Class-D amplifier
A class - D amplifier (English class -D amplifier ) or switching amplifier, digital amplifier or digital amplifier also called, is an electronic amplifier, which is used among other things as a power amplifier or power amplifier. It is characteristic of the audio signal by a suitable method - for example, by pulse width modulation ( PWM), - is brought into a series of pulses. This allows the power amplifier be operated in switching mode, whereby the switching elements ( practically always transistors ) are either up -conductive or maximum insulating ( so have only two states ) - these are ( in contrast to in conventional class -A, -B or - AB amplifiers used between states of the linear operation ), the two working areas, in which only little power loss is generated. Through a reconstruction filter ( low-pass ) after the power setting the input signal corresponding continuous voltage waveform is generated.
The common name in digital amplifier or digital amplifier gives the false impression that a PWM amplifier would amplify a digital signal "only". Indeed, the development of an amplifier for digital signals is a challenge to analog circuit designers to control the required power switching transistors.
Applications
Common uses find these amps as audio amplifier with high -performance with high energy efficiency, mainly in the field of sound systems ( " PA systems " ) and as a modulation amplifier with amplitude- modulated broadcast transmitters. Further wherever at low power, high efficiency is important, such as power amplifiers for headphones in battery-powered devices such as mobile phones and MP3 players. Due to the combination of low bandwidth requirement and increased compared to higher frequencies the power requirement they are also used in amplifiers for subwoofers. Another use is the final stage of sine wave voltage inverters high performance.
Construction
Hereinafter, an amplifier using pulse-width modulation (PWM) and analog control is described. There are various other analog and digital methods and refinements, which however have in common that a signal having two voltage states is produced which corresponds to a time average of the input signal. Examples are the pulse density modulation ( pulse frequency modulation ), the delta-sigma modulation or sliding mode control.
In contrast to the class AB push-pull amplifier that can amplify a signal similar to the original signal as far until it reaches the maximum voltage standing from the power supply to disposal, the PWM output stage operates on a different principle: a symmetrical working triangle generator oscillates with a typical frequency of approximately 250 kHz to several MHz. The applied audio signal is compared by a comparator with the triangle signal. The construction as the comparator circuit changes the analog sound signal into a square wave, as can be seen in the illustration.
The triangular signal is greater than the audio signal, the output will jump to "high". If it is smaller, it jumps to "low". The sound is now available in the duty cycle of the PWM signal. The mean value is approximately proportional to the average by the sound signal. This PWM signal is the final stage (comprising the power transistor in the switching mode ) is supplied, which takes place in the actual gain.
Due to the high frequency switching operation resulting amplified noise in the PWM frequency or its harmonics, which are preferably emitted by the speaker cables and make the increased interference suppression and low-pass filter required. By low-pass filtering the electromagnetic compatibility is guaranteed or radio interference is avoided.
In addition to the possibility for filtering the PWM switching frequency can also spread spectrum ( Spread Spectrum Data Sheet ) apply, whereby the noise over a wider frequency range can be spread. This unrealized Class D amplifiers are available as so-called spread-spectrum class D amplifier and require no low-pass filter at the output.
Benefits
A digital amplifier has a particularly high efficiency of typically about 70%. It derives an advantage over an analog amplifier lower heat loss from which may be discharged through smaller heatsink. The user brings this a smaller, lighter and more economical device that can be equipped in particular for mobile applications with a smaller battery, or with the same battery capacity gets a longer service life.
Disadvantages
Due to the PWM frequency of the audio signal to distortion, which are expressed as total harmonic distortion (THD ) and have their cause in the intermodulation between the audio signal and switching frequency result. Class D amplifiers have over other types of amplifiers to a higher THD is typically 1%, which can rise at maximum power up to 10%. The frequency of the pulse width modulation can lead to disturbances in other modules may, if it is not properly screened. In amplitude- modulated broadcast transmitters with NF- digital amplifier this may lead to unwanted radiations.
Digital amplifier
With switching amplifiers, it is possible to digitally perform most functions. The input signal is then usually a pulse-code- modulated signal is converted by a signal processor or a specialized digital modulator circuit into a drive signal for the power amplifier. In addition to the pulse width modulation described herein has the delta -sigma modulation is used. Because of the caused by the quantization of the digital processing power amp come methods for noise shaping used. Only when the control of the output stage will leave the digital domain - therefore provides a digital amplifier, in principle, a " performance digital -to-analog converter " dar.