comparator problem

Hey
I'm actually designing a comparator circuit


|--------feedback resistor------|
| |
| |---------------------| |
RC CIRCUIT ----|------|+ | |
| COMPARATOR CIRCUIT | |
INPUT----------|- |--|---------OUT
|---------------------|
The task is to design this cirucit in such a way that when an input
pulse (0.5,-2.5) with a pulse width of 8micro seconds is given it
should produce me an output which remains high for atleast like
64micro seconds..i tried using R=2248.5k and C=8500pF and the feedback
resistor i used was 90k..i was able to get an o/p which remains high
for 64microseconds but when looked at it carefuly i was able to figure
out that the outout remained high just beacuse of the large difference
between the resistors(the storage capacitance is not much observed in
the o/p graph)...it should basically work like a monostable
multivibrator (without using ic 555 timer though)...it would be of
great help to me if u could give me some suggestions as how could i do
this..

thanks
sk

sk