Signal Chain Basics 49: Extending Battery Life with Class-G Audio Amplifiers (2)

By ShreHarsha Rao, Portable Audio Product Marketing Manager, Texas Instruments (TI)

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 part of the output current, which is given by Equation IBATT = IDD x VDD/VBATTexpress.

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

Then, the calculation method of the total power drawn by the class AB amplifier is as follows (assuming a 4.2V lithium-ion or Li-Ion battery):

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

In the case of class G amplifiers, 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 that the output voltage of the DC/DC converter is 1.3V, the calculation equation is:

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

The total supplied power is the DC/DC converter output power divided by the DC/DC converter efficiency. Assuming 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 time, a class G headphone amplifier draws about 3 times less power than a class AB amplifier under the same conditions. The reduction in power consumption is related to VBATT/VDDproportional relationship. In our case it is (4.2/1.3)*Converter-efficiency = (4.2/1.3)*0.9 = ~3

Battery power saving is 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 we have observed, the battery life of a class G headphone amplifier (150 hours) is more than 2 times longer than that of a class AB amplifier (70 hours). For end-users using portable audio equipment, this translates to longer music playback time and longer talk time.

Figure 1. The battery discharge curve shows that the class G amplifier has a longer operating time than the class AB amplifier

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.


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About the Author
ShreHarsha Rao is Product Marketing Manager for Portable Audio at TI. He graduated from the University of Texas with a master’s degree in electrical engineering.

Author: Yoyokuo