That's it? No details or other observations? With nothing else to go on, I can share that GM has had a major problem with their generators starting with the current design that showed up in '87 models. Due to their design they develop huge voltage spikes that can destroy the internal diodes, internal voltage regulator, and can interfere with computer sensor signals. When one of the six diodes is defective the generator will only develop a maximum of one third of its rated current. That's barely enough to run the car's electrical system with little left over to keep the battery charged. "Ripple" will also be very high and that can be heard in the radio. The clue is the noise will go away when you unplug the small connector on the rear of the generator while the engine is running. To verify a bad diode, have the charging system professionally load-tested. The typical current rating for these generators is around 90 to 120 amps. If all yours can deliver is around 30 to 40 amps, and ripple is high, replace the generator AND the battery unless it is less than about two years old.
It is common to go through four to six replacement generators in the life of a GM vehicle. To reduce the number of repeat failures, replace the battery too. As it ages it loses its ability to dampen and absorb those harmful voltage spikes. The old battery will work fine in an '86 or older model.