Amplifier design considerations of the hottest dri

2022-10-23
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Amplifier design considerations for driving ceramic speakers

today's portable devices drive the demand for smaller, thinner and more efficient electronic devices. Now the shape of the honeycomb has become so thin that traditional electric speakers have become a limiting factor in how thin manufacturers will be able to make. Ceramic or piezoelectric speakers are rapidly becoming a viable alternative to electric speakers. These ceramic speakers (drivers) can provide highly competitive sound pressure level (SPL) in a thin and compact package, and they are very likely to replace the traditional voice coil electric speakers. Please refer to table 1 (the end of the article) for the main differences between electric and ceramic speakers

the amplifier circuit driving ceramic speakers has different output driving requirements compared with the amplifier circuit driving traditional electric speakers. The structure of ceramic loudspeaker requires that the amplifier can drive a larger capacitive load, provide more current at high frequency, and maintain a higher output voltage

characteristics of ceramic speakers

the technology used by ceramic speaker manufacturers is very similar to the technology used to manufacture multilayer ceramic capacitors. Compared with electric speakers, this manufacturing technology enables speaker manufacturers to control speaker tolerances more strictly. Strict structural tolerances are important in unifying loudspeakers and achieving consistent acoustic characteristics

from the driving amplifier, it can be seen that the impedance of the ceramic loudspeaker can be modeled as an RLC circuit with a large capacitance, which is the main component of the model, as shown in Figure 1, which improves the ability to satisfy customers in all aspects of service and even the company. For most audio frequencies, ceramic speakers are mainly capacitive. This capacitive characteristic of loudspeaker indicates that its impedance will decrease with the increase of frequency. Figure 2 shows the impedance and frequency characteristics of ceramic speakers similar to 1uF capacitors

Figure 1: model of ceramic loudspeaker

Figure 2: impedance and frequency characteristics of ceramic loudspeaker and 1uF capacitor

the above impedance also has a resonance point. The loudspeaker has the highest sound production efficiency above the resonance point frequency. The impedance drop at 1kHz indicates the resonant frequency of the loudspeaker

relationship between sound pressure and frequency and amplitude

applying an alternating voltage at both ends of the ceramic speaker can deform and vibrate the piezoelectric film inside the speaker, and its displacement distance is proportional to the input signal. The vibrating piezoelectric film pushes the surrounding air, producing sound. Increasing the voltage on the loudspeaker can increase the deflection amplitude of the piezoelectric element, thus producing a greater sound pressure, which can increase the volume

ceramic speaker manufacturers usually mark their speakers with the maximum terminal voltage, which is generally about 15vp-p. Indicates that the ceramic device reaches the maximum extension under this maximum voltage. If a voltage higher than the rated voltage is applied, it will not produce a higher sound pressure, but will increase the distortion of the output signal, as shown in Figure 3

Figure 3: relationship between SPL and frequency of ceramic loudspeaker

by comparing the curve of SPL and frequency and impedance and frequency, it can be seen that piezoelectric loudspeaker has the highest efficiency of generating large SPL above self resonant frequency

amplifier requirements

ceramic speaker manufacturers usually specify that the maximum sound pressure value is generated at the maximum voltage point of 14 to 15vp-p. Now the problem becomes how to generate these voltages from the single battery supply voltage

one method is to use a switching regulator to raise the battery voltage to 5V. With a stable 5V voltage, the system designer can choose a single power amplifier that requires a bridged load (BTL). Connecting the load by bridging can automatically double the voltage "seen" by the speaker

but BTL amplifier with 5V can only increase the output swing to 10vp-p in theory. This voltage is not enough to make the ceramic speaker output the highest SPL value. In order to produce a higher sound pressure value, the power supply voltage needs to be adjusted to a higher level

another method is to use a boost converter to raise the battery voltage to 5V or higher, but this has its own problems, such as the size of components. The high peak inductance current will restrict the overall solution, because the final inductance volume must be large to make the magnetic core unsaturated. Although inductors with high current and small volume are also available in the market, the rated saturation current of the magnetic core may not be large enough to meet the load current requirements for driving speakers with high voltage at high frequencies

because the ceramic loudspeaker has a very low impedance at high frequencies, it is necessary to consider high current driving and avoid current limiting when driving this ceramic device

the amplifier selected to drive the ceramic speaker must have enough driving current to avoid driving the speaker into the current limiting mode when driving a large number of high-frequency components

Figure 4 is an application circuit using class G amplifier. Class G amplifiers have several voltage rails available: a high voltage and a low voltage. The low-voltage rail is used when outputting small signals. When the output signal requires a higher voltage swing, the high-voltage rail is switched to the output stage circuit

