PWM bridge amplifier with input network configurable for analog or digital input not needing a triangular wave generator

Abstract: A low frequency PWM output bridge amplifier having an input network configurable for standard PWM digital input signals, phase shift PWM digital input signals or analog input signals and for standard PWM output or phase shift PWM output, comprises two identical amplifying modules (1, 2), one (1) for the amplifying channel relative to the direct or positive PWM output (Vo-If+) and the other (2) for the amplifying channel relative to the inverted or negative PWM output (Vo-If+). Each modules includes a switching output operational amplifier (O1), having a voltage mode noninverting input (In+), a current mode inverting input (In-) and a loop filter implementing a single or multiple slope integrator outputting a signal of a substantially triangular waveform, a logic inverter or a cascade of logic inverters (C1) coupled in cascade to the output of the integrator (O1, LOOP FILTER) and outputting a logic PWM signal, an output power stage (P1) converting the logic PWM signal output by said inverter or cascade of logic inverters (C1) in a PWM signal, switching between the potentials of the two supply rails of the circuit, and a feedback resistor (Rf) connecting the output of power stage (P1) to the inverting input (In-) of said operational amplifier (O1).

Description: FIELD OF THE INVENTION The present invention relates to low frequency/high performance amplifiers, commonly referred to as Class-D amplifiers and particularly though not exclusively to audio amplifiers. BACKGROUND OF THE INVENTION The efforts for reducing energy consumption, weight and size of heat dissipators of manufacturers of consumer apparatuses, for example in the field of car entertainment, have generated a demand for power amplifiers with a greater efficiency than Class-AB amplifiers. So-called Class-D amplifiers have been proposed to respond to these requisites. Substantially, these amplifiers include a DC-AC converter circuit which produces a PWM output signal. This PWM signal drives power switches which in turn switch a load which may implement a passive lowpass filter for reconstructing the amplified audio signal, of which the actual load (for example a speaker) may be a part. The analysis of the behavior of a single ended amplifier with an analog input and a PWM output (a Class-D amplifier) is described in the paper "Analysis of a quality Class-D amplifier", F. A. Himmelstoss, et al., I.E.E.E. Transactions on Consumer Electronics, Vol. 42, No. 3, August 1996. On the other hand, the increasing interest in digital audio signal processing, will make more convenient to use digital amplifiers rather than analog amplifiers. Therefore, many manufacturers of audio equipment require Class-D power amplifiers capable of handling analog audio signals and digital audio signals. According to a common technique, to process an analog input signal requires the generation of a reference waveform, commonly a triangular or a saw-tooth signal, whose generation is relatively critical and at any rate implies a certain circuital complexity. An example of a Class-D amplifier capable of selectively handle analog audio signals and digital audio signals, is described in the patent No. US-A-5,672,998. The amplifier uses a circuit for generating a PWM signal from an analog audio signal, separated from the feedback loop of the power amplifier itself. Even in this case, in order to generate a PWM signal, a triangular reference signal, produced by a dedicated generator (24), is employed. Another example of class-D amplifier is disclosed in the US Patent No. 4,949,048 in the name of Pioneer Electronic Corporation. Also this amplifier needs a generator of a triangular reference signal. The document GB 2 096 850 discloses a PWM amplifier having a feedback loop, that becomes effective after a predetermined time from the time when the power source is switched on, so as to prevent undesired oscillations of the output signal. The US patent No. 4,249,136 discloses a signal power amplifier including an input terminal supplied with a PWM signal directly, and a negative feedback circuit to reduce output distortion even if the operating voltage of the pulse amplifier undergoes to relevant fluctuations. OBJECT AND SUMMARY OF THE INVENTION The objective of the invention is to devise a Class-D power amplifier capable of handling analog and/or digital input signals (PWM), without requiring generation of a reference (triangular) wave, according to a traditional analog/PWM conversion scheme, or any selfoscillating structures (such as of the so-called BOSE type). According to an embodiment of the amplifier of the invention, outstandingly simple to adapt to particular user conditions, an input network configurable through a pair of four-position switches, allows for the selection of the input signal, analog or digital (PWM), and a further selection between a PWM-standard digital input or a phase shift PWM digital input, as well as a selection between a standard PWM output or a phase shift PWM output. Of course, the configuration of the appropriate input network may be carried out in a permanent manner or even by the use of external networks without employing any selection switch. Even according to these alternative embodiments, the core of the amplifier of the invention remains the same. The advantage of a phase shift PWM modulation, wherein the inverted signal is shifted by a half clock period in respect of the noninverted signal, is that the differential output carrier frequency of the PWM power amplifier is twice the clock