Re: Frequency counters
The "JFET Source-follower" does work - I've used it extensively. You need to take care to accept and acknowledge the wide variability in individual JFET pinch-off characteristics and adjust the bias accordingly.
[There's something to be said for lifting the earthy-end of the gate-to-ground resistor and feeding it from a potential-divider across the supply rails: then adjust this potential-divider to get approximately half the DC supply-voltage at the source]
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