I haven't had much luck finding anything much different from the others. But, after I figure out what gain I'm after, I want to use something like the "Ultra high Z in unity gain amplifier" shown at the top of page two, in the this link. >
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We want as little load on the cap as possible. Mikek
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Yes, and I don't have a full understanding of that, I don't know if they are trying to match the reciprocal non-linearity of the thermocouple in the circuit. Also the meter it drives has a non-linear scale. (On purpose) The overall plan is to measure the injection voltage with a modern RF millivolt meter, and forget the thermocouple output. Then have a linear output from the Fet and drive a meter with a linear scale. It's all a little pie in the sky, because everything I've tried has been non-linear.
The problem I'm trying to solve is as follows. The Boonton 260A only measures Qs up to 625. but if you lower the injection voltage (now you need to measure with an RF millivolt meter) you can measure higher Qs, using a little math. Injection Voltage is the multiplier, 20mV is 1X, 10mV is 2X 5mV is 4X and 2mV is 10X. The problem we're having is, it goes non-linear as you drop the injection voltage, were it starts depends on the tube. I'm hoping the Fet is not non-linear. Mikek
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Yes, there are multiple ways to measure Q, My 6.5" diameter by 6" length coil won't fit on your pcb! Also, if I get within 2 ft of the coil it changes the characteristic. Mikek
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Ya, I'm past that, I want to use a linear scale if that will work. Maybe a peak detector and a little hold, then scaled. Maybe need to amplify it before detecting to get linearity. That's a question! I found that the jellybean 2N3819 has a 3pf Ciss, lowest I've found. But I'm not sure how much that matters with the bootstrap circuit on the Ti site I linked to. Tell me this, the TI "Ultra high Z in unity gain amplifier" has an input of 0.25pf and 100Meg, if I use a capacative divider of 0.3pf in and 3 pf to ground, does that raise my input Resistance 10 times? Assumes incorrectly the divider will not add leakage resistance. Mikek
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You can't bootstrap that source follower detector* since the source no longer follows the AC signal but if you really wanted to lower its already pretty low input capacitance you could precede that detector with another fet wired as a bootstrapped follower.
*That source follower circuit with capacitor bypass and high value load resistor is what was sometimes called an "infite impedence detector" popular in regen TRF receivers way back. Your 12k load seems rather low though. Fet circuits I have seen used much higher values, I think >100k. As you have correctly figured there will be a standing bias DC voltage at the source that your meter circuit will have to zero out.
I have done that hundreds of times. That's not the point, I want a replacement for the hand selected 535 tube, it was hand selected for certain characteristics in the 1950s. They just aren't available anymore. Right now, I know of 4 tubes, only one of them still meets those specs.
So, my questions still stands, with an addition.
Can I use the "Ultra high Z in unity gain amplifier" shown at the top of page two, in the this link. >
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For a gain of one, then add a precision rectifier with a hold cap. If everything is perfect* I get a gain of 1.4 because I'm holding the peak voltage. I can easily scale it down as needed. I'm over my head here, remember I'm the fish monger, but I can fabricate a decent pcb. With a little help, I can get this done. Can you recommend an opamp, that will work as a precision rectifier up to 10 Mhz? Oh, I probably need some fast diodes too.
On another note, the 2n4416 has a 3pf Ciss, but it costs $6 each! I want a through hole fet here because it is to easy to over drive and pop it in normal usage. "I've been warned about that." Through will be easier to change.
I know they won't be perfect
Thanks for your time, Mikek
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