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Input FET stage

A project log for Active scope probe with no dedicated power supply

Using the probe test point to power a ultra-low capacitance, high-input-impedance amplifier, essential for accurate analog measurements!

Yann Guidon / YGDESYann Guidon / YGDES 04/28/2016 at 02:040 Comments

I just received some BF256B and BF245C. I heard the 245 came after the 256 so I'll be able to test which works better, with enough supply to match a few pairs.

I haven't decided yet which instrumentation amplifier I'll use. This part of the analog domain still sounds like black magic to me but I know it must draw the least possible current and have a reasonable bandwidth (10MHz ?)

In case I can't find a suitable part, I can still put together a discrete op-amp with JFETs...

Interesting texts:
http://www.analog.com/library/analogdialogue/archives/47-10/discrete_amplifier.html (I wouldn't use high gain, unity is OK)
https://www.fairchildsemi.com/application-notes/AN/AN-6601.pdf (less advanced version but useful for estimating the noises)
http://www.forsselltech.com/media/attachments/JFET_Opamp.PDF (pretty cool tricks in the input stage but the impedance must be modified and I don't need the high voltage gain)
http://contifret.free.fr/ (discusses complementary vs non-complementary input topologies and JFET selection)

This discussion is a direct consequence of my discovery of this circuit (I can't remember where)

The advantage of a full discrete solution is the ease to evaluate power consumption, for example. And there is no hard-to-find dedicated circuit.

So maybe the next log will focus on how to enhance a similar circuit (that is not dedicated to audio applications). I know already that it misses an offset compensation (with a simple trimmer) and a constant current source (another FET to replace R1).

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