High DC impedance is one thing, but 100kHz with good
waveshape fidelity would need a fair bit of output current
if there's any significant load capacitance. This needs to
be expressed before you can say that a Class A output
will be acceptable power-wise.
Figuring that, say, I wanted a 1MHz 3dB full power
bandwidth to ensure 100kHz is clean, and I had a 50pF
load, something like 1E-6s/50E-12F=>Rout=20Kohms
(leading to a 2% DC amplitude error if outside the
feedback loop) and a Class A idle current of 2mA
or so (higher than the 40uA that the DC load and
supply voltage alone, suggest). What is real Cload,
though?