Electronic design knowledge - Impedance
1. Definition of impedance
Impedance is defined instantaneous voltage divided by the current, with the definition of the resistance like, in addition to the impedance difference is outside there resistive capacitive, inductive. It is essentially capacitive electric field in the space in the form of stored energy or dielectric, is the essence of the inductive magnetic field in the space or magnetic energy storage medium . Both cases storing electric energy can be released at other times, rather than the barrier properties as converted to heat energy dissipated. However, capacitive and inductive voltage circuit has a current ratio of a time of a great impact, defined impedance i.e. a combination of resistive, capacitive and inductive in a synthesis parameter, the impedance of the resistor has the same units, are Ω .
Since the unit still ohm impedance, the impedance is also applicable to Ohm's law, and therefore, at the same voltage, the higher the impedance, the less current flows through the impedance, the lower the more current will flow through. Because of capacitive impedance DC infinite and the DC impedance of the inductor is zero, therefore, the impedance for blocking action described more alternating current circuit, the impedance of the expression is a complex (Complex):
Representative of the real part of the complex energy dissipation resistor (Resistance), representative of the imaginary part of the electrical energy storage reactance (Reactance).
Why, the two parts may be treated with a plurality of parameters respectively with a plurality of, two devices may dissipation capacity computing, storage capacity can be operational, but the storage capacity dissipation capacity can not be directly operation.
Since the reactance (capacitance and inductance) is a magnetic field energy in the form of reversible conversion, the electromagnetic field theory, this conversion process is the rate of change of the electric field or the magnetic field associated with, the signal is reflected in the frequency signal. That part of the impedance of the reactance is frequency dependent, the following section is the resistance, capacitor portion, the impedance of the inductance section of the expression:
To Learn More: resistance
Here does not refer to the resistance of said resistance device, but a description of devices or materials hinder current flowing therein, which is essentially irreversible will convert electrical energy into other forms of energy. Such resistance in the circuit, electric blanket heating wire, the electrical energy into heat is dissipated out. Incandescent convert thermal energy to heat and light. The essence of this process is the movement of material at a voltage of electron-atom collisions and energy to the atoms or re-radiation or conduction to dissipate the energy.
Not just energy conversion "hit electrons atom" This way, such as capacitor ESR (Equivalent Series Resistance) there is a part called the dielectric loss, dielectric particles can be power at an alternating electric field due to continue flipping thermal energy conversion.
2. characteristic impedance
Also known as "characteristic impedance", the characteristic impedance is not a basic concept, but the concept is applied to the transmission line. In the high-speed scenarios, the transmission line has a signal over the wire can not be seen, can not ignore some parasitic transmission lines, such as parasitic resistance, parasitic capacitance, parasitic inductance. Synthesis of these parameters is a characteristic impedance of the transmission line integrated scene.
Is the characteristic impedance of the RF transmission line voltage affect the radio wave, the inherent characteristics of amplitude and phase of the current change is equal to the ratio of voltage and current throughout, with the V / I expressed. In RF circuits, resistors, capacitors, inductors will hinder the flow of alternating current, impedance collectively. Absorption of electromagnetic energy is the resistance, capacitance and inductance over the electromagnetic energy is not consumed. Together influence the impedance magnitude and phase of the radio wave voltage and current. The characteristic impedance of the coaxial cable and the conductor, the dielectric constant between the conductors and the outer diameter of the medium, but not with the working frequency of the transmission line connected to radio frequency devices and independent of the length of the transmission line. That is, the ratio of the voltage and current of the entire radio frequency transmission line is constant , the characteristic impedance is constant.
The wireless communication system has two RF characteristic impedance device, one is used for military microwave 50Ω, GSM, WCDMA, etc. system; the other is a 75-ohm, a cable television system, less general application.
3. Common "impedance" - Speaker
. (1) Speaker Overview:
An electric energy is converted into speaker sound an apparatus, when the different currents transmitted to the coil, the excitation coil loudspeaker magnetic field, interacting with the magnetic field of the permanent magnet, thereby bringing the diaphragm vibration sound.
. (2) Speaker impedance characteristics:
Marked on the speaker impedance units of Ω. Impedance of the speaker unit usually it's rated impedance and impedance characteristics are represented, but the two are two completely different concepts. Specific understanding with pictures:
In Figure 1, a sharp rise in the impedance near the resonance frequency, the high frequency portion of the voice coil inductance increases with the increased impedance of the loudspeaker unit with the law of signal frequency changes referred speaker unit impedance characteristics . In Figure 1, the peak of the curve is caused by the resonance vibration system, and the minimum value of this curve is the center speaker rated impedance , also known as "nominal impedance."
Speaker impedance is actually composed of three parts, as shown two, line A represents the DC resistance of the voice coil with frequency; line B represents the inductance section, the inductance rises with increasing frequency, and a voice coil winding method, number of turns of the relevant; C trans line represents an electric part, generates a counter electromotive force, the current direction opposite to the input current and the counter electromotive force is generated, i.e., the vibration of the voice coil impedance increases. When the resonance frequency of the maximum vibration speed, the maximum counter electromotive force is equal to the maximum impedance.
Our national standard GB9399-88 "speaker main technical parameters" in the provisions of the speaker unit rated impedance value is preferably series: 2,4,8,16 and 32Ω, currently rated impedance speaker nationally and internationally most value 4Ω or 8Ω .
. (3) Impedance & Use:
①. When the amplifier circuit to output a fixed voltage to the speaker, in accordance with Ohm's law, four ohm speakers will be more than the current flowing through the double eight ohm speakers, theoretically, an eight ohm output one hundred watts crystal fraction circuit, when connected to four ohm speaker will automatically change to two hundred watts. But when the resistance value of the horn all the way down, the stage outputs a fixed voltage, the current flowing through it will be greater and greater, it is equivalent to the last speaker wire directly short-circuited, resulting in the output stage circuit from overcurrents "burn" .
②. In front of the headset thrust is sufficient, headphone impedance as high as possible, after the high-impedance headphones away, better sound field, suitable for high-quality equipment, but requires high-power amplifier output, and high-current amplifier to be relatively more difficult to design. On the desktop or amplifier, TV and other equipment, is used to a high-impedance headphones, and even some professional headphone impedance at 200 ohms.
③. If the player is Walkman, then we would choose low-impedance headphones, because the low-impedance headphones easier to drive, if you select high impedance headphones, even if the equipment at maximum volume, the sound will be small, but if the impedance is too low, will result in less damping and bass degradation and so on.
4. References
On the loudspeaker | speaker impedance curve
