This invention relates to audio amplifiers, particularly audio amplifiers where efficiency is important, such as battery-operated portable applications, for example portable amplifiers connected to headphones.
Minimizing unnecessary amplifier power dissipation is important in portable applications, such as headphones with a battery operated amplifier, such as a portable music player or telephones. In portable music players, such as MP3 players, designs are leading to decreasing MP3 encoding power dissipation as small feature size processes are used for the digital sections. This leaves the headphone as a major power dissipation contributor. In cellular telephones, especially where MP3 functions are integrated, the percentage of the power dissipated compared to the power dissipated in the transmitter and receiver is small. However, the headphones are used for a much longer period of time, making accumulated battery drain important.
Often very large AC-coupling capacitors are used to allow ground-referenced headphone return paths, i.e., headphone connections, where one of the terminals is grounded. It is desirable to remove these large capacitors and to achieve high power efficiency.
In the past, audio amplifiers have employed bipolar power supplies for AB amplifiers with a reference voltage, typically ground reference, between high and low (positive and negative) voltage rails. However, in relatively low-cost, low-power applications, demands have made it far more costly to provide a negative power supply voltage than to add large AC-coupling capacitors.
Some types of audio amplifiers with bipolar power supplies employ an integrated negative-supply generating charge pump in order to make it more cost effective to include the negative power rail, thereby making ground-referenced headphones easier for customers to use. Known art uses a charge-pump to generate a negative rail.
Reference is made to U.S. Pat. Nos. 7,061,327, 7,061,328, and 7,183,857 for background. Referring to FIG. 1, the techniques described therein use inefficient class AB amplifiers 12, 14 that employ two fixed voltage rails 16, 18 at +VCC and −VCC) with a battery 20 (at +VCC=1.8 VDC from ground) and a charge pump 22 and therefore suffer from high power dissipation at the loads, which are stereo earphone speakers 24, 26 referenced to ground. (The power dissipation in a class AB amplifier with a sinusoidal signal is minimal at the zero crossing and at the peak but maximum at points in between.) An alternative scheme described therein uses a single fixed voltage rail, which is likewise inefficient and requires a coupling capacitor to couple a signal to a ground-referenced load.
Class G amplifiers are known in the art. Referring to FIG. 2, a conventional class G amplifier 30 employs two parallel class AB amplifiers 32, 34 operating with complementary fixed voltage rail pairs 36, 38 and 40, 42 with power supplies at different maximum voltages (e.g. 3.0 VDC and 2.25 VDC), where connection to the rails are alternately switched by an equivalent ganged switch 44 (such as transistor switches) during each power cycle so that each amplifier 32, 34 operates only during a segment of different parts of the power cycle. The proper selection of the cycle parts improves efficiency to the output load, a loudspeaker 46. To accommodate the d.c. voltage shift of the a.c. signal, a coupling capacitor 48 is needed.
It is important to consider how battery voltage maps into power supply requirements. For example, where a Li-Ion battery is used, the output voltage is a nominal 3.6V. However, producing +/−3.6V and the associated rails can be costly due to the high 7.2V requirements. Efficiency degrades when many rails are employed. Supplying the input voltage from the output of a +DC, −DC power source is possible but at added cost.
More efficient multiple-rail power supplies are needed.