Output impedance
The output resistance R, also referred to as an internal resistance or source resistance, characterized the output of an electronic component, an assembly or a device for load change. In this case, there is no single value, because:
- With slow, small changes in load of the differential resistance is significantly
- Rapid changes it comes to the dynamic internal resistance to
- The maximum current is determined with the static internal resistance.
Only in rare cases are the same, the three results. A measurement with a resistance meter is usually impossible. When it comes to complex resistances, which also contain inductors and capacitors, called the output resistance is an output impedance.
Causes the internal resistance
Static internal resistance
In each electric device, the current passing through the copper wires, which contribute to the internal resistance. In a dynamic microphone, it can be 200 Ω, in a power transformer but only 0.01 Ω. When batteries of the current passing through conductors of substantially poorer conductivity than copper, which also may decrease when the battery is discharged. Precise terms, a battery is not "empty", but the internal resistance is due to chemical processes such that the required current can not be taken out.
Which - in comparison to copper - most poor conductivity of the electrodes and the electrolyte is compensated in batteries with large cross sections. If the active surface by decomposition ( dry battery ), or coating ( sulfation or memory effect) becomes smaller over time, the internal resistance of the cell increases. Other factors are temperature, age and size of the cell. In the lithium battery, the electrolyte begins after production slowly " eat up " the positive plate and disintegrates. This chemical modification can increase the internal resistance, it is not reversible. This limits the lifetime of two to three years, regardless of whether it is needed or not.
Differential internal resistance
When monitoring the output voltage electronically, they can be kept nearly constant, if a measure against controls as a result of the deviation and the static internal resistance changes fast enough. For example, there are low-cost fixed-voltage regulator. The internal resistance of laboratory power supplies can reach in extreme cases even slightly negative values , which means that the output voltage with increasing load and increases somewhat compensates for the voltage loss due to the ohmic resistance of a longer connecting cables to the load. However, an exaggerated negative resistance can take care of unwanted oscillations.
In laboratory power supplies, the internal resistance is current dependent: up to a certain maximum current it is very small, so that the output voltage under load hardly changes. If this is exceeded, a modified internal monitoring circuit the internal resistance to very large values. Laboratory power supplies then operate as a constant current source, with decreasing external resistance (up to the short circuit), the voltage output is always smaller without destroying the power supply.
Dynamic internal resistance
The power consumption is rarely constant, in particular for electronic circuits. In computers, the power requirements of individual integrated circuits may change in the nanosecond clock. Because this corresponds to a frequency in gigahertz range, the inductance of the power supply lines can not be ignored, even if they are only a few inches short. The inductive resistance of the wire increases the internal resistance of the voltage source the frequency increases considerably. As a result, the voltage at the device during power changes, for example, between 2 V and 10 V fluctuate and disrupt the IC, possibly even destroy it. A regulation does not respond fast enough, so low-inductance capacitors are used as an antidote directly to the IC terminals. Since capacitors have a certain amount of self-inductance and not the entire frequency range between zero and 5 GHz filter equally well, one electrolytic capacitors and ceramic capacitors with various dielectrics often as possible in parallel.
A classic example of what can be achieved by reducing the internal resistance, the electronic flash light device. The small built-in battery has high internal resistance so that you can get out at a maximum of about 0.5 W power adjustment. Therefore you load with the help of a DC-DC converter on a capacitor, which you can then refer to a peak power of several kilowatts because of its significantly lower dynamic internal resistance.
Determination of the static internal resistance
You can not measure with an ohmmeter, but only indirectly determine the source resistance Ri:
To measure, for example, the output voltage at no load, and then with a known load Ra. Here, when the output voltage is as half as large as idle, Ri Ra = true ( It refers to the module as a black box ).
For example, if the starter battery of a car with the open circuit voltage U0 = 12 V outputs when connected to a 0.5 - Ω resistor only Uk = 10 V, the internal voltage drop of 2 volts and consequently Ri ≈ 0.1 Ω. The internal resistance may vary as a function of the state of charge and is the sum of the resistance of the lead plates, and the boundary layers of the electrolyte ( acid fill).
If a 1.5 -V single cell maximum, ie at short circuit, only Ik = 10 mA outputs, it has an internal resistance of 150 Ω. Usually they say then, the battery was empty, which saw electrical engineering manifests itself as an increase of the internal resistance.
For the internal resistance Ri of the formulas apply:
With
Practical procedure
When working on live parts, the applicable safety regulations must be observed. See low voltage. Furthermore it must be ensured that the operation of amplifiers without appropriate terminating resistor can possibly pull their destruction by itself. It is essential to observe the operating instructions.
It is not practical in most cases, to measure the current ( above all with increasing frequency) with sufficient accuracy. The following approach saves a second measuring instrument because only the voltage is measured. Also, to avoid errors with current-or spannungsrichtigem fairs. One needs to carry out a resistor, the following conditions must meet:
- The resistance value should not deviate from the expected extreme internal resistance as low as possible in order to keep the measurement error.
