Class AB is out and Class G is in, according to Texas Instrument's Christoph Gromann, discussing headphone amplifiers for MP3 players and mobile phones. Class G amplifiers are able to prolong battery life by adjusting the voltage supplies within the audio signal to minimise power dissipation, digitally. Texas Instruments has just introduced two 25mW versions that consume 0.6mA/channel of quiescent current whilst delivering 5mW per channel into a 32ohm load. Intended for stereo headphone applications, every mW saved means another few precious minutes of standby time.
Benchmarking these Class G parts against a ground-referenced Class-AB part in a standalone Lithium-Ion battery test, TI claims that the Class G TPA6140A2 and TPA6141A2 amplifiers reduced power consumption by 30 percent, resulting in a system-level music playback time improvement of 20 percent.
These amplifiers also integrate charge pumps, or DirectPath technology as TI terms it. Eliminating the need for DC-blocking capacitors at the amplifier outputs, the charge-pump architecture generates a negative supply rail, so that the amplifier can reference true ground. There was a time when designers might have looked askance at the idea of integrating a charge pump architecture with Class G, as Gromann, who is TI's EMEA business development manager for high performance analog, explains: “Class G involves switching in a short amount of time, and audio being true analog, you want to avoid injecting any kind of noise. Switching the supply voltage without creating distortion in the audio signal path is a challenge.” He noted that Class G parts integrating charge pumps had previously been announced by competitors, but some have not yet been released.
Gromann adds that a capless architecture results in greater low-frequency audio fidelity, because of-course, power is not the only metric by which amplifier performance is judged. Sound quality is paramount, with features such as pop and click suppression coming equally high up the agenda. The TPA6140A2 and TPA6141A2 include built-in pop suppression circuitry to eliminate pop noise when the amplifier powers on. Other audio quality features include power supply rejection of greater than 100dB and differential inputs that improve noise rejection. These devices are said to boast 100dB signal-to-noise ratio.
The TPA6140A2 provides a variable gain range of -59 dB to +4 dB, which is programmable via a 32-step I2C volume control feature. The TPA6141A2 has two fixed gain settings of 0dB and +6dB for customers that don't need the same level of flexibility. As with other TI parts, an evaluation board for these parts is available, with an easy to follow gui where users can set impedence levels and channels.