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Class G amps key in driving ceramic speakers

Posted: 16 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:ceramic speakers? piezoelectric speaker? audio design? electronic components? multilayer ceramic capacitors?

By Mark Cherry
Maxim Integrated Products Inc.

Today's portable devices drive a need for smaller, thinner and more power-efficient electronic components. Cellphone form factors have become so thin that the traditional dynamic speaker has become the limiting factor in how thin manufacturers can make their handsets. Ceramic or piezoelectric speakers are quickly becoming a viable alternative to dynamic speakers. Ceramic speakers (drivers) can deliver competitive sound pressure levels (SPLs) in a thin and compact package, potentially replacing traditional, voice-coil dynamic speakers. Table 1 sums up some of the differences between dynamic and ceramic speakers.

Amplifier circuits that drive ceramic speakers have different output-drive requirements from those that drive traditional dynamic speakers. The construction of the ceramic speaker requires that the amplifier be able to drive a large capacitive load and supply increasingly larger currents at higher frequencies, while maintaining high output voltage.

Attributes
Ceramic-speaker manufacturers use technology similar to that used for building multilayer ceramic capacitors. This manufacturing technique gives speaker manufacturers tighter control over the speaker tolerances compared with dynamic speakers. Tight construction tolerances become important when attempting to equalize the speaker and in getting repeatable sonic characteristics from unit to unit.

The ceramic speaker impedance, as seen by a driving amplifier, can be modeled as resonant or RLC circuit with a large capacitance as its main element. Across most audio frequencies, the ceramic speaker is mostly capacitive. The capacitive nature of the speaker dictates that impedance decline as frequency increases.

The impedance also has a point of resonance. Above that resonance is where the speaker is most efficient at producing sound. The dip in impedance around 1kHz indicates the speaker's resonant frequency.

RF elements
Placing an alternating voltage across the terminals of the ceramic speaker causes the piezoelectric film inside the speaker to deform and vibrate, with the amount of displacement proportional to the input signal. The vibrating piezoelectric film moves the surrounding air, thus producing sound. Increasing the voltage across the speaker increases the piezo element deflection, creating more sound pressure and thus, increased audio volume.

Ceramic-speaker manufacturers typically rate their speakers with a maximum terminal voltage, typically around 15Vp-p (peak-to-peak voltage). This maximum voltage is the point at which the ceramic element will reach its maximum extensions. Applying a voltage greater than the rated voltage will not result in more sound pressure, but will increase the amount of distortion present in the output signal (Figure 1a).

By comparing the SPL vs. frequency and impedance vs. frequency graphs, we see that the piezo speaker is most efficient at producing high SPLs above its self-resonant frequency.

Amplifier requirements
Ceramic-speaker manufacturers specify a maximum voltage of 14Vp-p to 15Vp-p to produce the highest levels of sound pressure. The question quickly becomes how to generate those voltages from a single battery supply.

One solution is to use a switching regulator to boost the battery voltage to 5V. Armed with a regulated 5V supply, the system designer could choose a single-supply amplifier that requires a bridge-tied load (BTL). Bridge tying the load automatically doubles the voltage that the speaker "sees."

But supplying a BTL amplifier with 5V will only allow the output to swing theoretically to 10Vp-p. This voltage will not allow the ceramic speaker to output its highest SPL. To create higher sound pressure levels, the power supply would need to regulate to a higher voltage.

Another approach is to use a boost converter to regulate the battery voltage up to 5V or more, but that has its own set of issues (i.e. the size of components needed). Large peak inductor currents can quickly limit how small a total solution can be because the inductor ends up having to be physically large so that the core doesn't saturate. High current, small-profile inductors are available but the saturation-current rating of the core may not be high enough to handle the load current needed to drive the speaker with high voltage at high frequency.

Table 1: Ceramic speakers can deliver competitive SPLs in a thin and compact package, potentially replacing voice-coil dynamic speakers.

High current drive and current-limit avoidance are needed to drive the ceramic element. That's because the ceramic speakers have very low impedance at high frequencies.

The amplifier chosen to drive the ceramic speaker must have sufficient current drive available to avoid going into a current-limit mode when a lot of high-frequency content is being driven into the speaker.

Class G solution
Figure 2 shows an application circuit using a Class G amplifier, which has multiple voltage rails available: one high voltage and one low. The low-voltage rail is used when the output signal is small. The high-voltage rail is switched onto the output stage when the output signal demands a higher voltage swing.

As a consequence, the amplifier is more efficient than a Class AB amplifier when the output signal is small, directly because of the lower power-supply rail. The Class G amplifier can still handle peak transients because of the higher available rail.

The MAX9788 uses an on-chip charge pump to generate a negative rail that is the inverse of VDD. This negative rail is only applied to the output stage when the output signal demands the higher rail. The device provides a more efficient method of driving a ceramic speaker over traditional Class AB with boost converter methods.

