Transistor amplifier

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Block diagram of a basic switching or PWM (Class-D) amplifier.

A switching amplifier or class-D amplifier is a power amplifier where the active devices (especially in the output stage) are operated in on/off mode (i.e., as switches). The term "Class-D" is often assumed to mean "digital" amplifier. The quantization of the output signal implies sampling like that done in A/D conversion, but such amplifiers' input and output signals are still analog.

Output stages such as those used in pulse generators are examples of class D amplifiers. However, the term mostly applies to devices intended to reproduce signals with a bandwidth well below the switching frequency. These amplifiers use pulse width modulation (PWM), pulse density modulation (sometimes referred to as pulse frequency modulation) or more advanced forms of modulation such as Sigma delta modulation (see for example Analog Devices AD1990 Class-D audio power amplifier). The input signal is converted to a sequence of pulses whose average value is directly proportional to the amplitude of the signal at that time. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The final switching output consists of a train of pulses whose width is a function of the amplitude & frequency of the signal being amplified, and hence these amplifiers are also called PWM amplifiers. The output contains, in addition to the required amplified signal, unwanted spectral components (i.e. the pulse frequency and its harmonics) that must be removed by a passive filter. The filter is usually made with (theoretically) lossless components like inductors and capacitors in order to maintain efficiency.

A PWM amplifier operates similar to a switching power supply (SMPS), except that a PWM amplifier is feeding a varying audio signal voltage into a relatively fixed load, while a switching power supply feeds a fixed voltage into a varying load. A switching amplifier must not be confused with an amplifier that uses a SMPS. A switching amplifier may use any type of power supply but the amplifier itself uses switching of output devices though to achieve amplification.

One way to create the PWM signal is to use a high speed comparator ("C" in the block-diagram above) that compares a high frequency triangular wave and the audio input and generates a series of pulses such that the width of the pulses corresponds to the amplitude and frequency of the audio signal. The comparator then drives a switching controller which in turn drives a high-power switch (usually comprised of MOSFETs) which generates a high-power replica of the comparator's PWM signal. This PWM output is fed to a low-pass filter which removes the high-frequency switching components of the PWM signal to recover the audio information and feeds it to a loudspeaker. A suitably high switching frequency (or triangular waveform) is mandatory in order to obtain reasonably good frequency response and low distortion. Most class-D amplifiers use switching frequencies greater than 100 kHz. These high frequencies require most of the components in the amplifier to be capable of high speed operation. Another way to create the PWM signal is adopted when a SPDIF signal or other form of digital feed is available. The digital signal is fed to a DSP that uses software to create the PWM signal. The PWM signal is not usually fed directly to the MOSFETs but to some kind of MOSFET driver (inside the switching controller) that can deliver the high currents required to make the MOSFETs work in the non-linear area (i.e., as switches rather than amplifiers).

Usual problems encountered when designing class-D amps are, too short dead times and/or operation in the saturation area (for MOSFETs). Both the conditions typically results in catastrophic failure of the MOSFETs.

The final frequency response and distortion depends not only on the switching frequency and the output filter but also on the load (or speaker system) connected to the amplifier's output. A speaker system may contain a single driver (loudspeaker) or multiple ones with a passive crossover. Loudspeaker impedance is not fixed and changes with audio frequency and this compounds with the passive crossover's own problems. This means that the load presented to the amplifier is not purely resistive and changes with the frequency of the audio signal that the amplifier outputs, thereby causing anomalies in the final frequency response (including peaking, oscillation and distortion). Hence many high-end class-D amplifiers employ negative feedback to correct for phase/frequency anomalies due to speaker impedance and the crossover. This makes the design of a class-D amplifier even more complex. But despite the complexity involved, a properly designed class-D amplifier offers the following benefits:

Reduction in size and weight,
Reduced power dissipation (heat) and hence smaller (or no) heatsinks,
Reduction in cost due to smaller heat sink and compact circuitry,
Very high power conversion efficiency, usually ≥ 90%.

The high efficiency of a class-D amplifier stems from the fact that the switching output stage is never operated in the active (or linear for bipolar junction transistors) region. Instead, the output devices are either ON or OFF - both states representing minimum power dissipation in the output devices. When the devices are ON, the current through them is maximum but the voltage across the devices is (theoretically) zero and when the devices are OFF, the voltage across the devices is maximum but the current is zero. In both cases, the power dissipated (V x I) is zero. All these calculations are based on ideal circumstances. In practice, there are always losses, due to leakage, voltage drop, etc. However, these are still small enough to keep efficiency very high.

This still leaves the signal with a great amount of energy in the form of harmonics, which must be filtered out. To maintain a high efficiency, the filtering is done with purely reactive components (inductors and capacitors), which store the excess energy until it is needed instead of converting it into heat.

There are a number of commercial Class-D modules available that can easily substitute former linear power stages in order to update an amplifier system. (see for example coldamp)