Help with high input impedance amp.

Perhaps some of all that bootstrapping is getting back into the LC tuned circuit - you have recreated the old-time Q-multiplier. Or the input levels are so high the buffer is overdriven?

Padding down the input with a capacitive attentuator like Kleijer did could be a good way to reduce interaction. Also I guess at resonance the volatge is pretty high so overdriving is a real risk.

piglet

I'll try a tiny input capacitor. If I create a 10x capacitive divider, I'll need a 10X amp to get back to a gain of almost 1. I put a series 0.3pf cap in series with the input. Same as the Kleijer amp. Turns out that is a 10x was close it's 9.8 to 1, the output drops by a factor 9.8. The input capacitance is 0.45pf and the input resistance measures 180MΩ. This compares to the 200MΩ of the Kleijer amp, a little surprising sense the databook amp has bootstrapping and the Kleijer amp doesn't. I need to get some or make some smaller inductors so I can do these tests at 1, 10, 20, and 30MHz, to see if or how much the input impedance drops. Thanks, Mikek