# Input resistance

The input resistance , also called external resistance , terminating resistance or load resistance , is a property of the inputs of electrical devices. It loads the device that applies a voltage to this input. ${\ displaystyle R _ {\ mathrm {a}}}$ The feeding device has a source resistance (source impedance) , which is usually at voltage adjustment is much smaller than the input resistance is the powered device: . ${\ displaystyle R _ {\ mathrm {i}}}$ ${\ displaystyle R _ {\ mathrm {a}}}$ ${\ displaystyle R _ {\ mathrm {i}} \ ll R _ {\ mathrm {a}}}$ For DC voltages, but especially for AC and signal voltages and especially in high frequency applies to power adjustment - that output resistance and input resistance have the same value impedance matching : . ${\ displaystyle R _ {\ mathrm {i}} = R _ {\ mathrm {a}}}$ With Terminator (also dummy load , dummy load or short completion , the technical design is called) (mostly in the form of a plug) suitable high frequency of resistance referred to the z. B. is connected as a load to a signal source or line for measurement purposes or to avoid reflections . To test transmitters, terminating resistors with a correspondingly high output are required ("artificial antennas"). Such terminating resistors must have a particularly low induction, so only non-coiled sheet resistors or ground resistors are used.

Examples

• An oscilloscope usually has an input resistance of 1 MΩ, so that the measuring point (<< 1 MΩ) is only slightly loaded. By connecting a probe head , this input resistance can be increased to 10 MΩ or more; at the same time the influence of the measuring line is avoided.
• For the same reason, voltmeter and multimeter have the highest possible input resistance; the latter usually have an input resistance of 10 MΩ.
• If a signal is fed to a high-frequency device via a cable (e.g.  LAN , radio transmitter and receiver), this must have an input resistance suitable for the cable impedance (often 50 Ω) in order to avoid pulse reflections (see adapted cable ). The input resistance of antennas is called the base point resistance .
• Input resistances are always "passive", while output resistances are predominantly "active", as can be clearly seen in the figure. A passive input cannot be “short-circuited”; short-circuited sources serve only as a model. The internal resistance Ri, output resistance or source resistance can also be passive, for example in the case of attenuation and impedance matching elements.
• Often the load, external or input resistance is referred to as and the source, internal or output resistance as , which always leads to misunderstandings because external resistance (load) cannot be the output resistance (source). The designations and are to be avoided because only the external, load or input resistance can be.${\ displaystyle R _ {\ mathrm {e}}}$ ${\ displaystyle R _ {\ mathrm {a}}}$ ${\ displaystyle R _ {\ mathrm {e}}}$ ${\ displaystyle R _ {\ mathrm {a}}}$ ${\ displaystyle R _ {\ mathrm {a}}}$ • In mixing consoles by IRT - specification 3/5 (1989) (input impedance) of normal level inputs within the entire frequency range (40 Hz to 15 kHz) to be greater than 5 kOhm. For microphone amplifier inputs (input impedance) should be greater than 1 kΩ. The deviations should remain below 20% over the entire frequency range.${\ displaystyle R _ {\ mathrm {a}}}$ ${\ displaystyle R _ {\ mathrm {a}}}$ • A power amplifier should have the lowest possible source impedance (<0.1 Ω) in order to provide the loudspeaker with a nominal load impedance of e.g. B. to attenuate 4 or 8 Ω as well as possible; see damping factor .

With alternating current , the input resistance is often a complex resistance that also contains inductive and capacitive components. Complex resistances are called impedance .

Input and output resistances do not necessarily have to be represented by real resistances , but can result from the behavior (impedance) of a complex active or passive circuit.

In the case of operational amplifier outputs, the terminating resistor is often determined by a passive resistor - the operational amplifier itself usually has too low a source resistance or must not be subjected to too much load in order to function as calculated.

The value for a load resistance Ra should not be measured with an ohmmeter, but only determined indirectly.

## The impedances and their different names

${\ displaystyle R _ {\ mathrm {i}}}$ ${\ displaystyle R _ {\ mathrm {a}}}$ Internal resistance External resistance
Output resistance Input resistance
- Terminating resistor

## literature

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• Hans Fricke, Kurt Lamberts, Ernst Patzelt: Basics of electrical communication . BG Teubner Verlag, Stuttgart 1979, ISBN 3-322-94046-2 .
• Thomas Mühl: Introduction to electrical measurement technology. Basics - measuring methods - devices. Springer Fachmedien, Wiesbaden 2014, ISBN 978-3-8348-0899-8 .
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• Rainer Felderhoff: Electrical and electronic measurement technology. Basics - procedures - devices and systems. 8th edition. Carl Hanser Verlag, Munich 2007, ISBN 978-3-446-40571-4 .
• Dietmar Benda: Electronics without ballast. Basics of electronics easy to understand. Franzis Verlag, Poing 2008, ISBN 978-3-7723-5380-2 .
• Harald Schumny: Signal transmission. 2nd, revised edition. Friedrich Vieweg & Sohn Verlag, Braunschweig 1987, ISBN 3-528-14072-0 .