Headphone amplifier architecture sets a new solution

With the emergence of more and more portable audio devices such as DVD, MP3, MP4, smart phones, etc., the circuit board design space of these devices is becoming increasingly insufficient. Today, it is increasingly important to design the size of the solution according to the specific function and minimize the number of components required under the conditions of the expected function. In addition to the traditional use of DC blocking capacitors to transmit audio signals to headphones, other alternative solutions either have inherent limitations or are too simplistic and unrealistic, and are not recognized and accepted by the market.

This article focuses specifically on headphone amplifier architectures, explaining their pros and cons, and also introduces new solutions that can address the problems caused by certain headphone amplifier architectures.

Different headphone amplifier configurations

One of the traditional ways to drive headphones without large DC blocking capacitors is to offset the ground pin of the connector to mid-rail, which is VDD/2 (VBIAS). Since most consumer headphone amplifiers are single supply, the only way to achieve good dynamic range is to DC offset the audio to VDD/2 so that the signal can swing to ground and VDD. Since the ground pin is connected to VDD/2, the main disadvantage is that it will cause ground loop problems and cause unwanted noise or design problems when connected to external devices such as Hi-Fi amplifiers or power-driven speakers that are grounded to true ground (i.e. 0V).

Figure 1. Single-ended headphone amplifier with offset ground sleeve output.

As shown in Figure 1, the most traditional headphone amplifier architecture is a single-ended amplifier with a DC blocking capacitor.

Figure 2. Single-ended headphone amplifier with DC blocking capacitors.

It can be seen that the output of the headphone driver is offset to VDD/2 (VOUT), and the audio swings from VDD to ground. A DC blocking capacitor is required to remove this bias and allow the signal to effectively swing around ground, that is, between –VDD/2 and +VDD/2. The advantage of this architecture is that a standard headphone jack can be used, however, the main problem with this approach is the low frequency response. The headphone impedance is generally 16Ω or 32Ω, and the output capacitor and the headphone speaker impedance together form a high-pass filter with a cutoff frequency of 3dB, as shown in Equation 1:

(Equation 1)

The cutoff frequency must be within the audio frequency band of the headphones, which varies from manufacturer to manufacturer, but is generally between 20Hz and 20kHz. In order not to attenuate low audio frequencies, the high-pass filter cutoff frequency must be at least approximately 500Hz or less.

Rewriting Equation 1 into Equation 2 yields:

(Equation 2)

For a cutoff frequency of 100Hz and a headphone speaker impedance of 16Ω, the capacitor must be 110μF. This makes the capacitor value and physical size too large for a small size, and makes the cost too high. Many engineers can only use a smaller capacitor of 22μF, but this will affect the low-frequency fidelity of the headphone and result in poor bass response.

Each implementation has its pros and cons, but for designers who need better audio and avoid potential ground loop issues or large DC blocking capacitors, a newer architecture called ground neutral or “capless” is beginning to gain traction.

Ground-centered or DirectPathTM headphone amplifiers from Texas Instruments, such as the TPA4411, TPA6130A2 and TPA6132A2, use an innovative approach to eliminate the commonly used DC blocking output capacitors. Rather than offsetting the audio to VDD/2 within the device, a charge pump is integrated and a negative rail is provided, allowing the headphone amplifier to swing between the positive rail (VDD) and the negative supply voltage (VSS). This eliminates the need for any offset at all, so high-pass filtering of the output is no longer required. This allows the headphone speakers to play the entire audio frequency band, providing better sound quality.

Figure 3. Ground-centered DirectPathTM headphone amplifier with integrated charge pump.

Figure 4 shows how the frequency response of the high-pass filter changes with different DC blocking capacitors. For a fixed load impedance of 16Ω, the cutoff frequency changes simply by changing the output DC blocking capacitor. The result is that as the capacitance value decreases, the cutoff frequency increases and less of the audio bass content is transmitted to the headphone speakers.

Figure 4. Output frequency response comparison

This approach seems ideal, but due to the inefficiency of the integrated charge pump, the ground-centered headphone amplifier consumes more power than traditional headphone amplifiers with offset ground sleeves or large DC blocking capacitors, which slightly reduces the system's battery life. An innovative approach to solve this problem is to use improved Class-G technology.

Class-G Technology

In the ground-centered architecture of a Class AB amplifier, the amplifier always operates at the highest supply voltage, which means that for the noise-free phase of the audio, the voltage drop across the output FET is quite large. Taking a lithium-ion battery as an example, the typical battery voltage range is 3.0V to 4.2V. Assuming the battery supplies 3.6V, the red arrow in Figure 5 shows the voltage drop across the output FET when the output audio is playing.

Figure 5. Class AB grounded centered headphone amplifier operation.

Assuming that the amplifier's quiescent current is very small compared to the current flowing to the load, it can be inferred that the battery current is proportional to the output current.

(Equation 3)

Figure 6 shows a simplified schematic of a Class AB grounded centered headphone. As the audio frequency changes, the voltage drop across the output FET changes. The power loss in the device is the product of the voltage drop times the battery current (IBATT).