Class D amplifier for subwoofer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The article describes the features of the UMZCH class D and presents two options for amplifiers of different power, suitable for use mainly in the low-frequency channel for a subwoofer. As noted in the article, it is advisable to use such an amplifier with a pulse converter of the supply voltage both from a battery of batteries and from an alternating current network. When developing the design of the amplifier, attention should be paid to the shielding of these blocks.

In connection with the development of digital technology and digital audio equipment, there is a growing interest in amplifying devices operating in a key mode, known as class D amplifiers. Such devices perform pulse-width modulation (PWM) of the signal. Their advantages include, first of all, high efficiency, which actually reaches 98%. In addition, these cost-effective amplifiers are easily integrated with other elements of the digital audio path, even on the same chip.

Now, some companies are already producing microcircuits that exclude any analog processing of the audio signal, and the digital signal is converted to analog form directly in the output stage of the power amplifier. Such microcircuits are mainly used in portable and wearable equipment with autonomous power supply.

In periodicals [1] and the Internet [2,3], you can find descriptions of amplifiers (mainly automotive) operating in the "class T". On closer examination, it turns out that transistors in such devices also operate in a switching mode, while the introduction of a "new" class of amplifier operation is apparently dictated by marketing requirements. Although in some sources "class T" appears as "modified Class-D", that is, a modified class D and there are no fundamental differences from class D. It's just that in "class T" amplifiers a SHI modulator is used, in which both the frequency and the duty cycle of the output pulses depend on the input voltage. There is also information about amplifiers of a certain "class N" [4]. In fact, this is also an amplifier operating in switching mode, only it is combined with a mains power supply. All these patent and marketing moves should not mislead readers. This article outlines some of the advantages and disadvantages of a class D amplifier with two circuit examples.

The basis of an amplifier operating in class D mode is powerful switches, usually on MOS transistors, which are characterized by high speed and low channel resistance in the open state. Thanks to the use of transistors in the key mode (either on or off), a high efficiency of such amplifiers is achieved.

The transistors of powerful keys of such an UMZCH are controlled by a pulse-width (SHI) modulator, which converts the instantaneous values ​​of the input signal into the corresponding duty cycle of the pulses, and the duty cycle equal to 2 (meander) corresponds to the zero value of the signal. Typically, the pulses have an amplitude close to the power supply voltage. The output signal level can be adjusted by changing the parameters of the PWM modulator and the supply voltage of the key stage.

From the output stage, the signal enters the load (loudspeaker) through an LC low-pass filter (LPF), which separates the components of the audio frequency range from the PWM signal.

It should be noted that the quality of the output signal, determined by the level of distortion and the bandwidth of the amplified frequencies, strongly depends on the switching frequency, and high-frequency signals are always amplified with greater distortion than low-frequency ones. Increasing the switching frequency leads to a decrease in distortion, which is mainly intermodulation in nature. On fig. 1 shows waveforms of signals typical for a class T amplifier.

At present, the industry produces a wide range of semiconductor devices that make it possible to achieve operating switching frequencies of up to several hundred kilohertz and even units of megahertz. Amplifiers with such switching frequencies are capable of amplifying full audio range signals with an acceptable level of distortion.

The amplifiers described here have a relatively low switching frequency, so they are suitable for amplifying low-frequency audio signals (typically below 200 Hz). Such an amplifier can be an integral part of an active "home theater" subwoofer. With reduced signal quality requirements, it can be used as part of the equipment and for the full audio frequency range.

A diagram of the simplest amplifier operating in class D mode is shown in fig. 2.

In this device, a powerful amplifier stage is actually combined with a SHI modulator. Let's consider the scheme in more detail.

Operational amplifier DA1 is used as a comparator that compares the signals at the input and output of the amplifier. Signal-modulated pulses from the output of the op-amp are fed to a push-pull cascade with field-effect transistors VT1, VT2. Zener diodes VD1, VD2 are necessary to exclude through current through powerful transistors of the output stage. This current occurs during switch switching and is a major source of efficiency degradation in miscalculated push-pull stages. Resistors R3, R4 are needed to discharge the gate capacitance of field-effect transistors. Inductor L1 and capacitor C4 form a low-pass filter, from which the output signal goes to the loudspeaker. In addition, the output signal is fed to the non-inverting input of the op-amp DA1 through the divider R5R1, which determines the gain of the amplifier. Resistor R2 sets the input impedance of the amplifier; it can be increased if necessary. Chips DA2 and DA3 stabilize the supply voltage of the device.

