How are mutually exclusive goals achieved together without major cost? In the realm of portable audio, where small space, high performance and low cost are all sought, the system designer is usually forced to forsake one benefit in order to realize another. Most audio converters (DAC's, CODEC's) in the mixed-signal audio industry operate from only a single, external positive supply voltage and require a DC bias on the output of the integrated headphone amplifier. This DC-biased amplifier presents a challenge to the designer trying to achieve ideal goals of size, performance and cost without tradeoff.
This article highlights how all the tradeoffs associated with using a DC-biased amplifier are eliminated when a ground-centered amplifier is used instead. This amplifier configuration provides optimal board space, ideal frequency response and cost reduction without any tradeoff. Because of the nature of its topology, it also provides the greatest suppression of clicks and pops and outputs a large signal at low supply voltages. To understand these benefits, however, the limitations of the DC-Biased amplifier must be understood.
The DC-biased amplifier
The headphone amplifier integrated inside the typical audio converter chip derives its power from a single, positive analog source. This supply source defines the one, and only, maximum rail of the audio signal. To avoid clipping, the peak-to-peak signal must swing between this positive supply and ground.
To take advantage of the full range and produce the largest voltage on its output, the amplifier centers the audio signal on a DC voltage offset midway between the supply and ground. To create this quiescent or bias point, a DC voltage one-half the supply is applied to the amplifier's input and transferred to the output. Consequently, the audio signal from the DC-biased amplifier is made up of both AC (audio) and DC (bias) components, Figure 1 .
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Figure 1: Both AC and DC components are in the resulting audio signal of a DC-biased amplifier
Since the DC-biased amplifier has both AC and DC signal components, the portable system must also implement an RC high-pass filter (HPF) to block the DC component from the load. Without this circuit, DC current flow through the headphone’s voice coil can cause permanent damage.
The simplest HPF design is made up of a DC-blocking capacitor and the headphone load. The maximum load is typically 16Ω and the associated capacitor is generally selected according to a desired low frequency cutoff point, fc , where fc represents the point at which the output amplitude is 3 dB below full scale. This cutoff point determines the bass performance (bass represents the low frequency content that adds fullness and depth to musical sounds). The cutoff point also dictates the size of the DC-blocking capacitor. To achieve an ideal bass response down to 20 Hz, a large capacitor, approximately 5,000 μF, is required. This physically large capacitor is obviously impractical, especially in space-constrained portable systems.
Alternatively, the HPF may be designed with a smaller capacitor to mitigate the space requirement. But when combined with the small resistance of a headphone load, the HPF severely cuts into the audio band. For example, a 3.3 μF capacitor moves the -3 dB cutoff point, fc , to approximately 3 kHz. Evidently, the path toward one benefit (low fc ) is, inherently, a departure from the other (more board space). This is the nature of a tradeoff that must lead to a compromise of some sort.
Achieving some benefits with a compromise
A compromise (or an acceptable tradeoff) has traditionally been to use a 220 μF capacitor for the HPF design. This capacitor places the corner frequency (fc ) at approximately 45 Hz and consumes less board area. While this compromise has traditionally been acceptable in many designs, it becomes less tolerable in newer portable designs as designers require less and less space for external components. And since most headphones achieve full response across the entire audio band from 20 Hz to 20 kHz, a 3-dB attenuation at frequencies around the typical bass band is also less tolerable. The need to evaluate the tradeoff between capacitor size and corner frequency for adequate bass performance is still present.
The advent of newer capacitor technology also adds another dimension to traditional tradeoff options. Large-valued capacitors are now available in smaller sizes at higher costs. The portable system designer may achieve slightly more board area with a small-sized capacitor and still maintain a modest low frequency response; but this comes with much higher component costs. Here again, the path toward one benefit (better fc and capacitor size) is a departure from another (cost effectiveness).
The ground-centered amplifier
The ground-centered amplifier operates from two voltage sources: a positive and negative supply. Since portable systems only provide a positive supply, newer audio converters derive the negative supply from the applied positive supply using an internal, integrated charge pump. This audio converter provides the amplifier with the necessary dual rail that defines the maximum swing of the peak-to-peak audio output voltage. Because the amplifier now operates from an additional supply below ground, the largest output voltage attainable is one that is centered on ground and has a full scale output from +VA_HP to -VA_HP, double the voltage swing relative to the DC-biased amplifier.
From this ground-centered amplifier, no DC voltage is transferred and the audio output signal contains only an AC (audio) component. Figure 2 shows an example of a new audio converter with an integrated ground-centered amplifier.
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Figure 2: Integrated, ground-centered amplifier
The ground-centered amplifier does not require an HPF. This simple fact frees the system designer from constraining tradeoffs and optimizes board space and frequency response performance without the added costs of smaller, more expensive capacitors.
Achieving the benefits without compromise
This approach yields multiple attributes:
- Optimal Board Space. Since this amplifier circuit has no DC bias on its output, a high pass filter with a large series capacitor is not needed. Figure 3 compares an actual PCB layout of an audio converter requiring DC-blocking capacitors and an audio converter with an integrated ground-centered amplifier.
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Figure 3: PCB layouts for both approaches
Removing capacitors C3 and C4 provided more than 50 mm2 of extra board area. NOTE: Capacitors C5 and C6 are required for the internal switches of the charge pump and act as a charge reservoir and filter for the negative supply. The required values are usually comparable to typical power supply de-coupling.
- Optimal Low Frequency Performance. Since a HPF with a large series capacitor is not needed after the ground-centered amplifier, it achieves optimal response at low frequency bass bands. Figure 4 compares the actual frequency response of an audio converter requiring DC-blocking capacitors and an audio converter with an integrated ground-centered amplifier. Note that the fc is well below the audio band.
- Cost Effectiveness Without Tradeoff. Since there is no DC-bias and consequently no HPF on its output, the ground-centered amplifier is the most cost-effective solution for a portable system design. All of the benefits described above are realized without any major cost. Additional components include only two low-valued ceramic capacitors for the charge pump and negative supply.
- Other Ground-Centered Amp Benefits: In addition to all the benefits that would normally require a tradeoff in a DC-biased amp configuration, the ground-centered configuration is also not prone to the clicks and pops associated with charging and discharging to different DC levels (0 V and the bias voltage). And since the output signal swings between dual supply rails, the portable system can operate at low voltages (prolonging battery life) and still achieve significant output power into headphone loads.
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Figure 4: Frequency response shows cutoff vs. audio band
So how can the portable system designer increase available board space, improve bass, power and noise performance with minimal cost and no tradeoffs? With the elimination of a DC bias and bulky DC-blocking capacitors, the ground-centered amplifier achieves the greatest suppression of clicks and pops, provides a dramatic reduction in the area required for the audio converter and maintains nearly perfect low frequency response. As it operates from a positive and negative supply, it allows the portable system to operate at low voltages and still deliver more power to headphone loads than the DC-biased amplifier of typical audio converters. With all these benefits, the integrated ground-centered headphone amplifier truly offers the best solution to what used to seem like a never-ending circle of tradeoffs and compromises.
About the Author
Kevin Russell is an applications engineer for Mixed-Signal Audio Converters for the Cirrus Logic Mixed-Signal Audio Products Division. He has a B.S. in Electrical Engineering from the University of Miami and is responsible for portable CODECs, multichannel CODECs and single-ended DACs at Cirrus Logic.