I've been attempting to build a VCO recently (around 50Mhz) and I've reached the part where I need a solid buffer for my oscillator. Looking around at various options I can't help noticing that many of the buffer amplifiers used in designs are actually of the CCVS type. Take a look at these:
Taken from here
What's intriguing about all of these configurations is that they have an intrinsically low input impedance. If you take the 2nd circuit (MMIC) as an example. The circuit aims for a high trans-conductance, that makes the trans-impedance close to 560. This gives a gain of approx 11 with a 50 ohm source. But, it has a very low input impedance and the higher you make the open-loop gain, the lower it gets. Even with a 50 ohm source, the impedance is very low, I think.
So, what am I missing here? Why are these shunt type configurations so attractive in RF circles? Does it have something to do with standing wave reflection?