Power operational amplifiers (POA) are used to increase the power of low-level signals in applications that drive low impedances or reactive loads. They dissipate excess energy as heat, deliver extensive current, and can sustain relatively high supply voltages. Most devices have impedances that are 10 to 100 times lower than small-signal operational amplifiers and can deliver larger amounts of current and dissipate more power. There are several operational classes for power operational amplifiers. Class A devices are the most linear and the least efficient. Class AB designs provide increased efficiency and excellent linearity. Class B amplifiers are used almost exclusively in low-power applications. Class C devices are used with radio frequency (RF) transmissions. Class D amplifiers switch on and off at least two times per cycle while Class E units are designed for rectangular input pulses rather than sinusoidal audio waveforms. Class F power operational amplifiers include both RF and microwave devices. Glass G designs change the power supply voltage from a lower level to a higher level when larger output swings are required. Class H models modulate the higher power supply voltage through the input signal.