Differential probes are active devices in some cases and passive in others. They are based on an amplifier with a positive and negative input. The active differential probes are transistor based. Differential probes are useful for measuring signals when there is no ground reference, for example PCI express is a differential data path which would benefit from the use of differential probes. There are also high voltage differential probes that are typically passive type devices that are able to handle very high voltages. The final type of probe is the current probe. There are three main types of current probes: transformer based, Hall Effect device based, or a combination of both transformer and Hall effect device. The majority of current probes are clamp type devices, and most of them are Hall Effect.