Complex sentence: As the photodiode, the phototransistor has a very short time response, respond to fast light changes, but gives much greater power variations due to an amplification of the photodiode that does not have.
Split into simpler sentences: As the photodiode, the phototransistor has a very short response time, so responds to fast light changes. Unlike a photodiode it gives much greater power variations due to an amplification that the photodiode does not have.