The principle of operation of the amplifier is quite simple. In fact, the amplifier is an oscillator whose frequency is determined by the resonant frequency of the L1C4 circuit. Due to the fact that the amplitude of the pulse signal at the output of the amplifier is constant (and almost equal to the supply voltage), a change in the voltage at the inverting input of the op-amp leads to a change in the duty cycle and frequency of the pulses entering the circuit.

It should be noted that when the load is connected, the nature of the oscillations in the circuit acquires an aperiodic character. In this case, the switching frequency is determined not only by the parameters of the L1C4 circuit, but also by the gain of the entire device with an open feedback loop, as well as the stage switching delay.

A feature of the amplifier is the presence of high-frequency voltage fluctuations at the load. For a subwoofer, this is not a disadvantage, since the head impedance at ultrasonic frequencies is quite high. However, when connecting multi-band speakers to the amplifier, a noticeable current through the HF head can occur. To eliminate this current at the amplifier output, it is necessary to turn on an additional low-pass filter that is not included in the feedback loop.

The amplifier is powered by a transformer power supply with a nominal output voltage of ±20 V and a ripple range of up to 4 V at full load. If the transformer does not have a secondary winding with a midpoint, a bipolar power supply can be assembled according to the half-wave rectifier circuit (Fig. 3), doubling the capacitance of the filter capacitors. The maximum allowable reverse voltage of the rectifier diodes must be more than twice the open-circuit voltage of the source.

The amplifier is assembled on a breadboard. Capacitors C1, C4 - K73-17, C2, C3 - any oxide for an operating voltage of at least 16 V. It is useful to shunt them with ceramic or film capacitors with a capacity of 0,1 ... 0,47 μF. The design uses imported analogues of zener diodes VD1, VD2 for a voltage of 15 V. Choke L1 has 100 turns of 0,6 mm PETV wire in the BZO armored magnetic circuit made of M2000NM-1 ferrite. The cups of the magnetic circuit are assembled with a non-magnetic gap of about 0,5 mm. Resistors - any metal-dielectric, for example, MSC C2-33.

Although running the amplifier at full power into a 4 ohm load does not result in noticeable heating of any elements, with an increased rectifier voltage of the DAI integrated stabilizer microcircuit, DA2 must be installed on heat sinks.

Instead of the K544UD2 op amp, it is possible to use the KR574UD1, KR140UD11 (LM318), AD817 or other microcircuits with a unity gain frequency of at least 10 MHz. The output transistors used provide a load current of up to 2 A, but if desired, the amplifier power can be increased to the maximum possible by replacing transistors with IRFZ34 (VT1) and IRFZ24 (VT2). With such a replacement, the output power of the amplifier is limited only by the maximum allowable supply voltage of the operational amplifier.

Correctly assembled from serviceable parts, the amplifier does not need to be adjusted. However, in order to avoid damage to the output transistors due to improper installation, it is recommended to initially connect their source to the power supply through resistors with a resistance of 33 ... 51 Ohm and a power of 1 W.

When adjusting, you need to connect a load equivalent to the output of the amplifier - a resistor with a resistance of 4 ... 8 Ohms and a power of 10 W and close the input of the amplifier. When the power is turned on at the input of the low-pass filter, almost rectangular pulses with a duty cycle of about 2 and a frequency of about 80 kHz can be observed using an oscilloscope. The deviation of the duty cycle from the specified indicates the asymmetry of the cascade. Current pulses through additional resistors should have a shape close to triangular. The presence of significant surges in the pulses indicates the flow of through current through the output transistors. In this case, you need to replace the zener diodes VD1, VD2 with others with a high stabilization voltage.

The advantages of this amplifier include the almost complete absence of a transient process when turned on. The efficiency of the amplifier is determined mainly by the efficiency of the supply voltage stabilizers. An increase in efficiency can be achieved by using switching voltage stabilizers.

The amplifier according to the scheme in fig. 4 differs from that described above in a more complex output stage circuit. It can be powered from a source with a voltage greater than the maximum allowable supply voltage of the operational amplifier.

(click to enlarge)

A feature of this UMZCH is the use of the output stage of the op-amp in the output current limiting mode. This avoids saturation of the transistors of its output stage and, as a result, increases the clock frequency of the device.

With the output transistors indicated in the diagram, and transistors VT1, VT2 with a large allowable voltage Uke max (at least 60 V), the supply voltage can reach ± ​​50 V, which contributes to the operation of a high-resistance load.

