ground and a
> pair
> >of chip bypass capacitors, which is still a very
> >common technique. If you are familiar with GaAs FET
> >preamps, you will know exactly what I am talking
> >about. If not, skip to the last paragraph!
> >
> >I don't have an AG-20/25 to open up and look at,
> but I
> >suspect that when looking at the installed device,
> it
> >will probably resemble the installation of a more
> >current 4-lead single-gate GaAsFET where the two
> >opposing self-biased leads are the source leads
> with
> >both sitting on a chip cap pedestal with a chip R
> to
> >ground for the self-bias. In the AG-20/25 case, the
> >two opposed "biased" leads would not both both be
> >source, one would be G1 and the other would be
> source.
> >
> >Again, I have not verified this, but it would be
> the
> >most logical layout. It should be trivial to
> examine
> >the unit and identify the G2 and Drain leads, as
> they
> >are in the RF path. The other two leads would be G1
> >and Source, and should be opposed and both
> >self-biased, as above, with no other connection to
> >them.
> >
> >The bottom line is that a current style device
> should
> >be able to be installed in an identical fashion, as
> >long as the source bias R values are adjusted
> >accordingly, if necessary, for the new device. A
> much
> >better NF and somewhat better gain might be
> realized
> >by doing this. ICOM published the gain of the
> AG-20/25
> >but never the NF. I suspect that the NF is in the
> >neighborhood of 1 dB, not too great at 144 MHz
> >nowadays for serious weak-signal work. That NF
> guess
> >is just a gut feeling, based on the device and the
> >time it was produced. Perhaps someone here has
> >actually measured one on a Noise Figure meter, and
> >knows what it is.
> >
> >I did a quick check on Mouser, Digikey and Newark,
> and
> >there was nothing there. Someone said that RF Parts
> >had them about a year ago, but that they weren't
> >listed on the web site, so maybe call them too. Who
> >knows, maybe ICOM could pull a rabbit out on this
> one.
> >
> >
> >Hope this helps. Please let us know your luck in
> >finding a direct replacement.
> >
> >73, Frank W3UHF
> >
> >
> >
> >--- "Frederick W. Holmes, W1SKU"
> <w1sku at direcway.com>
> >wrote:
> > > Thanks to all who replied to my post about the
> > > GasFET in the AG-25. Thanks
> > > especially to Frank, W3UHF, who came up with a
> > > device number.
> > >
> > > 73 de Fred, W1SKU
> > > Ex-Net Manager for the CCA
> > > Chairman, Technical Review Committee, CCA
> > > CCA Member # 793
> > > Board Member, USECA ARC
> > > Technical Committee, USECA ARC
> > > Net Control Operator, Hurricane Watch Net
> > >
> > > ----
> > > Your Moderator: Dick Flanagan K7VC,
> > > icom-owner at mailman.qth.net
> > > Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > > Icom FAQ: http://www.qsl.net/icom/
> > >
> >
> >__________________________________________________
> >Do You Yahoo!?
> >Tired of spam?  Yahoo! Mail has the best spam
> protection around
> >http://mail.yahoo.com
> >----
> >Your Moderator: Dick Flanagan K7VC,
> icom-owner at mailman.qth.net
> >Icom Users Net: Sundays, 1700Z, 14.316 MHz
> >Icom FAQ: http://www.qsl.net/icom/
> 
> Gary Fiber K8IZ
> GROL PG-19-6691
> Washington State Resident
> 
> 
> ----
> Your Moderator: Dick Flanagan K7VC,
> icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
> 

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 