For IC couplers, there is a requirement related to common-mode transient immunity. However, for transistor couplers, such a requirement does not exist. Why?

Common-mode transient immunity refers to the maximum tolerable rate of rise (fall) of input/output common-mode voltage at which the specified High (Low) level can be maintained. A photocoupler has a small capacitance (0.6 to 0.8pF) between the input and output terminals. When an abrupt change in voltage occurs between the input and output terminals, a displacement current that occurs through this capacitance flows into the photodetector chip circuit, eventually causing malfunction. This phenomenon is likely to be felt more clearly in high-speed and high-sensitivity photocouplers. For transistor couplers, since their device responsiveness is about several times to tens of times lower than that of IC couplers, this phenomenon is unlikely to manifest itself easily.