Impedance bridging
Encyclopedia
In electronics, especially audio
and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection
is one which maximizes transfer of a voltage signal to the load. The other typical configuration is an impedance matching
connection, which maximizes power
delivered to the load.
When the output (output impedance
) Zout of a device (the source, ZS in illustration) is connected to the input (input impedance
) Zin of another device (the load, ZL in the illustration), it is a bridging connection if the second device does not appreciably load the previous device.
Essentially no power is transferred. The second device is sensitive to the output voltage of the first device, and this is maximized when loading is minimized. All good line level
audio connections are bridging, to ensure the high fidelity transfer of a voltage signal from one device in the chain to the next. If the bridging criterion is not satisfied, there may be distortion, altered frequency response or both.
A connection is commonly said to be bridged if the load impedance is at least ten times the source impedance.
, DF, which is:
Now it is easy to calculate Zsource
In audio systems, Zload is typically the nominal impedance of an 8 ohm loudspeaker. The output impedance of the amplifier is typically in the same order of magnitude of the impedance of the cables connecting it to the speaker (<0.1 ohm), so DF will drop rather easily.
Applying negative feedback
also increases the damping factor, because the source resistance is effectively reduced. In this way, damping factors into the hundreds can be reached.
Although the load impedance is significantly greater than the source impedance in this situation, the purpose here is not to transmit a voltage signal with great fidelity from input to output. Therefore, this situation is not an instance of impedance bridging. (If voltage transfer mattered in driving a loudspeaker, it would be easy to simply design the loudspeaker to have a high impedance.) Furthermore, the goal is to transfer power to the speaker, whereas the goal of impedance bridging is the exact opposite: to minimize transmitted power, allowing devices not to require powerful current sources to drive their outputs.
The reason an amplifier can transmit maximum power to a loudspeaker even though the speaker's impedance is much higher is that the power amplifier amplifies current. Its output impedance is kept as low as possible to minimize the current's power dissipation in the amplifier itself, thereby maximizing dissipation in the speaker. The low output impedance also dampens the speaker's motion.
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection
Electrical connection
An electrical connection between discrete points allows the flow of electrons . A pair of connections is needed for a circuit.Between points with a low voltage difference, direct current can be controlled by a switch...
is one which maximizes transfer of a voltage signal to the load. The other typical configuration is an impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
connection, which maximizes power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
delivered to the load.
When the output (output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
) Zout of a device (the source, ZS in illustration) is connected to the input (input impedance
Input impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
) Zin of another device (the load, ZL in the illustration), it is a bridging connection if the second device does not appreciably load the previous device.
Essentially no power is transferred. The second device is sensitive to the output voltage of the first device, and this is maximized when loading is minimized. All good line level
Line level
Line level is a term used to denote the strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles, and sometimes MP3 players....
audio connections are bridging, to ensure the high fidelity transfer of a voltage signal from one device in the chain to the next. If the bridging criterion is not satisfied, there may be distortion, altered frequency response or both.
A connection is commonly said to be bridged if the load impedance is at least ten times the source impedance.
Audio amplifiers
In audio system specifications, the value of the low output impedance is described by the damping factorDamping factor
In audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
, DF, which is:
Now it is easy to calculate Zsource
In audio systems, Zload is typically the nominal impedance of an 8 ohm loudspeaker. The output impedance of the amplifier is typically in the same order of magnitude of the impedance of the cables connecting it to the speaker (<0.1 ohm), so DF will drop rather easily.
Applying negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
also increases the damping factor, because the source resistance is effectively reduced. In this way, damping factors into the hundreds can be reached.
Although the load impedance is significantly greater than the source impedance in this situation, the purpose here is not to transmit a voltage signal with great fidelity from input to output. Therefore, this situation is not an instance of impedance bridging. (If voltage transfer mattered in driving a loudspeaker, it would be easy to simply design the loudspeaker to have a high impedance.) Furthermore, the goal is to transfer power to the speaker, whereas the goal of impedance bridging is the exact opposite: to minimize transmitted power, allowing devices not to require powerful current sources to drive their outputs.
The reason an amplifier can transmit maximum power to a loudspeaker even though the speaker's impedance is much higher is that the power amplifier amplifies current. Its output impedance is kept as low as possible to minimize the current's power dissipation in the amplifier itself, thereby maximizing dissipation in the speaker. The low output impedance also dampens the speaker's motion.