Not all doorbell transformers are the same. The different types out there are fixed voltage, center-tapped and multiple-tapped transformers. However, all bell transformers are step-down or fixed voltage transformers.
Fixed voltage transformers operate at 120 volts, center-tapped transformers operate at a dual voltage of anywhere between 5 to 14 volts and multiple tapped transformers are universal and offer various voltages.
In the event your doorbell isn't working, you'll need to check the transformer, but the challenge is finding it first. It is usually located in a nearby closet, high on a garage wall, below doorbell chimes, in the attic or a crawlspace. So simply follow the wires, and you'll find the location of the transformer.
Next, test the transformer to ensure that it matches the devices voltage requirements and that it isn't too low or doesn't exceed 16 volts. If it does, it's time to replace it along with a doorbell transformer cover.