Bipolar op amp retrofit with JFET preamp?

[Scott Gravenhorst]

Is it possible (even desirable) to retrofit a bipolar opamp with 
a JFET preamp to increase the input impedance?

I realize that there are ready made JFET input opamps.  What I 
am looking to do is increase the input impedance of one of the 4
amps in an LM324.  The application is a unity gain control voltage 
amplifier whose range is from about +.03 volts to about +6 volts.  
Can I simply pass the CV thru a source follower with say a 1 meg 
gate bias resistor to ground and then into the opamp input?  I look
at that and think that the JFET will be forward biased and won't work.
A 1 meg resistor to -v for gate bias instead?

-- Scott (who is obviously not an EE)

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