Editing Input impedance (section)

===Signal processing===

In modern [[signal processing]], devices, such as [[amplifiers|operational amplifiers]], are designed to have an input impedance several orders of magnitude higher than the [[output impedance]] of the source device connected to that input. This is called [[impedance bridging]]. The losses due to input impedance (loss)  in these circuits will be minimized, and the voltage at the input of the amplifier will be close to voltage as if the amplifier circuit was not connected. When a device whose input impedance could cause significant degradation of the signal is used, often a device with a high input impedance and a low output impedance is used to minimize its effects.  [[Buffer amplifier|Voltage follower]] or impedance-matching transformers are often used for these effects.

The input impedance for high-impedance amplifiers (such as [[vacuum tubes]], [[field effect transistor]] amplifiers and [[Operational amplifiers|op-amps]]) is often specified as a resistance ''in parallel with'' a capacitance (e.g., 2.2{{nbsp}}[[megohm|MΩ]] ∥ 1{{nbsp}}[[picofarad|pF]]). Pre-amplifiers designed for high input impedance may have a slightly higher effective noise voltage at the input (while providing a low effective noise current), and so slightly more noisy than an amplifier designed for a specific low-impedance source, but in general a relatively low-impedance source configuration will be more resistant to noise (particularly [[mains hum]]).