I have a rf signal generator (50 ohm, 3.2mw max power, going to be transmitting 13.56mhz. It's a HackRF SDR) , which I want to connect to a homebrew RF power amp. As a newbie to RF design, I know that whatever I do, I'm going to let some magic smoke out on my first try building this amp. In this case, I'd rather that the smoke not be emitted from my $400 RF gen. Thus, I'd like to somehow isolate the output of my RF source from the input of my rf amp, so that I can tinker around with the amp (adjust base currents to the amp transistors, for instance) without being nervous. Keeping the same impedance would be a plus. Is there some way to do this?
There are a couple of things that you can do.
1) insert a low-power buffer amp between the output of the signal generator and the power amp. Quite frankly, you are going to need multiple amplifier stages anyway. So make the first amp stage function as a buffer.
Have a look at Mini Circuits for really easy-to-use single-chip solutions.
2) Add a couple of big-ass diodes back-to-back across the output of the signal generator. 3.2 mW is about 0.4V P-P, so a pair of silicon diodes connected back-to-back from the signal pin to ground should protect the generator should something bad come back towards it.
Be sure to choose diodes that have low capacitance. I know that it can be hard to find the combination of low capacitance and high-current capability but I know they exist. Unfortunately, I don't recall where I saw them.