This is a basic differential amplifier.
I want to modify it in order to have high input impedance (a few \$M\Omega\$ from DC to at least \$200~kHz\$)
I thought about replacing the CE pair input with a CC-CC cascade pair, or a Darlington pair, but I don't think they have good frequency response.
Any suggestion?
I don't really care about the gain, because I will use another stage for that purpose.