- The maximum current carrying capacity of the source must not be exceeded.
With
- Amplifiers at the prescribed degree or nominal resistance value should be used.
- At increasing frequencies Composition Resistors shall be used to avoid inductive reactances. As an "alternative" solution and 10 to 20 are connected in parallel metal film resistors. This results in the value of the individual resistances from the multiplication of the number of Rmeas individual resistors.
- The power dissipation of the load resistance is calculated from
In case of parallel resistors, the power loss divided by the number of resistors.
First, the exact value of the load resistance Rmeas is determined. Now, the open-circuit voltage V0 is measured. Then the measurement of load on the source ( Ul ) by means of the load resistor repeated. From the thus determined values of the internal resistance can be computed using the formula 2).
Impact on parallel
In an ideal voltage source, that a voltage source without any internal resistance, it can be a plurality of loads connected in parallel to each other, without changing the voltage and hence the current to the current consumers. Only the total current in the circuit increases. However, since an internal resistance exists in a real voltage source, the increase of the total current will cause the voltage across the parallel-connected consumers decreases ( because even the voltage drop across the internal resistance increases), and thereby the single-stream of the current consumers through the added parallel additional load considered on its decreases. Despite this decrease in the individual currents of the total current increases with each new load to the limit and, consequently, the voltage across the parallel-connected consumers with the threshold value V 0 from. Due to this fact the parallel connection of devices is only possible within certain limits, because although the voltage at each parallel load is the same, but this decreases with each new parallel branch and at some point no longer sufficient to provide a consumer with its respective minimum power P.
Designation
Often the load, external or input resistance with and referred to the source, internal or output resistor, resulting in more misunderstandings because external resistance (load ) Output resistance (source) can not be. The terms and are to be avoided because only the outer, load or input resistance can be.
There are two approaches (see figure at right ):
- As " interface " for two interconnected devices and
- As a device with input and output.
The external resistor is the load resistance and the output resistance is the source impedance and the internal resistance.
Outputs are also referred to as active, inputs as passive. Both, however, can automatically adjust itself in special cases to the respective level or load impedances.
If an output is short-circuited, short-circuit current flows, which can be calculated from the open-circuit voltage and the output impedance.
Values of the output resistance
In general, a circuit then the maximum power is taken when the external resistance is equal to the output resistance ( impedance matching ). In communication engineering, this case is often sought when it comes, for example, to fully exploit the smallest benefits of receiving antennas. Also in the telecommunications and communications technology applies: The maximum power can be transferred if the output impedance matches the input impedance of the next module. This often there is the conventional power adjustment, with the result that the output voltage is half as large as the open circuit voltage.
In power engineering, the output resistance of the transformers is held (that is, the equivalent resistance of all consumers ) are very small compared to the external resistance. The reasons for this are:
- Voltage stability
- Low thermal load on the source
It also says a power grid works almost idle.
An amplifier has an input on the side of an input resistor (load resistance, external resistance or terminator of him feeding source) and on the output side of an output resistor ( source resistance or internal resistance of the amplifier output).
In the Hi -Fi technology and sound engineering is that the output resistance of a device has to be smaller than the input resistance of the following equipment, which is also referred to as voltage adjustment. reasons:
- One wishes to measure or enhance, so this should be significantly higher compared to the falling voltage to the voltage drop over. This ensures a good signal -to-noise ratio.
- A speaker better damped (who then has better transmission characteristics ), when energized by a source of low source impedance.
- In a dynamic microphone, the output resistance is relatively small; in the studio equipment is less than 200th
- In a condenser microphone, the output resistance is at the site of the membrane capacitor is very large ( magnitude gigaohm ), but at the microphone output impedance he is conversant with studio microphones about 50 microns.
- In a battery, the output resistance should be as small as possible, so that can be used effectively in their contained energy; it increases towards the end of the service life and increasing discharge.
In the recording studio facilities after the IRT Issue 3 /5 ( mixing consoles ) the internal resistance has to be less than 40 ohms over the entire frequency range of 40 Hz to 15 kHz. The outputs are also balanced and floating.
High voltage sources for laboratory purposes, however, usually have a specifically high output resistance in order to limit the current to 20 mA.
When connecting several modules of the respective internal resistance is observed.
The internal resistance of speaker power amplifiers is rarely indicated in the data sheets, but it should be as small compared to the load impedance (2, 4 or 8 ohm minimum speaker impedance ) be. Is the damping factor DF is known, can be determined by:
The attenuation factor is particularly high for the speaker if the power amplifier has a low source impedance. Usual transistor amplifiers have source impedances of < 0.1 ohm. In order not to increase the output resistance through leads, cables ( depending on their length and the load impedance, but not performance ) have a sufficiently large cross-section.
At each intersection of the output resistance of the source is connected to the input resistance of the load to adjust damping.