frequency which is therefore more easily filtered. Moreover, under null modulation conditions, ideally there are no residues of the carrier signal on the load terminals. Basically, the amplifier of the invention is composed of two identical amplifying modules, one amplifying channel for the direct (noninverted) PWM signal and the other for the inverted PWM signal. Each module comprises: a switching output operational amplifier, having a first noninverting voltage input, a second inverting current input and a loop filter forming with the amplifier a single slope or a two-slope integrator; a logic inverter (or cascade of logic inverters) coupled in cascade to the output of the integrator; an output power stage converting the logic level PWM signal output by said inverter or cascade of logic inverters to an analog signal switching between the potentials of the two supply rails; and a negative feedback resistor coupled between the output of the power stage and the inverting input of the operational amplifier. BRIEF DESCRIPTION OF THE DRAWINGS The various aspects and advantages of the invention will become even more evident through the following description of several embodiments and by referring to the annexed drawings, wherein: Figure 1 is a functional diagram of each of the two identical amplifying modules that comprise the amplifier of the invention, according to a first embodiment; Figure 2 is a functional diagram of each of the two identical amplifying modules that compose the amplifier of the invention, according to an alternative embodiment; Figure 3 shows the two alternative forms, respectively A and B, of the loop filter of the input integrator of each of the two modules that compose the amplifier of the invention; Figure 4 shows the configuration of the input network for an analog input signal and a standard PWM output; Figure 5 shows the configuration of the input network for an analog input signal and a phase shift PWM output; Figure 6 shows the configuration of the input network for an analog input signal and phase shift PWM output, according to an alternative embodiment; Figure 7 shows the configuration of the input network for a standard PWM digital input signal and a standard PWM output; Figure 8 shows the configuration of the input network for a phase shift PWM digital input signal and a phase shift PWM output; Figure 9 shows the architecture for selecting the desired input network configuration of the amplifier of the invention; Figure 10 shows an alternative embodiment for the case of a digital input signal (Vin_d) using two current generators switched by switches driven by the digital input signal in substitution of the Rq resistors of the input network; Figure A1 illustrates the significative signals in the case of a standard PWM bridge output configuration; Figure A2 illustrates the significant signals in the case of a phase shift PWM bridge output configuration. DESCRIPTION OF SEVERAL EMBODIMENTS OF THE INVENTION Fig. 1 shows the functional diagram of one of the two identical amplifying modules 1 and 2 that compose the amplifier of the invention, as will be better explained later in this description. Each of the two modules comprises an operational amplifier O1 having a lowpass loop filter thus implementing an integrating stage. The noninverting input (In+) is fed in a voltage mode while the inverting input of the amplifier (In-) is fed in a current mode. As shown in Fig. 3, the lowpass filter may be constituted by a single integrating capacitor CI connected between the output of the operational amplifier and its inverting input In-, thus implementing a single slope integrator, as depicted in the diagram A of Fig. 3. Alternatively, the lowpass filter may be a passive network constituted by two capacitors CI1 and CI2 and a resistor RI connected between the intermediate node between the two capacitors in series and the circuit's ground, as shown in the diagram B of Fig. 3, thus implementing a two-slopes integrator. Use of a multiple slope integrator allows for higher loop gains and therefore for a lower harmonic distortion. The Rf resistor (Fig. 1) represents a negative feedback resistor while the current generator Io, generating a current whose value depends on the value of the supply voltage, allows for a maximum voltage excursion of the undistorted signal on the filtered outputs (that is, downstream of the analog signal reconstructing filter) of the PWM amplifier. By referring the diagram of the basic module of Fig. 1, on the node Tri, indicated with an arrow, that is, at the output of the integrator, there exist a signal of an approximately triangular waveform the slopes of which depend on the input current, Iin, on the feedback current, Irf, and on the type of loop filter employed. This triangular waveform is compared by the logic block C 1 which, as mentioned above, may be a simple NOT logic gate or an inverter or a cascade of them, at the output of which a logic PWM signal is obtained, as indicated by the arrow in the diagram of Fig. 1. An alternative embodiment of the two basic amplifying modules that compose the amplifier of the invention is shown in Fig. 2. The sole difference in respect to the embodiment of Fig. 1 is represented by the use of a resistive divider, Ra-Rb, connected between the output node of the module and the ground node of the circuit and to the intermediate node of which is connected the feedback resistor Rf. This alternative configuration is particularly useful in case of an embodiment of the amplifier of the invention in the form of a monolithic integrated circuit, because it allows a reduction by a factor equal to the