[ICOM] AG-25 Pre-amp

[Frank A. Ellis]

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
> 
> ----
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