Headphone amplifier selection criteria for portable audio applications

For hand-held battery operated systems such as media players and mobile communication devices, the system designer has to carefully factor in each of these criteria when designing headphone amplifier and output stage circuitry that connects to the headphone load.

Class AB headphone amplifiers
A common amplifier topology is the Class AB amplifier. The efficiency of a Class AB amplifier changes with the nature of the signal being amplified. Losses such as non-ideal transistor behavior, transistor bias current, power supply efficiency and reactive components of the load impedance mean that a Class AB amplifier can have a maximum efficiency of approximately 70% for a full-scale sine wave.

However, typical audio signal levels vary greatly, with occasional peaks amongst a signal with lower average amplitude. At lower output signal levels, the average voltage drop across the amplifier output stage transistors increases; hence the power dissipated increases and the efficiency decreases.

A large proportion of portable audio ICs typically operate from a unipolar supply voltage in the region of 1.8 V to 5 V, with respect to ground. This means that the Class AB headphone amplifier output is biased at a DC level equal to one half of the supply range. This is often referred to as the mid-rail reference voltage (VMID). As the audio signal is centered at VMID, the amplifier outputs are capacitor coupled to the headphone load (Figure 1).

Figure 1: Class AB headphone amplifier external component circuit and output signal illustration. The amplifier audio signal is centered around VMID.

This method of AC coupling via a capacitor prevents the DC-biased amplifier dropping a DC voltage across the headphone load, which is typically connected to ground. Removing this bias prevents current from being drawn from the amplifier in its quiescent state. These DC blocking capacitors are often 100 - 220 µF tantalum capacitors over 1mm high, which is not ideal in space-constrained portable applications.

The AC coupling capacitor also forms a high-pass filter with the headphone load and can limit the bass response of the system. These capacitors are often expensive in relative terms compared to the cost of the IC containing the amplifier. They can also degrade over time, causing reliability issues.

The DC blocking capacitors can be removed from the system by biasing one end of the headphone load to the mid-rail voltage reference (VMID) (Figure 2). This is achieved by using an additional amplifier to generate the mid rail DC reference voltage, whilst the primary output amplifiers drives the audio signal into the load.

Figure 2: Capless Class AB headphone amplifier circuit with mid rail bias amplifier and output signal illustration.

This design has the advantage of removing the large and sometimes unreliable capacitors; however the additional amplifier requires more current, which increases power consumption and, ultimately, reduces battery life. Channel separation is often also degraded in this configuration as the negative headphone load terminals share a common DC level return connection, rather than ground. The mid-rail-referenced topology also has the drawback that the headphone sleeve is biased above ground to the mid rail reference, which can cause problems if the headphone jack is connected to another system which is referenced to ground, effectively short circuiting the mid rail amplifier to ground and damaging the IC.