[volt-nuts] Why discrete JFET differential pair before opams?

[Attila Kinali]

Hi,

I've been again reading a bit and stumbled over a few application
notes by Jim Williams. What stuck out on those, that he uses at times
JFET differnetial pairs as a kind of pre-amplifier before opamps (e.g. [1], [2])

The only reason I can think of using JFETs is low gate input leakage.
But in [1], the LSK389 has a specified max input leakage of 200pA
while the LTC6241 has 75pA.

Could someone be so kind and enlighten me, why a JFET diff pair
makes sense in such applications?


			Attila Kinali

[1] "40nVpp Noise, 0.05uV/°C Drift, Chopped FET Amplifier",
by Jim Williams, 2006
http://www.linear.com/product/LTC6241 (page 39)

[2] "1Hz to 10MHz V->F Converter",
http://cds.linear.com/docs/en/application-note/an13f.pdf (page 8)
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