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So if an amplifier is an electronic circuit(/device) that boosts the voltage and/or power of an AC electrical signal, what do you call such a device that decrease the voltage without increasing the current or better yet reduces the power of an electrical signal?

What I'm thinking of is essentially an amplifier with a voltage or power gain less than unity(in terms of magnitude).

Such a device might be used at a power substation to reduce both the voltage and current from a HV power source. Or when measuring a very high voltage current or signal(with high wattage) by means of a computer where the signal carries enough power to fry the low-power components. As step-down transformer reduces the voltage but increases the current, so I'm not talking about those.

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  • \$\begingroup\$ In DC terms, reducing the voltage without reducing the current is accomplished relatively easily with a linear regulator - however, they are avoided because they literally waste power - that is what it means to lower voltage without increasing current. If you just want this, but with AC instead, a voltage divider (i.e. two resistors) could work up to a certain level, as could a lossy (i.e. inefficient) transformer - might I ask for what? more amps available is generally not a bad thing (unless you need to explicitly limit current, such as when working with LEDs) \$\endgroup\$ – user2813274 Dec 10 '15 at 1:10
  • \$\begingroup\$ "Such a device might be used at a power substation to reduce both the voltage and current from a HV power source." Not really, because doing so would be dubious at best other than measurement purposes, which the next sentence covers. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 10 '15 at 1:27
  • \$\begingroup\$ The term you are looking for is attenuation or sampling but you need to indicate the practical use and this may mean you are looking for a current transformer or a HV probe or something other still. \$\endgroup\$ – KalleMP Dec 10 '15 at 8:37
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Such a device could be called an attenuator (defined as "an electronic device that reduces the power of a signal without appreciably distorting its waveform.")[1].

For specialized RF situations, there are specific designs based usually around passives of carefully-selected values for a given frequency or frequency range.

For low-frequency applications, or where extreme precision isn't necessary, it's often common (at least for hobbyists, not sure about professional applications) to use simple voltage dividers with the divided voltage connected an an analog input, for example on a microcontroller.

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