IMPORTANT >> Make square wave lag by 90 degrees?

Why is this thread locked? As I said, there is an easy way to do this. No one seemed to recognize that the output waveform (see below) is the derivative of the input. A differentiator is very simple. I have posted one below, followed by a logic level translator (the transistor). The value of the capacitor is selected depending on the slew rate of the input.

C1=200nF/Vpp, where Vpp is the peak to peak amplitude of the triangle wave. This will yield +/-2V at the output of the op amp, which is used to switch the transistor.

differentiator

I noticed, but the thread was locked (why??). A differentiator followed by an inverter gives the correct phase, as Ron pointed out.

http://www.ecircuitcenter.com/Circuits/opdfr/opdfr.htm

My thought was to use a dual, single supply op-amp like the LM358 for this, it's hard to get much simpler than Ron's circuit. The trouble with using a comparator like the LM339 is that the signal phasing comes out wrong (the comparator switches at mid-point of the triangle wave).