Editing Input impedance (section)

==Applications==

===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]]).

===Radio frequency power systems===

Signal reflections caused by an impedance mismatch at the end of a transmission line can result in distortion and potential damage to the driving circuitry.

In analog video circuits, impedance mismatch can cause "ghosting", where the time-delayed echo of the principal image appears as a weak and displaced image (typically to the right of the principal image). In high-speed digital systems, such as HD video, reflections result in interference and potentially corrupt signal. 

The standing waves created by the mismatch are periodic regions of higher than normal voltage. If this voltage exceeds the [[dielectric breakdown]] strength of the insulating material of the line then an [[Electric arc|arc]] will occur. This in turn can cause a reactive pulse of high voltage that can destroy the transmitter's final output stage.

In RF systems, typical values for line and termination impedance are [[50 Ω]] and [[75 Ω]].

To maximise power transmission{{what|reason=What is the relation between (non-conjugate) impedance matching at termination of a line, and conjugate-matching of source and load impedance (output and input) across a port? These appear to have two different effects, yet they are intermixed without any transition.|date=January 2017}} for radio frequency power systems the circuits should be ''[[complex conjugate]] matched'' throughout the [[power chain]], from the [[transmitter]] output, through the [[transmission line]] (a balanced pair, a coaxial cable, or a waveguide), to the [[antenna (radio)|antenna]] ''system'', which consists of an impedance matching device and the radiating element(s).