Signal Amplification on Smartphone´s Audio — Part I

Maximizing the speaker acoustic capabilities on smartphones has been for years one of the main development concerns. Recently, it has been fond out that the amplification is the key for a huge improvement.

Firstly, we will do an overview of the basic principles of the amplifiers before going into detail in the state of the art on audio amplifiers in smartphones industry.

What is an amplifier?

Amplifier´s target is increasing the signal level to adequate it to the next system in the signal path with the lowest signal perturbation. In the picture below we can see a very simple block diagram of an amplifier. Amplifiers replicate the input signal at a greater level. Most of amplifiers need a power supply as “raw material” source.

Types of amplifiers

There are many different types of amplifiers based on different parameters such as its input (signal nature, level, bandwidth, frequency,…), output, efficiency, quality and system limitations (size, cost, thermal performance , interferences shielding…). We can cover some of the most interesting amplifiers attending to the input signal nature:

• Acoustic amplifiers are very interesting since they can work without a power source. Acoustic signal reverberations within a cavity can amplify the original (input) signal. This is the physical phenomenon behind the speaker baffles.

• Electric amplifiers work with electric signals, and require an external power supply as source to output the increased signal. These are the most common amplifiers, and can be used for many different applications such as radio and audio (which will be our case of study).

Which parameters describe an amplifier?

• Input signal: Physical nature, level, spectrum

The physical nature of the signal describes which kind of input we have in the amplifier, it can be an optic, acoustic or electric signal among others.

Two of the most relevant parameters of an input signal are the amplitude and frequency. We can observe both either in the time domain or in the frequency domain. Time let us understand how the signal changes in time while frequency shows signal´s spectral content.

• Power Supply: level, noise, stability.

Raw material quality is essential in amplifying the input signal; we can’t construct a building if we lack a big amount of stable and pure concrete.

Boost level, is the maximum level the power supply can deliver. This level will obviously limit the maximum output level.

Power supply must be kept clean, with no interferences or instabilities, as every defect this signal has will be present in the output signal.

• Output: level, gain, load.

The output level will be defined by the gain set to the amplifier (as long as our boost voltage is high enough). The gain defines how many times bigger the output signal will be. Depending on which is the next system in the signal path (called load), our amplifier can be capable or not of supplying the output signal to it.

• Efficiency

Defines how efficient the amplifier is in terms of power. Which amount of the total used power is delivered in the output? Depending on the application, this parameter can be extremely relevant, or not.

Amplifier Quality

We already got the output signal! It is time to check how well it was amplified. These parameters describe the amplifier quality.

• Frequency Response (FR): Defines how constant the gain is across the different frequencies which compound the input signal, and which is the amplifier working bandwidth.

• Total Harmonic Distortion (THD): Distortion refers to “added” content in frequencies not present in the input. Harmonic signal is a type of distortion which appears on the harmonics of the original signal.

• Transient response: Besides spectral analysis, there are several time-dependent quality indicators, which define how the amplifier behaves in rapid signal changes. Slew Rate is the most common way of describing how fast the signal level can increase/decrease in the output.

Now that we have already explained amplifiers a bit, we can face a deeper analysis about which peculiarities audio amplifiers have, their types, which is the state of the art on smartphone´s industry and nowadays leading technology.

Hope you can all read the second part of the article soon!