Bridged and paralleled amplifiers
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.
Overview
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 priciples 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 paralllel mode provided that the common loudspeaker terminal (normally black) are connected together 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 automaticlly 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.
Bridged amplifier
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A bridged amplifier or H-bridge is a configuration for creating a larger output voltage swing than that possible with one amplifier by inverting a second amplifier and connecting the load (such as a loudspeaker) between the two outputs (BTL = bridge-tied load).
The image shows two identical amplifiers A1 and A2 connected in bridge configuration. If a single amplifier is able to produce ±10 V relative to ground (20 Vp-p), then the second amplifier will output the same signal, but inverted. If the load is connected between the positive ("hot") outputs of the two amplifiers (instead of connecting between one output and ground), it can see up to 10 V − (−10 V) = ±20 V (or 40 Vp-p) total, which is twice what each individual amplifier can put out. Driving the load in antiphase makes each amplifier see only half the load's impedance.
Because the available voltage swing across the load is doubled for the same power supply voltage, bridged output enables the design of an amplifier producing 4x the power output on the same supply voltage, since power varies as the square of the voltage:
- .
This is often provided as an option on power amplifiers for audio, allowing the two channels to be used in a stereo configuration, or in a single channel bridged configuration to increase the power available. For example, if an amplifier can deliver a maximum of 100 W each into two 8 Ω loads in stereo mode, the same amplifier when bridged will deliver 400 W into a single 8 Ω load. (the same amplifier will deliver 200watts into a 16ohm load.
This output configuration is useful in applications where battery size dictates a lower supply voltage, e.g., automotive or handheld applications.
However, if a loudspeaker is used that has the minimum impedance specified for just one of the pair, the bridged pair WILL see a load impedance of half that the amplifiers are designed for and it will attempt to supply up to four times the output. This is no different than saying you may double the output of a given single amplifier by working it into a load of half the rated output impedance. The effect of doing this will depend entirely on the design of amplifier; ie it's overload detection circuit, its power supply capability, it's heat sinking ability, the margin of safety designed into the output devices and what degree of harmonic distortion is acceptable.
There is just one circumstance when x4 power can be achieved. This is when single amplifiers are rated at full output for an impedance that is lower than the connected speaker. eg. 8 ohm speakers are very common and many amps are rated down to 4 ohms. An 8 ohm speaker connected to such an amp will only accept half of the available power. When two such amplifiers are bridged, the new output impedance now 8 ohms and full power can be output to a 8 ohm speaker.
Bridging is a common technique with car audio systems to increase the available power output, since the supply is limited to that of the alternator (14.4 VDC) unless a voltage step-up power supply is used.
Also, note that when connected in bridge, each amplifier sees half the load impedance and thus delivers double the current, thereby doubling the dissipation within the amplifier.
One requirement of this configuration is that the output DC offset voltage of the amplifiers (not applicable to capacitive or transformer output) must always be equal , preferably as close to zero as possible at no signal. Unequal offset will cause some direct current to flow through the load and the amplifiers, wasting power in all three. Practically, a small offset may be acceptable depending on final requirements or specifications.
Amplifiers that use a single supply with an internal DC offset (at half the supply voltage for maximum undistorted voltage swing) must not drive that direct current into the load, so they typically use output coupling capacitors so that the speaker sees zero volts DC. In the case of a bridged amplifier, however, this is not necessary, since if both amplifiers have the same offset, the net voltage across the load is zero.
Paralleled amplifier
A paralleled amplifier configuration uses multiple amplifiers in parallel, i.e., two or more amplifiers operating in-phase into a common load.
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.
Bridge-parallel amplifier
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.