Bridged and paralleled amplifiers
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Bridged and paralleled amplifiers are multiple amplifiers connected such that they drive a floating load (bridge) or a common load (parallel). This article is aimed more at IC amplifiers but can be applied to discrete ones as well.
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[edit] Overview
There are many situations where one amplifier may not be sufficient to drive a given load. For example, an amplifier rated for 100W into 8Ω will not be capable of driving any more power into that load unless the supply voltages are increased - this may not always be feasible in practice. So in a design situation which calls for 400W into 8Ω, the designer may opt for a bridged configuration using two 100W/4Ω amplifiers.
Another situation arises when it is desired to keep the power dissipation per amplifier unchanged whilst operating into a lower impedance load or, equivalently halve dissipation while operating into the rated load. So in a design situation which calls for using a 100W/8Ω amplifier into 4Ω, the designer may opt for a paralled amplifier configuration using two such amplifiers.
Yet another situation may require both the above design goals to be met. In such situations a bridge-parallel configuration is used.
Note: The schematics show the amplifiers using the op-amp symbol. Amplifiers (including power amplifiers) which have distinct inverting and non-inverting inputs (such as voltage or current feedback circuits), are often depicted using the op-amp symbol. Hence the reader must not assume that the discussion applies to standard op-amps alone. Further, any amplifier may be imagined in the schematics since the schematics are only representative of the concepts and not the actual or practical circuits.
[edit] Bridged amplifier
A bridged amplifier 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 to both outputs.
The image shows two identical amplifiers A1 and A2 connected in bridge configuration. The + and − signs on the input side represent the non-inverting and inverting inputs of each amplifier. Other components such as feedback network, input/output resistors/capacitors, etc. are not shown. If a single amplifier is able to produce ±10Vp-p relative to ground, a second amplifier can output the same signal, but inverted. If the load is connected across the positive or hot output (the other being ground) of both amplifiers, it sees +10Vp −(−10Vp) = 20Vp (or ±20Vp-p) total, which is the double of what each individual amplifier can put out. This is often an option on stereo power amplifiers for audio to increase the power available for a monaural signal or single channel. Bridging is a common technique with car audio systems where the supply is limited to that of the battery (12VDC) to increase the available power output. For example, if a single amplifier can deliver 100W into an 8Ω load at an input of 1Vp, the same amplifier when bridged (that means two such amplifiers) into 8Ω will deliver 400W for the same input. That is because power varies as the square of the voltage (P = V2/R). 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 for this configuration is that the output offset voltage of the amplifiers must always be equal (in magnitude and sign), preferably as close to zero as possible at no signalǂ. Unequal offset will cause some current to flow through the load and the amplifiers thereby dissipating unwanted power in all three. However, practically some (small) offset may be acceptable depending on final requirements or specifications.
ǂ – Amplifiers that use a single supply, such as car audio amplifiers may use bridging without output coupling capacitors. In this case, the output of each amplifier in the bridge will not be zero volts at no signal but will be at half the supply voltage for maximum undistorted voltage swing. This is acceptable since if both amplifiers have the same offset the net effect of bridging is to cancel out the offset.
[edit] 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. The + and − signs on the input side represent the non-inverting and inverting inputs of each amplifier. Other components such as feedback network, input/output resistors/capacitors, etc. are not shown. 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 4Ω) are paralleled into a 4Ω load, each amplifier sees an equivalent of 8Ω 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 completely identical to the other(s). Practically, each amplifier must satisfy the following:
- Each amplifier must have as little output 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.
[edit] 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.
