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SIGNAL CHAIN BASICS #49: Increasing battery life with Class-G audio amplifiers

(Editor's note : This is Part 2 of an article about Class-G audio headphone amplifiers. To read Part 1 , click here ; or click here for a complete, linked list of all previous installments of the Signal Chain Basics series.)

To illustrate the battery life savings with Class-G audio amplifiers, calculations are done based on the following values:

  • PBATT : Power drawn by the battery
  • VBATT : Battery supply voltage
  • IBATT : Battery supply current
  • VDD : DC/DC converter output voltage
  • PDD : DC/DC converter output power
  • VOUT : Voltage across the load
  • RL : Load impedance
  • POUT : Power dissipated at the load
  • IOUT : Current supplied to the load

 In a standard Class-AB amplifier, supply current equals output current (IBATT = IOUT ).

With Class-G (step-down converter), supply current (battery) is a fraction of the output current and is expressed by the formula:

     IBATT = IDD × VDD /VBATT.

Given an amplifier driving 200 mVRMS into a 32 Ω load, the output current to the load 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 (assuming a Lithium-Ion or Li-Ion battery at 4.2V):

     PBATT (Class-AB) = VBATT   ×  IBATT

                       = 4.2V × 7.25 mA = 30.45 mW

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

      PDD = VDD × IDD = 1.3v × 7.25mA = 9.425 mW

The total supplied power will be the DC/DC converter output power divided by the efficiency of the DC/DC converter. Assuming 90% step-down efficiency, total power supplied to the Class-G amplifier is:

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

 In this case the Class-G headphone amplifier draws approximately three times less power as compared to a Class-AB amplifier under identical conditions. The reduced power dissipation is proportional to the ratio of VBATT /VDD . In our example, this is:

     (4.2/1.3) × converter-efficiency = (4.2/1.3) × 0.9 = ~3

 This battery saving is shown in Figure 1 . Here two Class-AB and Class-G headphone amplifiers are compared using identical audio inputs powered by a Li-Ion battery. As can be observed, the Class-G headphone amplifiers have more than twice the battery life (150 hours) when compared to the Class-AB amplifiers (70 hours). This means more music playback time and longer conversation time for end-consumers using portable audio devices.

 

Figure 1: Battery discharge curves showing extended playing time for

Class-G amplifiers compared to Class-AB

In conclusion, Class-G audio amplifier topology is a variant of the Class-AB topology with adaptive power supplies that varies with the audio source. This kind of topology reduces power dissipation and increases the efficiency resulting in longer battery life for headphones using the Class–G amplifier topology.

References


About the Author
ShreHarsha Rao is the Portable Audio Marketing Manager at Texas Instruments. He has a Masters Degree in Electrical Engineering from the University of Texas. ShreHarsha can be reached at .

1 comment on “SIGNAL CHAIN BASICS #49: Increasing battery life with Class-G audio amplifiers

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    April 17, 2014

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