>>>A capacitive divider seems to be a requirement. Additional information
here:
10/120 pF divider work well.<<<
Yes and nothing wrong with using a capacitive divider, I'm just adding, there are high input impedance amps with bootstrapping
that will not add very little capacitance to your tuned circuit and have a higher parallel resistance.
I built a Kleijer high input impedance amp several year ago, it uses a 17 to 1 capacitive divider.
http://www.crystal-radio.eu/fetamp/enfetamp.htm
The input signal enters the amplifier via a 0.3 pF input capacitor, together with the input capacitance of the FET (T1) this forms a voltage divider, the input signal is attenuated 17 times by a capacitive divider, and then amplifier 17 times before a 50 ohm driver.
?.
If the write up can be believed, it was 0.3pf input and towards 5G¦¸ resistance.
? I've used it on high Q circuits, I can't see any change on my Q meter (after adjusting for the 0.3pf) when I add this in parallel with the resonating C.
?
"The 0.3 pF input capacitor is self-made of two copper plates of 1 square cm at a distance of 3 mm.
By changing the distance between the plates we can adjust the gain of the amplifier.
The plates must have at least 1 cm distance from the surrounding grounded box.
The input signal enters the box via a 1mm copper wire, through a 10 mm hole in the box.
The wire is supported by a piece of polyethylene, which is fixed with nylon screws.
The input amplifier (T1) is screened from the rest of the circuit.
Between the gate (input) of T1 and ground there is a 20 M.Ohm resistor.
But the input resistance of the amplifier is much higher then 20 M.Ohm, in theory even 17? times higher? (so, 5780 M.Ohm), this is because over the 20 M.Ohm resistor is only 1/17th part of the input voltage.
In practice the input resistance will be lower then 5780 M.Ohm because of dielectric losses e.g. in the gate of the FET."
?????????????????????????????????????? It's all fun!? Mikek
