low(est) frequency amplifier

[TimFox]

You didn't state where your actual pulse originates (not in the simulation), but I assume it will start from 0 V.  The input errors of the amplifier (DC offset, noise, etc.) require you to set an input threshold slightly above zero before your output rises:  this can be done with the op-amp circuit (with diode bounding), and then followed by a comparator to get a fast logic output.  Op amps have lower noise and offset than the comparator, but the latter is designed to swing between fixed levels.  You can't set the input threshold at zero for physical and mathematical reasons.  I recommend the two diodes plus a large resistor (all in parallel) for op-amp feedback;  the resistor keeps the feedback connected when the diodes are not yet conducting;  the resistor will give the high gain near zero that you need.

[jazzalex]

You didn't state where your actual pulse originates (not in the simulation), but I assume it will start from 0 V. 

In fact it starts with 0 V but the actual problem (realized by today) is that since my Gaussian pulse is extremely long it in turn starts with extremely low voltage which obviously is not high enough for the comparator circuit to trigger. Now I wonder what is the lowest threshold a comparator circuit can use as the trigger voltage.

Thanks

Alex

[TimFox]

The lowest threshold is a combination of input noise and offset.  The noise is a function of the resistance seen by the input, the amplifier noise, and the bandwidth.  The offset can be adjusted away, by adjusting the threshold, but will drift with temperature and time.  It’s not a simple question.  For best accuracy, I recommend the combination of bounded op amp and comparator to get a well-defined threshold.