Impedance converter

An impedance converter is an assembly that adapts the impedance of a source to that of a consumer.

Impedance converter in electrical engineering

In electrical engineering , the impedance converter is an electrical assembly, a quadrupole , which adapts the impedance (the alternating current resistance) of a source to that of a consumer .

Passive converters

Passive two-port as an impedance converter

A passive impedance converter in its equivalent circuit is a two-port with correspondingly dimensioned impedances . The input and output as well as the internal impedances can be arbitrary. A transformer is also a passive impedance converter, because it causes an impedance transformation at the same time due to the current and voltage transformation. An approximate ideal transformer is assumed, i.e. a lossless power transmission from the primary side 1 to the secondary side 2 , and the two powers P are then identical:

${\displaystyle P_{1}=P_{2}}$

About the reference that the power P on the amount of an impedance Z equals

${\displaystyle P={\frac {U^{2}}{Z}}}$

and with the transformation equation at an ideal transformer with the transformation ratio and the two numbers of turns N from the primary and secondary side: ${\displaystyle \gamma }$

${\displaystyle \gamma ={\frac {N_{1}}{N_{2}}}={\frac {U_{1}}{U_{2}}}}$

the transmission ratio results as a function of the impedances

${\displaystyle \gamma ={\sqrt {\frac {Z_{1}}{Z_{2}}}}}$

with which the impedances between the primary side and the secondary side on the transformer are transformed quadratically:

${\displaystyle \gamma ^{2}={\frac {Z_{1}}{Z_{2}}}}$

In high-frequency and antenna technology , this transformer is often a balun , in addition to which, among other things, resonance transformers , line sections of certain cable impedance and length are used as impedance transformers.

Active converters

Reduce the impedance with operational amplifier

Active impedance converters are always analog amplifier circuits . Since with active impedance converters electrical power is supplied to the circuit by a voltage source , almost any type of impedance conversion is possible.

The basic transistor circuits and circuits with operational amplifiers such as the voltage follower are mentioned as examples .

High frequency circuits

In devices used in high-frequency technology , different impedances of active components have to be matched to one another . In the case of wavelengths that significantly exceed the dimensions of the devices, resonance transformers are also used for this purpose, and for shorter wavelengths, lines of suitable length for impedance matching, for example strip lines , are used within the framework of line theory . Complex resistances can also be adapted with both arrangements.

Non-electrical converters

sound

The impedance conversion of sound is necessary for its effective transmission and radiation. The aim of a horn loudspeaker is to match the wave impedance of the actual loudspeaker to the characteristic sound impedance of the surrounding air . Here are more examples:

Middle ear

Since the outer ear is filled with air and the inner ear with fluid, there is an increase in acoustic impedance in the middle ear , i.e. the transition . This acts like a reverberant wall . To counteract this increase, there are two mechanisms in the middle ear: the leverage of the auditory ossicles and the increase in pressure, which is created by the difference in area between the oval window and the eardrum .

Bass reflex principle

The resonant bass reflex principle is often used in loudspeaker boxes in order to still achieve good radiation of low frequencies with a limited speaker volume. The impedance against which the loudspeaker works is reduced by exciting an acoustic resonator, which consists of the elastic volume of air in the box and the air mass moving in the bass reflex tube.

Sonotrode

Sonotrodes are used to adapt the impedance of an ultrasonic transducer to the workpiece during ultrasonic welding and bonding . In addition, they are tapered, i. H. inverted like an exponential horn, shaped.

Exponential horn

Exponential horns are found on some tweeters and as funnels on gramophones and the Edison apparatus . In the past, meter-sized exponential horns behind the screen were used for sound reinforcement in cinemas. In all these cases, they serve to match the impedance of the sound transducer to the free space impedance of the airborne sound. Today, exponential horns are still used for truck horns and air horns. See also horn (loudspeaker) and macrofon .

literature

• Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 12th edition. Springer, 2002, ISBN 3-540-42849-6 .