Extending Battery Life with Class G Audio Amplifiers

To illustrate the increase in battery life achieved 　　with a Class G audio amplifier , our calculations are based on the following values:

• 　　PBATT: Battery power
• 　　VBATT: Battery supply voltage
• 　　IBATT: Battery supply current
• 　　VDD: DC/DC converter output voltage
• 　　PDD: DC/DC converter output power
• 　　VOUT: load voltage
• 　　RL: load impedance
• 　　POUT: load power consumption
• 　　IOUT: load current

　　In a standard Class AB amplifier , the supply current is equal to the output current (IBATT = IOUT). When using Class G (buck converter), the supply current (battery) is a fraction of the output current, which is expressed by the formula IBATT = IDD x VDD/VBATT.

　　Assuming an amplifier driving 200 mVRMS into a 32 Ohm load, the load output current is: IOUT = VOUT/RL = 200mVRMS/32Ω = 6.25 mA. Assuming a quiescent current of 1 mA (IDDQ), the total current drawn by the amplifier is: IBATT = 7.25 mA.

　　The total power drawn by the Class AB amplifier is then calculated as follows (assuming a 4.2V Li-Ion battery):

PBATT (Class-AB)= VBATT x IBATT = 4.2V x 7.25 mA = 30.45 mW Eq. 1

　　For a Class G amplifier, the voltage rails are generated by a switching DC/DC converter, and the power supplied depends on the DC/DC converter output voltage and efficiency. Assuming the DC/DC converter output voltage is 1.3V, the calculation equation is:

PDD = VDD*IDD = 1.3v * 7.25mA = 9.425 mW   Eq. 2

　　The total power delivered is the DC/DC converter output power divided by the DC/DC converter efficiency. Assuming a 90% buck efficiency, the total power delivered to the Class G amplifier is:

PBATT (Class-G) = PDD/90% = 11.09 mW    Eq. 3

　　At this point, the Class G headphone amplifier draws about 3 times less power than the Class AB amplifier under the same conditions. The reduction in power consumption is proportional to VBATT/VDD. In our example, it is (4.2/1.3)*Converter-Efficiency = (4.2/1.3)*0.9 = ~3

　　The battery savings are shown in Figure 1. Here, we compare two Class AB and Class G headphone amplifiers using the exact same audio input powered by a Li-Ion battery. As can be observed, the Class G headphone amplifier provides more than 2 times longer battery life (150 hours) compared to the Class AB amplifier (70 hours). For the end user using a portable audio device, this means longer music playback and talk time.

Figure 1. Battery discharge curves showing that class G amplifiers have longer operating time than class AB amplifiers.

　　In summary, the Class G audio amplifier topology is a modified version of the Class AB topology with an adaptive power supply that changes with the audio source. This topology reduces power consumption and improves efficiency, resulting in longer battery life for headphones using the Class G amplifier topology.