Thursday, 10 October 2013

BFG591, ATV amplifier for 1.2 GHz

Some blog entries ago I informed you about my intention to build a 1.2 GHz amplifier with a CLY15 RF mosfet. I ordered 3 of them and did extensive trials. But in the end I had to conclude these devices are to delicate for me. I demolished 2 of them in 5 minutes time and the third provided almost no RF power in my circuit. 
I decided to look for another semiconductor that could help me in the GHz range. Hans, PE1DWA, informed me that a bi-polar transistor, the BFG591 could be interesting. This semiconductor has an Ft of 7 GHz and at 23cm I should easily be able to get 500 mW RF from it. I ordered 5 of them at eBay for about 4 euros.
Yesterday I built this circuit:


                                500 mW 1.2 GHz amplifier with BFG591

Although I have no device to measure the power at this rather high frequency I think half a Watt is a fair estimate that the circuit is producing.
With a bi-quad antenna I could get my signal to the local ATV repeater PI6ZTM. It was verified that the videosignal that came from a DVD-player  arrived at the local repeater. Although signal strength is not high enough to get a clear picture I recognized the transmitted pictures, and they were even in color.



  PI6ZTM repeater picture; in the left hand upper corner my video signal at 1255 MHz

                                          Snapshot of the 1.2 GHz  \  500mW amplifier

This is a good step forwards. In the mean time I ordered an MRF284 which is able to provide 5 Watts at 1255 MHz. Another 3 weeks to wait until it arrives from China...

Note: As you can see, I drawed the circuit by hand. Although I have computer programs to generate a drawing I did it in the old-fashioned way: Pencil & paper. Including scanning this takes me about 10 minutes time. A computer drawing consumes in most cases more than half an hour.





2 comments:

  1. Ron,

    Thank you for posting the nice schematic of the 200 mW amp. I am just starting out in building RF-circuits, and I have a question in that regard. I understand most of the circuit design except the placement of the 5p trimmer caps in front- and at the end of the microstrips L1 and L2. What is their purpose? djusting the specific impedance of the microstrips? Inpu/utput impedance matching maybe?

    Thanks in advance!

    Best regards

    Anders Grinrød, Norway

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    Replies
    1. Hi Anders, thank you for your nice reply. In fact there are two pi-filters (C-L-C). One on input and one on the output side. You are correct; Main purpose is the match of impedance. 73 Ron

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