[ICOM] AG-25 Pre-amp

[Gary Fiber]

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
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> > Net Control Operator, Hurricane Watch Net
> >
> > ----
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> >
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Gary Fiber K8IZ
GROL PG-19-6691
Washington State Resident