Figure 4: typical ceramic loudspeaker application circuit

therefore, when the output signal is small, the efficiency of class G amplifier is higher than that of class AB amplifier, which is due to the lower voltage rail. Because of the high voltage rail, the class G amplifier can still process peak transient signals

the max9788 shown in the figure uses an on-chip charge pump to generate a negative voltage opposite to VDD. This negative voltage rail is only added to the output circuit when the output signal requires a higher voltage rail. Compared with the traditional class AB amplifier using the boost converter method, the device firmly surrounding the goal of building a powerful material country can drive ceramic speakers more efficiently

speaker manufacturers often recommend fixed resistors (RL) connected in series with ceramic speakers, as shown in Figure 4. This resistor can limit the output current of the amplifier when the signal contains a large number of high-frequency components

in some applications, if the bandwidth of the audio frequency response sent to the speaker is limited to ensure that the speaker will not be short circuited to the amplifier, this fixed resistance can be avoided. At present, the capacitance value of ceramic speakers on the market is about 1uF. The impedance of the loudspeaker is 20W at 8kHz and 10W at 16KHz. Ceramic speakers in the future may have larger capacitance values, which will force the amplifier to provide more current at the same signal frequency

efficiency of ceramic speakers and electric speakers

the efficiency of traditional electric speakers is easy to calculate. The decline in imports does not mean that the utilization rate of foreign equipment has fallen. Electrically, the voice coil winding can be modeled as a fixed resistance in series with a large inductance

the resistance value of the loudspeaker can be used and the power provided to the load can be calculated according to Ohm's Law:

p=i2r or p=vxi

this power is consumed as heat on the loudspeaker coil

due to the capacitive characteristics of ceramic speakers, they will not generate too much heat when consuming power. According to the dissipation coefficient of ceramic components, the so-called blind power consumed by this kind of loudspeaker is very small. Therefore, the heat generated when consuming reactive power is also very small

reactive power cannot be calculated with simple p=vxi

(reference 1)

reactive power should be calculated as follows:

[insert Formula 1]

p = (π fcv2) × (cos Φ + DF)

where:

-c= capacitance value of loudspeaker

-v=rms driving voltage

-f= frequency of driving voltage

-cos j= phase angle between current passing through loudspeaker and voltage on loudspeaker

-df= dissipation coefficient of loudspeaker, which depends on signal frequency and ESR of ceramic loudspeaker

for ideal capacitance, the phase angle between voltage and current should be 90 degrees, while ceramic loudspeaker is mainly capacitive characteristic, Therefore, cos J is equal to 0, that is, the capacitive part of the ceramic speaker has no power consumption. However, the non ideal characteristics of ceramic materials will cause the voltage on the loudspeaker to lag behind the current through the loudspeaker, and the phase angle between them is not completely equal to 90 degrees. The difference between the ideal 90 degree phase shift and the actual phase shift is the dissipation coefficient. DF in ceramic loudspeaker can be modeled as a small resistor, ESR and ideal capacitor in series. (don't confuse the series resistance with the isolation resistance between amplifier and loudspeaker)

df is the ratio of ESR to capacitive reactance at the target frequency (references 2 and 3):

[insert formula 2]

df = resr/xc

for example, the reactive power of a loudspeaker with 1.6uf capacitance and 1W ESR when driven by 5vrms and 5KHz signals is:

[insert formula 3]

real power consumption

therefore, although the ceramic speaker itself will not dissipate the actual power in a thermal manner like the electric speaker, heat will be generated on the output stage of the driving amplifier and the external resistance (RL) between the amplifier and the speaker (Fig. 4)

the larger the external resistance is, the more power consumption will be generated by the amplifier, which will affect the low-frequency display response according to the detection results

when driving the ceramic loudspeaker with 10W series resistance, we can see that the "reactive" power has little effect on the total load power. Most of the power is consumed on the external resistance. See the power versus frequency curve of the amplifier shown in Figure 5

Figure 5: relationship between required power and frequency

better low-frequency response requires smaller external resistance, but this will increase the power consumption of the output stage of the amplifier. Amplifier efficiency indicates how much power is consumed in the output stage of the amplifier. The power consumption of amplifiers drives the demand for more efficient solutions, including class D and class G amplifiers. Since the load is composed of many series resistors, certain power consumption will be generated on the load network rather than on the speaker. Even if the efficiency of the amplifier is 100%, the series resistance will consume the power originally given to the speaker

in this simple example, the total power supplied to the load at 5KHz is 515mw. An amplifier with an efficiency of 53% will consume 457mw of power. The necessary power consumption of the amplifier determines the package size of the device. If the speaker must be driven by high-frequency sine wave, it requires a lot of power consumption

in short, thinner and thinner portable devices drive the demand for small ceramic speakers. This kind of loudspeaker is different from the traditional electric loudspeaker, so designers need to consider different design elements. The capacitive characteristics of ceramic speakers require the amplifier to have the ability of high output voltage drive and large output current, so as to maintain high voltage in the whole frequency range

the amplifier selected to drive the ceramic loudspeaker must be able to provide reactive power and real power to the mixed load at the same time. The amplifier efficiency must be high enough to ensure a small size and low-cost solution

therefore, it is necessary to use an amplifier topology different from the traditional class AB amplifier. For example, more efficient solutions such as class G and class D amplifiers are becoming more and more attractive, in which class G amplifiers can provide the best efficiency. Table 1: advantages and disadvantages of ceramic and electric loudspeakers

references references

1 Sonitron NV, Application Note 02AN106, "Blind Power Dissipation in a Piezo Ceramic Load (for a sine wave)," the last two pages of

2. National Semiconductor Corp., "Piezoelectric Transducers: Distributed Mode Actuators

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