Transistor

[ksoni]

what is the maximum operating frequency of BDX33 and BDX34 ??

[Andrew Neil]

What do their datasheets say?

(it will also depend on your circuit, and what you consider to be the limiting factor(s) on performance)

[AScha.3]

These (old) darlingtons are more for small audio amps, see :

From ds i would recommend to use it at max. 30kHz (sine) , over 30kHz gain falling.