amplifying 555 astable multivibrator output
The below will operate adequately at 800kHz. 







simulate this circuit &ndash; Schematic created using CircuitLab

The 2N7002 level shifts the 7V output signal to 0/40V and the push-pull emitter follower output stage provides more drive current (and since the transistors cannot saturate, it switches quickly). Voltage swing will be a bit less than the supply voltage. 

If you don't need much current drive you might be able to eliminate the output stage, but even a small capacitive (tens of pF) loading will slow the output significantly. With the output stage it can drive 1000pF easily.