G4BAO 2.5W Amp

[G4KLB]

I have built the G4BAO 2.5W Amp as seen in the latest CQTV.
After a small driver stage my DigiLite gives 50mW out, I fed this into the G4BAO amp and had 1W out.
The recommended input is 100mW and this gave 2W out at 1249MHz and 2.5W out at 1296MHz.
Not recommended, but unable to resist the urge to tweak things I found 110mW in gave 2.5W at 1249MHz, I did have 3W out briefly before loosing my nerve!

No problems were found following the instructions, the only thing I found was performing the initial test below.

Rotate R2 fully ANTICLOCKWISE and using a thin wire, apply a regulated +13.6
volts to +ve end of C5 and ground to the ground plane. Measure the voltage at the
junction of R1 and C9 where it connects to the gate of the device. It should be 10.5
volts +/- 0.2

I was only able to measure 7V max.

Will give it a test on air in the next few days.

Colin
G4KLB

[G4GUO]

All very interesting Colin,
I have only tested my board by biasing it up to the 50mA.
Dr John G4BAO did ask that someone measured the QPSK spectral re-growth vs power sometime.
Eventually I will get around to doing it but I did find an IEEE article on spectral re-growth vs
dBs backed off from P1dB which seems to indicate that 1watt is about all you can expect
to get out of that amplifier on DATV if you want to transmit a half decent signal.
it will be interesting to compare theory and practice.

- Charles G4GUO

[G4KLB]

Just had this email from John G4BAO

For info:

Since I've now built a few of the new PCBs I've noticed that you get more output power at 1296MHz if you don't fit the 1.5pF cap C12 to the output line.
I'll update the documentation in due course.

73
John

Not shure what the effect will be at 1249MHz, if you build one let us know.

73
Colin G4KLB

[M0DTS]

I've just received a couple and got one built.
On mine i find having the 1p5 fitted helps improve gain at 1249 and 1296 (measuered with 150mW o/p) but i haven't driven it up to 2W level yet, gain and power rarely go hand in hand!
Currently i'm seeing around 13dB gain at 1296MHz 10dB at 1249MHz.

Will do some more tests soon when i buld a pre-driver amp as my synth only goes up to 20mW....

Rob

[PHYguy]

I made some ACPR measurements on this nice amplifier. Documented at: http://www.phyguy.net/pe1gta/DVB-S_ACPR.htm

Note that I made a typo in original doc.: fig.3. caption mentions that C1 is not mounted. This should read C12 is not mounted. (Fixed 20160131. F5 to refresh.)

PE1GTA
Anne Westra