I need to know a bipolar junction transistor which has an input impedence more than or close to 20 kOhms.

 

Input impedance in BJT is depending on circuit configuration.
For common-emitter Rin is equal
Rin ≈ hfe * (26mV/Ic)
And for common collector (emitter follower)
Rin ≈ hfe * ( 26mV/Ic + Re).
So for Rin> 20KΩ you need to use emitter follower circuit.

 

thankz dude