I got this high input impedance amp out again and redid my input capacitor. I have it at about 0.3pf.
The writer says the input impedance is near 5 gigaohms but, I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal. Then I put a ten Meg in series with the input and my output dropped to about 0.5Vpp. That certainly doesn't seem like it is anywhere close to 5 Gigaohm. So can someone calculate the input impedance, I want to figure out if I have something wrong or if the writer got it wrong.
Ok, I missed that, he's figuring additional capacitance from his input wires and input connection. Just the 1.4pf has an impedance of about
115K ohms. Then the 0.3pf in series with 20 Megaohm has an impedance not much more than 20 Megaohms, so we have 115k parallel 20 meg, so we are still about 115K. Then throw in the gate impedance and we are slightly less than 115k. So where did the writer come up with:
"But the input resistance of the amplifier is much higher then 20 M.Ohm, in theorie even 17² 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."
145k is certainly lower than 5780 Megaohms, but I would not call it high impedance.
I'm not sure how the 1.4 pf should be treated, the circuit is used to measure Q of LC circuits, so the 1.4pf could just lower the resonant frequency slightly and the Q would be affected only by the quality of the 1.4pf of capacitance. So maybe this isn't as bad as the 115 kohm impedance suggests, but I'm still stuck with my 10 megaohm experiment that started my question.
Yes, that would be very helpful, maybe when I retire. If I was starting over I would have studied math and physics. Thanks, Mikek
He didn't, that's a calculation. It's also been called goobledegook.
I'm thinking the capacitance just adds to the resonating capacitor of the LC being measured and should not be called part of the input impedance when measuring the Q of an LC circuit. The loss in the capacitor will show as a large parallel resistor.
First, the author is claiming that the circuit looks like 0.3pF in parallel with 5Gohm, not 5Gohm bare. That 0.3pF has a reactance of around 500kOhm, which is almost sorta-kinda consistent with what you're seeing.
Second, the author is using theory that doesn't match the part he's using. He needs to sit down, make his little FET all nice and comfy, and explain to the little critter how it is supposed to work and hope that it changes its ways. Or, he needs to understand what the hell he's talking about.
I'm not inclined to go through the math right now, but the effective input impedance at the gate of the FET is going to look like the FET gate capacitance in series with some resistance that's going to be in the kilo- ohms. I'm not sure _how many_ kilo-ohms, but it's going to be 100k or less.
Keep in mind, though, that the circuit ought to be doing pretty much what the author claims for it: making measurements on LC circuits without significantly affecting their Q. I do _not_ see him claiming that the resonant frequency won't be affected. So when you clip that thing onto a tank circuit you should expect that the resonant frequency _will_ change (probably more from the parasitic capacitance of the leads than the 0.3pF cap), but it shouldn't get loaded down to any significant degree.
Whether the rest of the amplifier maintains calibration close enough for even remotely useful measurements is left as an exercise for the reader
-- if you're using it for narrowband ratiometric stuff, like finding the bandwidth of the tank, then it'll probably work fine.
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
hmm... His writing is not real clear on that, (as I read it).
I have modified John S's LTspice file to include an LC driven through a
100Meg ohm. I then connect the (input circuit) .3pf in series with (paralleled) 20meg and 4.8pf to ground. This looks like an input impedance of 950K according to LTspice. If I add 0.7pf as my input stray capacitance the input impedance drops to 275k.
Here's where I want to get some understanding.
I think the .7pf becomes part of the resonating LC, so it should not be considered as part of the load on the LC. It is just additional capacitance that will lower the resonant frequency. Yes there will be losses in the stray capacitance.
That's what he designed it for.
I cannot find info on how to generate an LTspice netlist that I can post. Will someone walk me through it? Thanks, Mikek
It could be, here's what I did. I measured the different in the output voltage of my fet with and without a 10meg resistor in series with the input. I plugged the circuit into LTspice and juggled the stray capacitance number until it matched my measurements. .7pf is the number that matched. I will post the netlist when I find out how to. Thanks, Mikek
That's not a netlist, .asc files are the schematic itself. You can't import that into another simulator. In the LTspice _View menu, select Netlist, the ctrl-A (copy all), then open your usual text editor and ctrl-V to paste the netlist into a file. ...Jim Thompson
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