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Pushing the Limits of Audio Power Amplifiers
By Eric Droge,
Audio Power Amplifier Product Specialist, Texas Instruments, Inc.

Introduction
Will advances in amplifier technology, like Class-D, bring 10 to 20 W audio performance to handheld electronics that operate from a single cell battery? As great as this would sound, no pun intended, it would directly conflict with the conclusions of a German physicist, George Simon Ohm, who published a pamphlet in 1827 entitled, "The Galvanic Circuit Investigated Mathematically." Nevertheless, supply voltages in portable, battery-powered applications are migrating to lower levels. A problem these handheld applications encounter is the audio power amplifier (APA) becoming supply-voltage limited.
While not a revolutionary discovery, it is surprising how many times a system requirement must be revised because it is theoretically impossible. In most cases this is merely an oversight that goes unchecked until the designer tries to select an APA to drive the speakers or headphones and cannot find a device to meet the requirement. Remembering Ohm's Law can greatly reduce the amount of time spent searching for an APA that does not exist. Given below are three audio systems all requiring a different APA to drive the speaker; only one will have a possible solution:

System A needs an APA capable of driving 2.75 W, bridge-tied load (BTL), into a 4-W speaker from a 5-V supply.
System B needs an APA capable of driving 2 W, single-ended (SE), into an 4-W load from a 5-V supply.
System C needs an APA capable of driving 10 W, BTL, into a 4-W load from a 12-V supply.
Single Ended and Bridged-Tied Loads
To determine which system is feasible a designer must know the power tradeoffs between SE and BTL configurations. Along with the speaker impedance and supply voltage, the configuration of the amplifier dramatically affects how much power can be delivered to the speaker. The two ways to design the output of an amplifier are SE and bridged (i.e. BTL.)
The SE configuration is most common for headphones or for applications when the speakers use a common ground. It is referred to as single ended because only one terminal of the speaker is connected to the amplifier, with the other terminal tied to ground (see Fig. 1.) This technique requires only three conductors between the amplifier and speaker for a stereo solution, left positive, right positive and the third for ground. In terms of power provided to the load, the equation is straight forward, just remember to convert the supply voltage to an rms value by dividing the peak-to-peak voltage by 2*(2)^1/2 or 2.83. Once Vrms is determined plug the value into the following equation to find the power delivered to the headphones:
P = (V_rms)²/R_LOAD.

Fig. 1: Single-ended Stereo Amplifier
A BTL amplifier consists of two amplifiers driving both ends of the load differentially, (see Fig. 2) which has several potential benefits. The single supply SE configuration (Fig. 1, again) used a coupling capacitor to block the dc offset from reaching the load. These capacitors can be quite large (in a range from 40 to 1000 µF), are expensive and have the additional drawback of limiting low-frequency performance.

Fig. 2: Monophonic BTL Amplifier with 700 mW into 8 W
The BTL configuration cancels the dc offsets which eliminates the need for the blocking capacitors, and low-frequency performance is then limited only by the input network, amplifier and speaker frequency response. The differential drive to the speaker means that as one side is slewing up the other side is slewing down and vice versa. This effectively doubles the available voltage swing on the load, which quadruples the power delivered to the speakers. This particular circuit is useful in wireless applications where only a single speaker is required and is capable of driving 700 mW into an 8-ohm speaker from a 5-V supply.
Avoid Clipping
Another catch to remember when determining the peak-power capability of an amplifier is the output swing. A few tenths-of-a-volt of headroom from the top supply rail significantly decreases distortion (clipping.) For example, an amplifier with a 5-V single supply, driving a 4-W speaker, has a typical peak-to-peak output swing around 4.5 V.
This translates into 1.59 V_rms (quite a difference in peak vs. rms power if the conversion is omitted.) Putting those numbers into our earlier power equation yields 0.63 W of rms power into a 4-W load from the 5-V power supply. If an 8-W speaker is used the power is cut in half to 0.32 W. If a 2-W speaker is used the power jumps to 1.26 W. If the speakers are 4 W and the supply voltage is 5 V the maximum output power from a SE and BTL configuration is:

P_SE = ((4.5V/2.83)²)/4-W P_BTL = ((9V/2.83)²)/4-W

P_SE = 0.63 W P_BTL = 2.53 W

A conclusion from this analysis is lower speaker impedance yields higher output power. However, speakers with an impedance lower than 4 W are generally not as efficient as 4- to 8-W speakers. Moreover, the APA's efficiency decreases as the speaker's impedance drops below 4 W. This degradation in speaker and APA efficiency below 4 W negates the increase in output power.
Determining the Real System
Beyond lowering the speaker impedance to 4 W, the best way to increase the output power in a given SE or BTL configuration is by increasing the supply voltage. Plots of the maximum theoretical output power vs. supply voltage for SE and BTL amplifier configurations driving 4-, 8- and 32-W speakers (see Figs. 3 & 4) help determine which of the previous systems are practical. It turns out that System A and System C are viable solutions, but only System C is practical.
As discussed earlier, the output swing of an APA operating from a 5-V supply is only 4.5 V. The 0.5-V headroom from the supply rail is needed to avoid clipping. Recognizing this, the real-world maximum output power of System A is only 2.53 W. On the other hand, the specified 10-W output power of System C is well below the maximum output power for a BTL amplifier driving a 4-W load from a 5-V supply. As for System B the maximum output power from an amplifier operating in a SE configuration (see Fig. 3, again), driving a 4-W load, from a 12-V supply is 781 mW. This is clearly below the 2-W output level specified. To achieve 2 W in System B, the supply voltage must be greater than 8 V.

Fig. 3: Maximum Theoretical Output Power from an SE Amplifier

Fig. 4: Maximum Theoretical Output Power from a BTL Amplifier
Conclusion
Knowing the maximum output power a given APA can deliver from a fixed supply voltage will save considerable time and effort when selecting a device. An APA with a BTL configuration will drive four times more power to the speaker than an APA in a SE configuration. Once the amplifier output configuration is selected there are basically two variables that limit the output power being supplied to the speaker; the APA's supply voltage and the speaker's impedance. Lowering the impedance of the speaker will increase the APA's output power, but the loss in speaker efficiency tends to offset the increase in output power. This means the only way to effectively increase the output power from a speaker is to increase the supply voltage to the amplifier. Put another way, before specifying an output power requirement confirm the amplifier is not limited by the supply voltage.
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