Fiber optic receivers are instruments that convert light into electrical signals. They contain a photodiode semiconductor, signal conditioning circuitry, and an amplifier. Fiber optic receivers use three types of photodiodes: positive-negative (PN) junctions, positive-intrinsic-negative (PIN) photodiodes, and avalanche photodiodes (APD). PIN photodiodes have a large, neutrally-doped region between the p-doped and n-doped regions. APDs are PIN photodiodes that operate with high reverse-bias voltages. In short wavelength fiber optic receivers (400 nm to 1100 nm), the photodiode is made of silicon (Si). In long wavelength systems (900 nm to 1700 nm), the photodiode is made of indium gallium arsenide (InGaAs). With low-impedance amplifiers, bandwidth and receiver noise decrease with resistance. With trans-impedance amplifiers, the bandwidth of the receiver is affected by the gain of the amplifier. Typically, fiber optic receivers include a removable adaptor for connections to other devices. Data outputs include transistor-transistor logic (TTL), emitter-coupled logic (ECL), video, radio frequency (RF), and complementary metal oxide semiconductor (CMOS) signals.