Speaker manufacturers always recommend a fixed resistance (RL) in series with the ceramic speaker. This resistor limits the current out of the amplifier when the signal has a lot of high-frequency content.

In some applications, the fixed resistance may not be needed if the frequency response of the audio passed to the speaker can be bandwidth-limited to ensure that the speaker does not look like a short-circuit to the amplifier. Current ceramic speakers have a capacitance on the order of 1?f. The impedance of the speaker is 20W at 8kHz and 10W at 16kHz. Future ceramic speakers may have a larger capacitance that will force the amplifiers to deliver even more current for the same signal frequency.

Ceramic vs. dynamic apps
Efficiency in a traditional dynamic speaker application is easy to calculate. The voice coil windings can be modeled electrically as a fixed resistance in series with a high-value inductance.

Calculating power delivered to the load is an Ohm's Law problem, using the resistance value of the speaker: P = I2R or P = V * I.

This power is dissipated as heat in the speaker coil.

Figure 1: Applying a voltage above the rated voltage will increase the output signal's distortion (a); when driving a ceramic speaker with a 10W series resistor, "blind" power is a small contribution to the overall load power (b).

Ceramic speakers don't generate very much heat when they dissipate power because of their capacitive nature. The speaker dissipates a so-called blind powera very small amount of power based on the dissipation factor of the ceramic element. Very little heat is generated when blind power is dissipated.

Calculating blind power is not as straightforward as P = V * I. Blind power is calculated as: P = (fCV2) * (cos + Df), where c = capacitance value of the speaker; V = is the rms drive voltage; f = the frequency of the drive voltage; cos = the phase angle between the current through the speaker and the voltage across the speaker; and Df = the dissipation factor of the speaker. This depends on the signal frequency and the ceramic speaker ESR.

Since the phase angle between the voltage and current is 90 in an ideal capacitor, and the ceramic speaker is mostly capacitive, cos is equal to zero, thus resulting in no power dissipation in the capacitive portion of the ceramic speaker. Imperfections in the ceramic material cause the voltage across the speaker to lag the current through the speaker by a phase angle that does not quite equal 90. The difference between the ideal, 90 phase shift and the actual phase shift is the dissipation factor. Df in a ceramic speaker can be modeled as a small resistance, ESR, in series with the ideal capacitor.

Df is the ratio of the ESR to the capacitive reactance at the frequency of interest: Df = RESR / XC

For example, a ceramic speaker with a capacitance of 1.6?f and ESR of 1W being driven by a 5Vrms, 5kHz signal would have a blind power of: P = ( * 5,000 * 1.6c-6 * 52) x (0 + 0.05) or 31.4mW.

Real power dissipation
Thus, although the ceramic speaker itself doesn't dissipate real power as heat, as dynamic speakers do, heat is generated in the output stage of the driving amplifier and in the external resistor (RL) placed between the amp and the speaker (Figure 2).

The larger the external resistor, the more power dissipation is moved off of the amplifier at the expense of low frequency response.

Figure 2: An application circuit uses a Class G amplifier, which offer multiple voltage rails: one high voltage and one low.

When driving a ceramic speaker with a 10W series resistor, one can see that "blind" power is a small contribution to the overall load power. Most of the power is dissipated in the external resistor (Figure 1b).

Better low-frequency response will require a smaller external resistor, but that will cause the output stage of the amplifier to dissipate more power. Amplifier efficiency will dictate how much power will be dissipated in the output stage of the amp. The need to dissipate power in the amplifier drives the need for more efficient solutions including Class D and Class G amplifiers. The load consists of some series resistance, which will lead to some power dissipation in the load network and not the speaker. Even with a 100 percent efficient amplifier, the series resistor will burn power that is intended for the speaker.

In this example, at 5kHz the total power delivered to the load is 515mW. An amplifier with 53 percent efficiency will dissipate 457mW. The amount of power that the amplifier must dissipate will dictate the package size that the application can use. A significant amount of power dissipation will be required if high-frequency sine waves must be driven into the speaker.

In summary, ever-thinner portable devices are driving a need for low-profile ceramic speakers. Such speakers differ from traditional dynamic speakers, so a different set of design considerations applies. The capacitive nature of the ceramic speaker requires the amplifier to have high output-voltage drive and a large output-current capability, so that the high voltage can be maintained over frequency.

An amplifier chosen to drive a ceramic speaker must be able to deliver both blind and real power to the complex load. Amplifier efficiency must be high enough to allow for a small solution size and low cost.

Such demands require the use of amplifier topologies that differ from the traditional Class AB. More efficient solutions such as Class G or Class D amps become more attractive, with Class G offering the best efficiency.

About the author
Mark Cherry
is audio corporate applications engineer of multimedia business unit at Maxim Integrated Products Inc.





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