In many applications and primarily in audio reproduction systems such as car radios, hi-fi audio systems and similar apparatuses that are intrinsically compact because of stringent installation requirements, as well as in portable apparatuses, power dissipation in the final power stages is often quadrupled to drive a pair of loudspeakers (front and rear) for each stereo channel. This may create heat balance problems. For example, four 20 W amplifiers may have a power dissipation of about 4×12=48 W, and because of the limited space available in certain apparatuses, such a relatively large power may be difficult to dissipate without a significant increase of temperature within the apparatus.
Relatively high temperatures of operation may degrade the magnetic tape of cassettes or optical disks (CD), the drives of which are often tightly fitted inside a single apparatus case. D-type switching amplifiers are highly efficient, and are considered the most appropriate type for these applications.
Unfortunately, switching amplifiers generate electromagnetic emissions (EMI) that in compact apparatuses interfere with the correct functioning of other systems, thus reducing their performance. For these reasons, audio signals are frequently amplified using a pair of class AB power amplifiers. The pair of class AB power amplifiers operate in a single-ended mode or in a bridge configuration depending on the level of the processed signal.
In fact, class AB power amplifiers are less efficient than switching amplifiers and a common technique for reducing power consumption of class AB amplifiers includes configuring them in a single-ended configuration instead of in a bridge configuration whenever it is possible to do so. In fact, these amplifiers dissipate more power in the bridge configuration than in the single-ended configuration as long as the level of the output signal remains smaller than the positive supply voltage. Unfortunately, it is not possible to use single-ended class AB amplifiers if the output surpasses this voltage because the output signal would be severely distorted by clipping.
Techniques for automatically switching from one configuration to the other as a function of the monitored level of the signal are disclosed in the following patents: U.S. Pat. Nos. 5,194,821; 5,365,188 and 5,654,688. These patents are incorporated herein by reference in their entirety, and are assigned to the current assignee of the present invention.
U.S. Pat. No. 5,194,821 discloses a bridge amplifier using positive and negative supply voltages that may function in a single-ended or in a differential or bridge output configuration depending on the level of the output signal. A comparator changes the output circuit configuration of the amplifier from a bridge configuration to a single-ended configuration or vice-versa by closing or opening configuring switches when the output signal becomes smaller than or greater than a certain threshold voltage.
U.S. Pat. Nos. 5,365,188 and 5,654,688 disclose a single supply dual bridge power amplifier. As depicted in FIGS. 1 and 2, each amplifier has a window comparator for sensing the level of input signals fed to the amplifier and driving the switches that coordinately configure the amplifier in either a bridge or in a single-ended configuration.
The switching from a single-ended to a bridge output configuration and vice-versa may cause distortions and EMI disturbances in view of the fact that, when functioning with relatively low signal levels, one of the operational amplifiers of the bridge output structure is configured to operate as a buffer. The buffer outputs a constant reference voltage that usually is equal to half the supply voltage when the amplifier is to function as a second operational amplifier for driving the load in a bridge mode configuration following an increase of the signal level. This operational amplifier should rapidly assume a different output voltage, that is, its output voltage undergoes a step variation.
A known approach to reduce this step variation of the output voltage of the operational amplifier that is configured to function as a buffer when switching to a single-ended configuration is disclosed in U.S. Pat. No. 5,654,688, and is based on the use of a common mode control loop employing a sample-and-hold circuit. Although this approach is satisfactory when the signals in the different channels of the amplifier are substantially correlated among each other, the effectiveness in reducing the output step variations upon changing the configuration and the above-mentioned consequences diminishes significantly if the signals in the channels of the amplifier functioning in a bridge mode are no longer correlated.
This phenomenon has become evident in investigating the reasons why in modern car audio systems the distortion would inexplicably increase under certain circumstances. It has been found that increased EMI and increased distortions occur when the correlation between the signals that are fed to the four channels of the audio system diminishes due to different settings of independent channel equalization controls that are customarily provided in quality car audio systems.
The independent equalization setting on the various channels cause different delays of propagation of signals through the channels, which thus become substantially uncorrelated. To better understand the problem to be addressed, reference is made to FIG. 3 that depicts sample diagrams of differential pairs of signals OUT_F+, OUT_F− and OUT_R+, OUT_R− output by the left OPA+F, OPA−F and right OPA+R, OPA−R channels. The signals are referred to a fixed voltage, which is generally half the supply voltage Vcc and are substantially uncorrelated.
When both outputs (OUT_F+)-(OUT_F−) and (OUT_R+)-(OUT_R−) are within the comparison window defined by upper and lower voltage levels VREF+ and VREF−, respectively, the power switch SW_C is on and the switch SW_F is off. Thus, both channels are single-ended. The opposite happens otherwise, and both channels are bridge configured.
The common mode voltages of the left and right channels, respectively, are sampled when the amplifier switches to the bridge configuration and are held as long as the amplifier switches to a single-ended configuration. When the input signals are substantially uncorrelated, it is very likely that the output signals undergo step variations when the amplifier switches from a bridge to a single-end configuration (instants t2 and t4).
These step variations are potentially dangerous because they may cause even an undue intervention of the protection circuit from short-circuits. In fact, both the amplifier OPA+R that is to switch in the bridge configuration and the amplifier OPA−F that is to be disconnected may output signals with an amplitude far larger than the fixed voltage VCC/2. Therefore, when the switch SW_C turns on, the outputs of both channels OPA+R and OPA−F must quickly reach the fixed voltage VCC/2, generating a step variation that produces EMI and distortions.
This step variation propagates also to the other channels OPA−R and OPA+F through the feedback lines. Even the power switch SW_C may turn on when it withstands is a high differential voltage, thus generating a cross-conduction current that is likely to cause the intervention of the protection circuit from short-circuits.
A multi-channel power amplifier capable of reducing EMI and distortions is disclosed in European patent application no. 03425357.5 which is assigned to the current assignee of the present invention. Its design ensures that one of the two operational amplifiers of the output bridge structure of each channel be kept at the design reference voltage (typically half of the supply voltage) for as long as the other operational amplifier of the output bridge structure does not begin to saturate. This is done by connecting in common the gate nodes of the P-type MOS transistors and the gate nodes of the N-type MOS transistors of the output half bridge stages of all the operational amplifiers of the output bridge pairs of all the channels that are eventually configured to function as a voltage reference buffer. This is when the multi-channel amplifier is configured to function in a single-ended configuration.
By connecting in common the gate nodes of the output transistor pairs of all the buffer configurable operational amplifiers of the output bridge structures of each channel, they are effectively prevented from delivering differential mode currents to the loads but exclusively common mode currents (that is, currents of equal sign) make them operate as a single buffer.
Of course, the switches that connect in common the gate nodes of the power transistors of the same type are controlled in phase with the switches that configure the multi-channel amplifier to function in a single-ended configuration, referred to a constant reference voltage.