bridgetied

Bridgetied, usually written as bridge-tied load (BTL), refers to an electrical amplifier topology used to drive a single load, such as a loudspeaker, from two amplifier outputs that are driven in opposite phase. In this arrangement the load is connected between the two outputs rather than between an output and ground, allowing greater voltage swing without increasing supply rails or using output coupling capacitors.

In operation, two identical amplifier channels generate differential signals that are 180 degrees out of phase. The load thus experiences the difference between the two outputs, effectively doubling the voltage swing across it compared with a single-ended connection. This differential drive helps cancel common-mode DC offsets, so, in ideal conditions, there is little or no net DC current through the load and no coupling capacitor is required at the output. The approach relies on carefully matched channels and balanced biasing to maintain symmetry.

Advantages of the bridge-tied load arrangement include a higher peak-to-peak voltage across the load and the potential for greater output power without larger supply rails or output capacitors. It can also improve low-frequency response by removing the need for output coupling capacitors. However, BTL requires precise matching of the two outputs, and any DC offset or imbalance can drive current through the load. It can also introduce added design complexity in feedback, stability, and EMI considerations, and is more sensitive to variations in load impedance.

Applications of BTL are widespread in consumer and automotive audio amplifiers, particularly in Class D and some Class AB designs. A related concept is multi-bridge-tied load (MBTL), which uses more than two outputs to drive a single load for higher power.