The harmonic distortion factor does not exceed 0,3% at a frequency of 1 kHz with an output sinusoidal power of 60 W at a load of 4 ohms. The amplifier is capable of operating in the full range of audio frequencies, while the sound has a certain "cassette" tone at high frequencies. Intermodulation distortion, measured by the double tone method at frequencies of 19 and 20 kHz, reaches 14%, but, oddly enough, this does not have a "destructive" effect on the sound.

The printed circuit board for this amplifier is shown in fig. 5.

The inductor used is the same as in the amplifier according to the circuit in Fig. 2. Long-term operation of the amplifier at a power of 80 W at a load of 4 ohms does not lead to significant heating of the active components. However, for the operation of the amplifier in hot climates, it is advisable to install powerful transistors and integrated stabilizers on small plate heat sinks.

If you intend to use a high-speed op amp, such as AD817, it is advisable to reduce the number of turns of the L1 coil by one and a half to two times. In this case, the operating switching frequency increases; accordingly, distortion at high frequencies of the audio range is reduced. The waveform at the output of the op-amp can be almost sinusoidal, while the output transistors continue to operate in switching mode due to the high gain of the pre-output stage. The main criterion for the operation of the PA in the switching mode in this case is the absence of a through current through the output transistors.

Recommendations for replacing components in this UMZCH are similar to those outlined for the first version of the circuit. But when using an op amp with a large load capacity, such as AD817, you should reduce the supply voltage of the op amp to avoid overheating. To do this, instead of integrated stabilizers 7815 and 7915, you need to use similar ones for a voltage of 9 ... 12 V. The output transistors must be complementary with close parameters. Preference should be given to transistors with the lowest possible input capacitance and low (and the same for transistors of different types of conductivity) channel resistance in the on state. In principle, bipolar output transistors can also be used; to do this, you need to reduce the resistance of resistors R4, R6 to 33 ohms. However, due to the significant excess of the turn-off time over the turn-on time of bipolar transistors, a through current occurs through the transistors of the push-pull stage and the main advantage of the amplifier is lost - high efficiency; in addition, non-linear distortions also increase. To eliminate the through current, it is necessary to use a special pause shaper between the moments of turning off one and turning on another powerful transistor.

The amplifier does not need to be tuned and, if serviceable parts are used, it starts working immediately.

As noted earlier, the described amplifiers are intended for use as part of an active subwoofer. The presence of high-frequency components in the input signal can lead to unpleasant intermodulation distortion, therefore, to limit the frequency band of the amplified signal above 150 Hz, a low-pass filter is recommended according to the circuit in Fig. 6.

(click to enlarge)

If the subwoofer is connected to the low-frequency channel output of an SB Live 5.1 or similar sound card, then there is no need for such a filter. In other cases (for example, when connected to a stereo system), such a filter may be necessary.

The low-pass filter consists of two parts: the input amplifier-adder and the second-order low-pass filter itself with a tunable cutoff frequency. The only feature of the device is the adjustment of the output signal level by changing the feedback coefficient of the adder using resistor R3. LPF with the Bessel characteristic of the second order has no singularities.

LPF assembled on a breadboard. Other op-amps recommended for use in amplifiers and corrected for unity gain can be used if an external correction circuit is needed. The variable sensitivity adjustment resistor R3 must have a B characteristic, and the cutoff frequency adjustment resistor R5 - A (linear). Resistors R1 and R2 can be placed directly in the connectors connected to the signal source. In this case, only one shielded wire will need to be pulled to the filter.

LPF does not need to be adjusted.

Amplifiers can be used as part of both automotive and computer (multimedia) audio complexes. The amplifier according to the scheme in fig. 4 is conveniently powered from a switching power supply described in [5]. The output voltage of the 30 V supply is enough to produce a peak output power of 100 W into an 8 ohm load. It is advisable to use such a source to stabilize the supply voltage of the first version of the amplifier; the efficiency of the system will be the highest.

It should be noted that turning on the described amplifiers for a long time without load can lead to overheating and failure of the output transistors. If it is assumed that the load can be turned off for some reason during the operation of the amplifier, a resistor with a resistance of 100 ohms and a power of at least 2 watts should be connected to the output of the low-pass filter.

Literature

1. Elyutin A. Exhibition "CES'2002". - Autosound, 2002, No. 4, pp. 11-14.
2. tripath.com/downloads/anl.pdf.
3. kensel-con.neVBizDev/pub-Msh/articia_9415.shtmi
4. http://1stquadrant.com/news_cisssN.htm.
5. Kolganov A. Automotive UMZCH with a power supply. - Radio, 2002, No. 7, p. 20-22.

Author: E. Saveliev, Tver

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