Harsh and noisy environments are commonplace for cell phones, personal digital assistants (PDAs), and other portable communications equipment. This fact has led to the development of new audio power amplifiers (PAs). These PAs offer fully differential architectures with good radio-frequency (RF), common-mode, and power-supply ripple rejection. This article will examine the architectures of the single-ended, typical bridge-tied-load and fully differential audio amplifiers. It also will look at the effects of noise on power supply and RF rectification.

Three main types of audio-power-amplifier architectures are used in this industry: single-ended, typical bridge-tied-load, and fully differential amplifiers. Single-ended (SE) audio power amplifiers tend to be the simplest of all of the architectures. In cell phones, however, they aren't commonly used to drive speakers for applications like polyphonic ringtones or hands-free mode. Typically, SE amplifiers are used to drive headphones for listening to music in MP3 format or for gaming audio (FIG. 1).

In the typical single-supply, single-ended configuration, an output coupling capacitor (C_OUT) is required to block the DC bias at the amplifier's output. This prevents DC currents in the load. The output coupling capacitor and load impedance form a high-pass filter, which is governed by the following equation:

where RL represents speaker impedance.

From a performance standpoint, the main disadvantage is that the typical small load impedances—in this case between 4-Ω and 8-Ω speakers—drive the low-frequency corner frequency (F_C) higher. Large values of C_OUT are required to pass low frequencies into the speaker. Consider a case in which the speaker load is 8 Ω. If one were to use a C_OUT of 68 µF, any frequencies less than 292 Hz would be attenuated.

To eliminate the output capacitor (C_OUT) with a single-ended amplifier, a split supply rail is necessary. This solution isn't good for the wireless environment. It would require cell-phone designers to add a DC-to-DC converter for the negative rail, thereby raising the cost and size of the solution. Furthermore, SE amplifiers are prone to "pop" when they are turned on, turned off, placed in shutdown, or taken out of shutdown. This unwanted noise occurs when there is a certain change of voltage (voltage pulse) across the speaker. It is related to the rise time, fall time, and width of the voltage pulse.

Most humans react to sounds from 20 Hz to 20 kHz. So if the pulse length is less than 50 ms, the ear won't be able to respond. At this point, the frequency will be greater than 20 kHz and no "pop" will be heard. If the pulse's rise time is greater than 50 ms, the ear will once again not be able to hear a "pop." (The frequency will be lower than 20 Hz.) The famous "pop" noise can be heard when the pulse width is greater than 20 ms. Here, the rise time of the pulse is less than 50 ms. Because the single-ended amplifier can only make a pulse if it is turned off immediately, the amplifier must ramp up in >50 ms. This speed is too slow for most smart-phone applications.

With a single-ended single supply, the "pop" also occurs because the output DC blocking capacitor holds charge. When a change occurs at the amplifier output, that voltage—combined with the voltage that's already on the capacitor—will be placed across the speaker. The result is a "pop."

Finally, delivering power to the load is a key concern when talking about audio amplifiers. When using an SE amplifier with a single supply, one end of the speaker is attached to the amplifier's output via an output capacitor. The other end is attached to ground. As a result, the potential across the speaker can only be between V_DD and ground. Use the equation for output power to a load:

The maximum peak-to-peak output voltage is the supply voltage. Assuming a sine-wave output, the maximum RMS output voltage is:

The maximum theoretical output power is:

Later, it will be shown that bridge-tied-load (BTL) and fully differential amplifiers can output four times the power of an SE amplifier from the same supply and load impedance.

Today's cell-phone and portable communication devices use a common type of audio-amplifier architecture: the single-ended input with a BTL output configuration (FIG. 2). The BTL amplifier comprises two single-ended amplifiers that drive both ends of the load. The first amplifier (A) sets the gain while the second amplifier (B) acts as a unity-gain inverter. The gain of this BTL amplifier is defined as:

Due to the unity-gain inverting amplifier (B), the gain is double. One of the main benefits of this differential-drive configuration is the power to the load. With the differential drive to the speaker, one side will slew down while the other side is slewing up and vice versa. This characteristic, in effect, doubles the load's voltage swing compared to a ground-referenced load. Because there is effectively twice the voltage swing across the load, the output-power equation becomes:

The maximum theoretical output power with BTL is:

When compared to the single-supply, single-ended audio power amplifier, this doubling of voltage across the speaker results in a quadrupling of the output power from the same supply rail and load impedance.

Another point to consider is the bypass capacitor (C_BYPASS). This capacitor is the most critical one in the circuit. It serves several important functions. First, C_BYPASS determines the rate at which the amplifier starts up. If the amplifier ramps up slowly, it can reduce the "pop" noise. The C_BYPASS and high-impedance resistor-divider network, which generate the mid-rail, result in an RC time constant. As mentioned previously, no "pop" will be heard if this time constant is greater than 50 ms.

The second function of C_BYPASS is to reduce the noise that's produced by the power supply. This noise is caused by coupling into the output drive signal. It is derived from the mid-rail generation circuit that's internal to the amplifier. The noise appears as the degraded power-supply rejection ratio (PSRR). In a system with a noisy supply, it can affect THD + N.

Compared to SE audio amplifiers, the advantage of this type of architecture is the amount of output power from the same supply rail. In addition, the output DC blocking capacitor can be removed. After all, the DC offsets are now canceled by both sides of the speaker biased around V_DD/2. Now, the low-frequency performance is only limited to the input network and the speaker response.

This type of configuration also has a clear disadvantage, however. If any noise is coupled into the single-ended input, it will still be present on the output and multiplied by the amplifier gain. Because amplifier B has no feedback to the input, any high-frequency noise coupled onto the outputs also will result in clicking and buzzing. This effect is called RF rectification.