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

[Dave Daniel]

Attila,

That is a most excellent signature.

I understood exactly to what you were referring after the first line.

DaveD

On 8/29/2015 12:06 PM, Attila Kinali wrote:
> 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)