Bridged and paralleled amplifiers
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Multiple electronic amplifiers can be connected such that they drive a single floating load (bridge) or a single common load (parallel), to increase the amount of power available in different situations. This is commonly encountered in audio applications.
Bridged or paralleled modes of working, normally involving audio power amplifiers, are methods of combining the output of two identical amplifiers to provide, what is in effect, a mono amplifier. Combining more than two amplifiers can be effected using the basic principles described, including the possibility of bridge and parallel modes in combination.
Two identical amplifiers are most often encountered in a common case, with a common power supply, and would normally be regarded as a stereo amplifier. Any conventional stereo amplifier can be operated in bridge or parallel mode provided that the common loudspeaker terminals (normally black) are connected and common to the ground rail within the amplifier.
Some two channel amplifiers, or stereo amplifiers, have the built in facility to operate in bridge mode by operating a switch and observing the input and output connections detailed on the back panel or in the manual. This option is most often found in high power PA equipment or amplifiers designed for car audio applications Operation in parallel mode requires no special facility and is implemented merely by the appropriate external connection.
Stereo amplifiers usually have a common control for gain and frequently bass/treble and when switched to bridge mode will automatically track each channel identically. Where two channel amplifiers have separate controls, and are switchable to bridge mode, only the controls on one channel will be operational.
Where the user implements his own connections for either bridge or parallel mode, and the amplifiers have individual controls, great care must be taken to ensure that both sets of controls are set identically.
Example. Two amplifiers, each rated 100 watts maximum into 4 ohms, in bridge mode they will appear as a mono amp rated 200 watts into 8 ohms. This is the most commonly misunderstood mode of operation and it requires additional circuitry to implement if the pair of amplifiers does not have the facility built in. The image shows two identical amplifiers A1 and A2 connected in bridge mode. The signals presented to each amplifier of the pair are caused to be in anti-phase. In other words, as the signal in one amplifier is swinging positively, the signal in the other is swinging negatively. If, for example the maximum output voltage swing of each amplifier is between a peak of + and – 10 volts, when the output of one amplifier is at + 10 volts the output of the other will be at –10 volts, which means that the load (a loudspeaker) now sees a 20 volt peak difference between the “hot” (normally red ) output terminals.
The provision of the anti-phase audio input signal can be provided in several ways:
- by the built in facility in an amplifier pair provided with the bridge mode option;
- by an internal modification such as that described by Rod Elliot at http://www.sound.westhost.com/project20.htm;
- by a simple active phase splitter circuit, external to the amplifier;
- by a phase splitting audio input transformer, external to the amplifier.
Options 2, 3 and 4 require appropriate knowledge and skill. The bridge mode option is often used in PA systems and especially in car audio applications to feed bass loudspeakers at high power. Car audio amplifiers commonly have only a 13.8 volt supply and obtaining the voltage levels in the amplifier circuit required for even modest powers is expensive. Bridge mode operation helps provide the power required at lower cost.
The × 4 myth
It is sometimes stated that operating an amplifier pair in bridge mode can give four times the power (of one of the pair). This statement makes reference to the fact that power is proportional to the square of the voltage, implying that if the output voltage is doubled – as it is in bridge mode – then the power available increases by a factor of four. This would be true if the amplifier in bridged mode were used to drive loudspeakers of the same impedance used in stereo mode. However, in this case, the current through the loudspeaker and the amplifier would also double, which could exceed the amplifier ratings and lead to overheating and finally destruction of the amplifier. In fact, the minimum impedance of the loudspeaker in bridged mode should be double the minimum impedance rated for stereo mode.
A paralleled amplifier configuration uses multiple amplifiers in parallel, i.e., two or more amplifiers operating in-phase into a common load.
In this mode the available output CURRENT is doubled but the output voltage remains the same. The output impedance of the pair is now halved.
The image shows two identical amplifiers A1 and A2 connected in parallel configuration. This configuration is often used when a single amplifier is incapable of being operated into a low impedance load or dissipation per amplifier is to be reduced without increasing the load impedance or reducing power delivered to the load. For example, if two identical amplifiers (each rated for operation into 4 Ω) are paralleled into a 4 Ω load, each amplifier sees an equivalent of 8 Ω since the output current is now shared by both amplifiers — each amplifier supplies half the load current, and the dissipation per amplifier is halved. This configuration (ideally or theoretically) requires each amplifier to be exactly identical to the other(s), or they will appear as loads to each other. Practically, each amplifier must satisfy the following:
- Each amplifier must have as little output DC offset as possible (ideally zero offset) at no signal, otherwise the amplifier with the higher offset will try to drive current into the one with lesser offset thereby increasing dissipation. Equal offsets are also not acceptable since this will cause unwanted current (and dissipation) in the load. These are taken care of by adding an offset nulling circuit to each amplifier.
- The gains of the amplifiers must be as closely matched as possible so that the outputs don't try to drive each other when signal is present.
In addition, small resistors (much less than the load impedance, not shown in the schematic) are added in series with each amplifier's output to enable proper current sharing between the amplifiers. These resistances are necessary, without them the amplifiers will in practice fight each other and overheat.
Another method of parallelling amplifiers is to use current drive. With this approach the close matching and resistances are not needed.
A bridge-parallel amplifier configuration uses a combination of the bridged and paralleled amplifier configurations. This is more commonly used with IC power amplifiers where it is desired to have a system capable of generating large power into the rated load impedance (i.e., the load impedance used is the one specified for a single amplifier) without exceeding the power dissipation per amplifier. From the preceding sections, it can be seen that a bridged configuration doubles the dissipation in each amplifier while a paralleled configuration with two amplifiers halves the dissipation in each amplifier when operating into the rated load impedance. So when both configurations are combined, assuming two amplifiers per configuration, the resulting dissipation per amplifier now remains unchanged while operating into the rated load impedance, but with nearly four times the power that each amplifier is individually capable of, being delivered to the load.