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High input impedance amplifier
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Qmavam:
Hi all, Here is a high impedance amp used to reduce the load on a High Q LC circuit, specifically when working on Crystal Radio tank circuits. I have built it and it does a good job, although I have no great measurement equipment. It's been a while, but I did set up a high Q coil (over 1000, very reduced drive to measure that high) on my Boonton 260A, measure the Q, then add the amp, make a small capacitance change to get back to resonance, then found very little change of the Q. Been to long I don't recall the exact numbers, but the addition of the amp resulted in little change to the measurement. It is a bit of different approach in that it starts with a 17 to 1 capacitive divider, consisting of a DIY air cap and the FET input capacitance, then a 17X amplifier driving a 50Ω output pushpull amp.  It will output 4Vpp into 50Ω, or 8Vpp unloaded, before getting distorted. 
I have had feedback that the Rin is not anything near what the designer suggests, but, my tests say it is very high. Quote from the write up. "In practice the input resistance will be lower then 5780 M.Ohm because of dielectric losses e.g. in the gate of the FET."
  Just curious how others see this approach, and how it could be improved.
http://crystal-radio.eu/fetamp/enfetamp.htm
                                      Mikek
TimFox:
The input impedance at the tip, from the (very small) coupling capacitor into 20 M\$\Omega\$ resistor and (larger) shunt capacitance, can be calculated using the series-parallel conversion algorithms for the RC circuits.
The capacitance at the tip stays approximately the same and will slightly detune an external circuit.
The trade-off, of course, is the reduction in voltage from the capacitive voltage divider.
Kleinstein:
For even higher input impedance one could bootstrap the 10 M resistor chain (e.g. a small capacitor from the source to the point between the two 10 M resistors).
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