Eliminating the audible ‘buzz’ in GSM phones

The fully differential amplifier can cancel RF rectification effects and silence cell-phone noise.

Today’s cell-phone PCBs are much more complex and internally “noisy” than ever before, due to added functionality and less board space. In many cases, the speaker is very close to the RF antenna, and the audio amplifier is closer to the RF power-amplifier (PA) section. That means the components on the board have to deal with higher levels of RF noise than in previous generations. In many cases, this noise causes RF rectification in the audio amplifier, which results in buzzing or clicking in the speaker.

Figure 1. Typical RF power-amplifier waveform

RF noise

One of the bigger noise contributors in the cell phone is the RF stage. In GSM cell phones, the RF PA switches on and off at 217 Hz. During the on state, the RF PA sends data to the base station and the off state allows the phone to conserve precious battery power. In laboratory test holding a GSM phone about 10 cm away from the audio amplifier, the signal that was picked up on the outputs of the audio amplifier could be seen. This noise looked like an 850-MHz-1.9-GHz RF signal gated by a 217-Hz square wave (see Figure 1).

That is the noise when coupled onto the outputs of the typical master/slave audio PA and is typically heard as a clicking or buzzing in the speaker. Fully differential architectures offer better immunity to this RF rectification effect.

Master/slave BTL audio PA architecture

The most common type of audio-amplifier architecture used today in cell-phone and portable communications devices is the single-ended amplifier with a BTL output configuration (see Figure 2).

This master/slave BTL amplifier consists of two linear amplifiers driving both ends of the load. The first amplifier (A) sets the gain, and the second amplifier (B) acts as a unity-gain inverter. The gain of this BTL amplifier is defined as:

The gain of the amplifier is double the gain of the first stage, due to the unity-gain inverting amplifier (B). One of the main benefits of this differential drive configuration is power to the load. The differential drive to the speaker means that as one side is slewing up, the other side is slewing down, and vice versa. That in effect doubles the voltage swing on the load as compared to a ground referenced load and as a result quadruples the output power from the same supply rail and load impedance.

Figure 2. Master/slave BTL audio power amplifier.

This master/slave architecture is not good in a noisy environment like a GSM phone because the input for amplifier B is taken from OUT+. Typically, a GSM phone would have a speaker near the antenna for the speaker-phone/melody ringer. The RF from the GSM phone shown in Figure 1 is picked up on OUT+ and OUT-. This signal itself is not audible because the RF component is centered at zero and the speaker and human ear cannot respond to the RF frequencies. OUT+ is actually the input for amplifier B that creates OUT-, as seen in Figure 2. An audio-amplifier bandwidth is much lower than the 850-MHz-1.9-GHz RF signal, but it still tries to respond. In these cases OUT- looks like a 217-Hz square wave in the audio band.

Figure 3. 217-Hz RF rectification with master/slave audio amplifiers.

By using the same GSM phone mentioned previously, which was held about 10 cm away from the audio amplifier, we used an oscilloscope to look at the effect of the RF noise coupled in the outputs of this audio amplifier. Looking at the full bandwidth (>20 MHz), the signal picked up on the outputs has no effect since the speaker cannot reproduce signals at that high a frequency. But when looking at the limited bandwidth (<20 MHz) in the master/slave BTL amplifier, the inverter follower (amplifier B in Figure 2) tries to respond to the gigahertz signal injected on OUT+ and causes dips on its output, resulting in clicking or buzzing on the speaker.

By using the same GSM phone mentioned previously, which was held about 10 cm away from the audio amplifier, we used an oscilloscope to look at the effect of the RF noise coupled in the outputs of this audio amplifier. Looking at the full bandwidth (>20 MHz), the signal picked up on the outputs has no effect since the speaker cannot reproduce signals at that high a frequency. But when looking at the limited bandwidth (<20 MHz) in the master/slave BTL amplifier, the inverter follower (amplifier B in Figure 2) tries to respond to the gigahertz signal injected on OUT+ and causes dips on its output, resulting in clicking or buzzing on the speaker.

Figure 4. Fully differential audio power amplifier.

The main advantage of the master/slave BTL architecture is that most of these audio amplifiers require only one input pin and therefore potentially a smaller package. However, the main disadvantage of this type of configuration is that any noise coupled into the input will be present on the output multiplied by the gain of the amplifier, and any RF noise coupled onto the outputs will result in clicking and buzzing (RF rectification).

Figure 5. 217-Hz RF rectification with fully differential audio amplifiers.

Fully differential BTL architecture
The newest type of audio PA architecture, which is in production today in many cell phones, smart phones, and other wireless devices, is the fully differential audio amplifier (see Figure 4). These types of audio PAs are available in both in Class-AB and Class-D technology, with Class-D starting to be the preferred solution due to its high efficiency.

In the case of the fully differential amplifier, the gain is defined as:

The fully differential amplifiers have differential inputs and outputs. These amplifiers consist of a differential amplifier and common-mode amplifier. The differential amplifier ensures the amplifier outputs a differential voltage equal to the differential input times the gain. The common-mode feedback, internal to the amplifier, ensures the common-mode voltage at the output is biased around VDD/2, regardless of the common-mode voltage at the input. This type of architecture offers much better RF immunity compared to the older master/slave approach.

Again, using the same test as for the master/slave audio PA, we used an oscilloscope to look at the effect of the RF noise being coupled in the outputs of the audio amplifier. As can be clearly seen from Figure 5, when an RF signal similar to the one in Figure 1 is coupled onto the outputs of a fully differential audio PA, RF rectification does not occur and the speaker is silent.

The fully differential amplifier does not use an output as an input for creating a differential signal, so the amplifier does not create a 217-Hz square wave on one of its outputs while trying to respond to the RF signal. That’s one of the reasons why many manufacturers of cell phones, especially GSM phones, are moving to this type of amplifier architecture.

Fully differential feedback

Audio power amplifiers are prone to pick up noise from the harsh environments of portable wireless communications devices. The master/slave BTL audio PA has several limitations, allowing noise coupled onto the amplifier outputs to cause clicking and buzzing. In comparison, the fully differential amplifier excels in this type of environment due to its fully differential feedback, canceling the effects of RF rectification and eliminating the “buzz” in cell phones.

Nick Holland works in audio power-amplifier product marketing and Mike Score is an audio power-amplifier systems engineer at Texas Instruments (Dallas-www.ti.com).