by pebe » Sun Apr 10, 2011 4:28 pm
Your 40kHz square wave signal has an 'on' time of 12.5µs, but the LT1013 has a slew rate of only 0.4V/µs so the output can only rise and fall relatively slowly - hence the sawtooth (triangular?) wave.
The 555 can give an output of 200mA, so do you really need a buffer?