Signal amplifiers are electronic devices that receive and amplify signals for processing or digitization. They produce only a small amount of internal noise and increase signal strength by a significant factor. Signal amplifiers often include field-effect and bipolar junction transistors, electronic components that amplify signals without producing significant amounts of thermal noise. They also include one or more operational amplifiers. Signal inputs include AC and DC voltages, AC and DC currents, charges from piezoelectric devices, and frequencies and other specialized waveforms. Some signal amplifiers receive inputs from accelerometers, thermocouples, thermistors, resistance temperature detectors (RDTs), strain gauges, or bridges. Other devices receive signals from linear rotary differential transformers (LVDTs) and rotary variable differential transformers (RVDTs).