partition ratio given by the respective values of Ra and Rb of the divider, the value of the feedback resistor Rf, thus a consequent reduction of the silicon area required for integrating it. Fig. 4 shows a complete scheme of the amplifier of the invention configured for the case of an analog input signal and a standard PWM output. Of course, each of the two amplifying modules 1 and 2 that compose the amplifier of the invention has an identical circuit diagram that may be that of Fig. 1 or Fig. 2. A square wave clock Vq is fed via the resistor Rq1 to the inverting input In- of the module 1, relative to the positive PWM output Vo-If+, while the inverted square wave clock output by the inverter I is fed to the inverting input In- of the second module 2, relative to the negative PWM output Vo-If-. Vref is a fixed reference voltage, for example of 2.5V, while the analog audio signal to be amplified is fed to the input Vin_a. The partition given by the input resistors Rin1 and Rin2, which feed the input signal to the noninverting channel, takes into consideration the difference of gain between the inverting channel and the noninverting channel, in order to ensure that the two channels saturate simultaneously. The same result could be obtained even by making Rin1=0 and by adequately modifying the value of the feedback resistor Rf of the two amplifying modules 1 and 2. By eliminating the logic inverter I of the square wave clock from the diagram of Fig. 4, an input network configuration of the amplifier as depicted in Fig. 5 is obtained, determining a phase-shift PWM output instead of a standard one (as will be described below). The considerations relative to match the gains of the two channels remain valid. Alternatively, a similar result of producing a phase shift PWM output may be achieved even with a different configuration of the input network, as shown in Fig. 6. According to this alternative embodiment, the amplifier of the invention provides for two identically inverting signal paths, thereby having the same gain provided that Rf=Rin . The inversion necessary to obtain a low frequency signal in phase opposition to the two amplifier's outputs (Vo-If+ and Vo-If-) is obtained by way of a unit gain inverting amplifier (-1) functionally introduced at the input of the channel relative to the positive output Vo-If+. Fig. 7 shows the configuration scheme of the input network of the amplifier of the invention for the case of a standard PWM digital input signal and a standard PWM output. By comparison, it may be seen how the circuit is identical to the circuit of Fig. 4, with the exception of the short circuiting of input Vin_a to the fixed reference voltage Vref. The square waveform of the input digital PWM signal is transferred to the output with the same frequency but with a changed duty cycle, depending on the difference between the logic voltage (for example 0-5V) and the supply voltage 0-Vcc. Ideally, the output duty cycle is approximately equal to that of the input in absence of a modulating signal (duty cycle = 50%). Fig. 8 shows an alternative diagram of the input network of the amplifier of the invention for the case of a phase shift PWM digital input and a phase shift PWM output. The circuit has the same structure as that of Fig. 5 and may be obtained by simply connecting the node Vin_a to the node Vref, and by applying the differential phase shift PWM digital signal Vin_PWM- and Vin_PWM+, separately via the input resistors Rq1 and Rq2, to the respective inverting inputs in the two amplifying modules 1 and 2. Fig. 9 shows a possible architecture of an input network for the amplifier of the invention, configurable by a simple selection made by means of switches for the three different types of input signals, and for the two different types of output signals: standard PWM and phase shift PWM. According to the embodiment of Fig. 9, each possible combination of the type of the input signal and of the type of PWM output signal is selectable by means of a pair of electronic four-position switches S1, S2. The so configurable amplifier of the invention has four input nodes A, B, C, and D to which input signals as specified in the scheme of Fig. 9 may be applied. The side table indicates, for each position of the switches S1 and S2, the selected circuit configuration for the respective types of input signal and/or of PWM output signal. According to an alternative embodiment, each of the resistors Rq1 and Rq2 of the input network (diagram A of Fig. 10) may be substituted by a pair of current generators switched by switches driven by the digital input signal, one directly and the other one via an inverter (diagram B of Fig. 10). The differences between a standard PWM bridge output and a phase shift PWM bridge output are more readily observed by comparing the waveforms of the signals shown in Fig. A1 for a standard PWM output and in Fig. A2 for a phase shift PWM output and wherein a differential output Vo is taken downstream of a reconstructing filter of the analog audio signal. For a standard PWM output, the PWM waveform present on the two Vo-If+ and Vo-If- outputs of the amplifier is such to present only one phase inversion per clock period. By contrast, for a phase shift PWM output, the inverted or negative output Vo-If-, besides been inverted is also shifted by half clock period in respect to the direct or positive Vo-If+ output. By considering a differential output, the frequency is twice the clock frequency. Therefore, the PWM carrier may be filtered more easily. Moreover, during null modulation conditions there are no carrier residues on the load terminal (in the ideal case).

