because you r designing a driver, if the input impedence is fixed and the load is 20pf. probably your only choice is increase the bias current. it is because the problem is introduced by the low gain or low driving capabilty of each stage.
it is very hard now, because you may be limited by the capability of the technology.
if the signal bandwidth is not large, you may be able to use inductive peaking technique to increase the gain, hence use less stages.
Added after 3 minutes:
there may be some technique or circui will increase the gain a little bit.
try using negtive resistance to increase the gain of each stage.
it might help.