modify preamp for bias-t

[bob808]

Hello,
I just got into SDR and bought a preamp from ebay, the cheap ones.
As I see, it has a separate 5V supply. Since my sdr dongle has bias-t support, I'd like to send the power through the coax to power the preamp.
After seeing some other devices online I figured I can just add a choke between the output to the sdr and the 5V input on the board.
It uses the SPF5189Z amplifier.
PDF here: http://www.rfmd.com/store/downloads/dl/file/id/28188/spf5189z_data_sheet.pdf
And the application on my board seems to be identical to the one in the datasheet. There's three 0 ohm resistors added, but the rest seems to be the same.
Now, I see two ways of powering the amp via the coax. One would be to remove the output capacitor and the inductor feeding the voltage to the chip, and bridge the capacitor pads. But that is not the recommended application.
The second way would be to add an inductor between the output sma pin and the 5V pad. I can wind an aircore inductor at about 2uH or so. Would that be ok?

[kg4arn]

....
Now, I see two ways of powering the amp via the coax. One would be to remove the output capacitor and the inductor feeding the voltage to the chip, and bridge the capacitor pads. But that is not the recommended application.
The second way would be to add an inductor between the output sma pin and the 5V pad. I can wind an aircore inductor at about 2uH or so. Would that be ok?

Would you draw a schematics of what you are proposing? 

[babysitter]

I recommend using a choke from output-coax center pin to a cap, feeding a low-noise 5V regulator from that voltage, and use the regulator output to feed the amplifier. Use a higher voltage on the feed line. This way you can supress some noise which gets introduced to your coax line, I guess one doesn't want the bias of a RF am to jump around.

[bob808]

Something like this

[bob808]

I recommend using a choke from output-coax center pin to a cap, feeding a low-noise 5V regulator from that voltage, and use the regulator output to feed the amplifier. Use a higher voltage on the feed line. This way you can supress some noise which gets introduced to your coax line, I guess one doesn't want the bias of a RF am to jump around.

I see why this would be recommended. But I have 4.5-5V barely to work with. And the amp chip is 5V at full power. I may need to experiment a bit, as maybe full gain would not help me and I could use 3.3V or so. A 3.3V regulator should work very good.

[babysitter]

If you are really fighting for every hundred mV, this might be the way to go:

http://www.wenzel.com/documents/finesse.html

(No experience with this,  just aware of its existence)

[whiyne]

Firstly, sorry for the thread revival - this was the only related topic I can find!

OP, did you even manage to successfully complete this mod? I have the exact same LNA and am hoping to do the same.

Or, if anyone else has advice on whether the mod shown above is a good idea, i'm all ears! Given the price of the LNA I'm leaning towards just having a go and seeing what happens.

Cheers

[Wolfgang]

Hint: try Mini Circuits. They have Tees for all kinds o frequencies and even ones with SMA connectors.