Claims: A low frequency PWM output bridge amplifier having an input network capable of processing in a selectable manner in operation of the amplifier different input formats among standard PWM digital input signals, phase shift PWM digital input signals or analog input signals and output formats between standard PWM output or phase shift PWM output, having an input node (A) destined to received a square waveform clock signal in case of an analog input signal (Vin_a), comprising two identical amplifying modules (1, 2), one (1) for the amplifying channel relative to the direct or positive PWM output (Vo-If+) and the other (2) for the amplifying channel relative to the inverted or negative PWM output (Vo-If+), each having a switching output operational amplifier (O1), having a noninverting input (In+), an inverting input (In-) and a loop filter implementing an integrator, circuit means coupled in cascade to the output of the integrator (O1, LOOP FILTER) and outputting a logic PWM signal, an output power stage (P1) converting the logic PWM signal output by an inverter or cascade of logic inverters (C1) in a PWM signal, switching between the potentials of the two supply rails of the circuit, a feedback resistor (Rf) connecting the output of power stage (P1) to the inverting input (In-) of said operational amplifier (O1), said integrator being a single or multiple slope integrator that output a signal of said substantially triangular waveform, and said circuit means consisting in a logic inverter or said cascade of logic inverters (C1). The amplifier according to claim 1, characterized in that said current mode inverting input (In-) of the operational amplifier (O1) of each module (1, 2) is biased by a current generator (Io) connected between said inverting input and the circuit's ground node, generating a current in function of a supply voltage of the circuit. The amplifier according to claim 1, characterized in that said feedback resistor (Rf) is connected to the intermediate node of a resistive divider (Ra, Rb) of the output voltage of said power stage (P1). The amplifier according to claim 1, characterized in that said configurable input network comprises a pair of identical four-position switches first (S1) and second (S2), the first switch (S1) coupling alternatively the first (A) or a second input node of the amplifier to the inverting input (IN-) of one (2) of said amplifying modules (1, 2), the second switch (S2) coupling alternatively a third (C) or a fourth (D) input node of the amplifier to the inverting input (In-) of said amplifying module (2), the first input node (A) being destined to received either a square waveform clock signal in case of an analog input signal (Vin_a) fed to said fourth input node (D) of the amplifier and of a constant reference voltage (Vref) applied to said third input node (C) of the amplifier, or a PWM input signal (Vin_PWM, Vin_PWM+) in case of a standard digital PWM input signal (Vin_PWM) and in case of a phase shift (Vin_PWM+), an inverted PWM digital signal (Vin_PVM-) being fed to said second input node (B) of the amplifier; in all said cases of a digital input signal said fourth input node (D) of the amplifier being connected in common with said third input node (C) to said reference voltage (Vref).

Inventors: BOTTI EDOARDO, GROSSO ANTONIO, MASINI MARCO, BOTTI, EDOARDO, GROSSO, ANTONIO, MASINI, MARCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H03F3
IPC codes: H03F3