What is the role of the diode in the circuit?
It looks like a common emitter amplifier using a PNP transistor so it has a negative power rail with respect to 0V. However there is a positive voltage biasing the base and this will reverse bias the base-emitter juntion turning the transistor off to a certain degree.

When an input signal is of sufficient peak-to-peak amplitude, part of the negative section of the input waveform will cause the base-emitter junction to be forward biased and the output will amplify this part of the input waveform and probably produce something akin to a pulse at the collector.

The diode prevents the base-emitter junction from becoming too reverse biased (either to prevent base-emitter reverse voltage breakdown or just to "shape" the input signal a little bit).