A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, and not as linear gain devices as in other amplifiers. They are rapidly switching back and forth between the supply rails, being fed by a modulator using pulse width, pulse density, or related techniques to encode the audio input into a pulse train. The audio escapes through a simple low-pass filter into the loudspeaker. The high-frequency pulses, which can be as high as 6 MHz, are blocked. Since the pairs of output transistors are never conducting at the same time, there is no other path for current flow apart from the low-pass filter/loudspeaker. For this reason, efficiency can exceed 90%.
Basic operation
Class-D amplifiers work by generating a train of square pulses of fixed amplitude but varying width and separation, or varying number per unit time, representing the amplitude variations of the analog audio input signal. It is also possible to synchronize the modulator clock with an incoming digital audio signal, thus removing the necessity to convert it to analog, The output of the modulator is then used to gate the output transistors on and off alternately. Great care is taken to ensure that the pair of transistors are never allowed to conduct together. This would cause a short circuit between the supply rails through the transistors. Since the transistors are either fully “on” or fully “off”, they spend very little time in the linear region, and dissipate very little power. This is the main reason for their high efficiency. A simple low-pass filter consisting of an inductor and a capacitor are used to provide a path for the low-frequencies of the audio signal, leaving the high-frequency pulses behind. In cost sensitive applications the output filter is sometimes omitted. The circuit then relies on the inductance of the loudspeaker to keep the HF component from heating up the voice coil.
The structure of a class-D power stage is somewhat comparable to that of a synchronously rectified buck converter (a type of non-isolated switched-mode power supply (SMPS)), but works backwards. Whereas buck converters usually function as voltage regulators, delivering a constant DC voltage into a variable load and can only source current (one-quadrant operation), a class-D amplifier delivers a constantly changing voltage into a fixed load, where current and voltage can independently change sign (four-quadrant operation). A switching amplifier must not be confused with linear amplifiers that use an SMPS as their source of DC power. A switching amplifier may use any type of power supply (e.g., a car battery or an internal SMPS), but the defining characteristic is that the amplification process itself operates by switching. Unlike a SMPS, the amplifier has a much more critical job to do, to keep unwanted artifacts out of the output. Feedback is almost always used, for the same reasons as in traditional analog amplifiers, to reduce noise and distortion.
Theoretical power efficiency of class-D amplifiers is 100%. That is to say, all of the power supplied to it is delivered to the load, none is turned to heat. This is because an ideal switch in its on state would conduct all the current but have no voltage loss across it, hence no heat would be dissipated. And when it is off, it would have the full supply voltage across it but no leak current flowing through it, and again no heat would be dissipated. Real-world power MOSFETs are not ideal switches, but practical efficiencies well over 90% are common. By contrast, linear AB-class amplifiers are always operated with both current flowing through and voltage standing across the power devices. An ideal class-B amplifier has a theoretical maximum efficiency of 78%. Class A amplifiers (purely linear, with the devices always “on”) have a theoretical maximum efficiency of 50% and some versions have efficiencies below 20%.
TPA3122
TPA3122 is from Texas Instruments. According to the TPA3122 datasheet, it is a 15-W (per channel) efficient, Class-D audio power amplifier for driving stereo single ended speakers or mono bridge tied load. The TPA3122D2 can drive stereo speakers as low as 4Ω. The efficiency of the TPA3122D2 eliminates the need for an external heat sink when playing music. The gain of the amplifier is controlled by two gain select pins. The gain selections are 20, 26, 32, and 36 dB.
Features
- 10-W/ch into an 4-Ω Load From a 17-V Supply
- 15-W/ch into an 8-Ω Load From a 28-V Supply
- Operates from 10 V to 30 V
- Efficient Class-D Operation
- Four Selectable, Fixed Gain Settings
- Internal Oscillator (No External Components Required)
- Single Ended Analog Inputs
- Thermal and Short-Circuit Protection with Auto Recovery Feature
- 20-pin DIP Package
DIP Package Pinout
Terminal Functions
Simplified Application Circuit
Complete Circuit
PCB Design
Bottom Side
Top Side
Gain Jumper Settings
Table 1
| GAIN1(J2) | GAIN0(J1) | Amplifier Gain(dB) |
| ON | ON | 20 |
| ON | OFF | 26 |
| OFF | ON | 32 |
| OFF | OFF | 36 |
OFF denotes jumper is REMOVED; ON denotes jumper INSTALLED.
Applying Power to the Circuit
- Verify correct voltage and input polarity for the external power supplies. Turn ON. The EVM starts operation.
- Adjust the input signal.
- Adjust the control inputs to the desired settings as described in the Control Inputs section.
- Adjust the amplifier gain by installing or removing the gain jumpers, J1 and J2 to yield the gain values described in Table 1.
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