New developments in audio power amplifier (APA) technology are increasing the sound quality in flat-panel displays to match the superb image quality. As DLP, LCD-TV, LCD monitor and plasma display TV screens increase in size, audio performance must continue to keep pace. Heat dissipation and power consumption are two inherent problems with traditional Class-AB, or linear APAs that prohibit them from being excellent solutions in the latest flat-panel displays.
Due to the inefficiencies of linear amplifiers, the flat-panel display industry is choosing Class-D APAs for its audio solutions. Class-D APAs operate much cooler and consume a lot less power compared to Class-AB amplifiers. Designers who use Class-D APAs can increase the audio power performance in their applications without increasing heat or power consumption, keeping transformers and voltage regulators small and eliminating heat sinks. In fact, Class-D APAs can lower heat and power consumption even while increasing power performance.
Inherent Problems with Linear APAs
Linear APAs are inherently inefficient due to the voltage drop from the power supply across the linear amplifier's output stage transistors. In most cases, they dissipate more power in heat than they supply to the speaker to produce sound. When the linear amplifier's output voltage signal is not equal to the supply voltage, there is internal power loss in the amplifier, which lowers the amplifier's efficiency. When the amplifier's output voltage signal equals the supply voltage, its efficiency is much higher; however, the amplifier is distorting because the voltage signal is being "clipped" by the supply rail (See Figure 1 below). Distortion caused by clipping the supply rail produces an unpleasant sound and can permanently damage the speaker.
Therefore, there will always be an internal voltage drop from the supply voltage to the output voltage signal. The voltage drop is calculated by subtracting the RMS value of the output voltage from the supply voltage (VDD). The voltage drop multiplied by the average supply current, IDD (avg), determines the internal power dissipation of the linear amplifier. The larger the voltage drop, the lower the amplifier's efficiency. A simple formula for calculating the efficiency of a differential output linear amplifier is:
1) η = (π *√ (2*PL *RL))/(4*VDD)
PL = Power delivered to the speaker
RL = Speaker resistance
VDD = Supply voltage
From Equation 1, the efficiency of a linear amplifier driving 3 W into an 8Ω speaker from a 12-V supply is only 45 percent, which means the total power consumption for a stereo solution is 13.3 W. The total power consumption of a typical 17-inch LCD monitor, which needs a 3-W amplifier, is 90 W. In this example, the linear amplifier consumes nearly 15 percent of the total power supplied to the LCD monitor. Furthermore, the amplifier is dissipating 7.3 W as heat, which requires a large heat-sink.
How Class-D APAs Solve Problems of Linear Amplifiers
Unlike linear APAs, the Class-D APA's efficiency is not dependent on PL or VDD. Theoretically, a Class-D APA is 100 percent efficient because the Class-D output transistors function as switches that turn on-and-off very quickly. When the transistors are switched on, the output voltage is equal to the supply voltage. If the transistor is ideal, no voltage drop occurs that would cause power dissipation when multiplied by the average supply current. Moreover, when the transistors are switched "off", they are an open circuit, through which no current flows and therefore no power is dissipated. The amplifier's four-stage Class-D switching modulation provides the advantages of lower noise floor and reduced filter requirements compared to other two-stage Class-D modulations. In each stage, two transistors are switched on and two are switched "off", and are switching on-and-off at 250 kHz. The 250 kHz switching frequency achieves the best balance of performance while minimizing switching losses.
Conclusion and Next Steps
Class-D technology provides several benefits as an audio power amplifier solution. The amplifier's efficient operation minimizes heat, thereby eliminating the need for heat sinks, and also reduces overall power consumption, which helps reduce the transformer and regulator size and cost.
About the Author:
Eric L. Droge joined Texas Instruments after obtaining a Bachelor of Science degree in Electrical Engineering from Kansas State University. He has held several product marketing roles in the High Performance Analog division of Texas Instruments. Currently he is the worldwide audio power amplifier marketing manager.