Abstract:
The invention is directed to an amplifier circuit based on the principle of a class D amplifier. To avoid unwanted convolution effects and to improve the power supply rejection ratio, provision is made for the amplitude of the ramp signal used for pulse width modulation to track proportionally the supply voltage for the amplifier circuit. For this purpose, the ramp signal generator has an amplitude control input suitably connected to supply and reference potentials. This ensures a constant duty ratio which is independent of the supply voltage. The present circuit may be used, for example, as a DC/DC converter or as an audio amplifier.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of PCT/DE03/02304, which was not published in English, that claims the benefit of the priority date of German Patent Application No. DE 102 31 183.8, filed on Jul. 10, 2002, the contents of which are herein incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to an amplifier circuit.  
       BACKGROUND OF THE INVENTION  
       [0003]     Audio signals are normally amplified using class D amplifiers, inter alia. In line with the underlying principle, a reference signal is first compared with a signal fed back from the output and a corresponding error signal is output. This signal is processed on a pulse width modulation basis using a sawtooth signal and is passed to an output amplifier stage. In this case, the output stage is operated so as to switch at a particular duty ratio. To maintain a flow of current at the output, a freewheeling diode and an inductor are provided at the output. It is thus possible to provide a constant output current at the output.  
         [0004]      FIG. 1  in the “Journal of the Audio Engineering Society”, Audio Engineering Society, New York, USA, vol. 39, No. 9, Sep. 1, 1991, pages 650, 662 shows an amplifier circuit with a differential amplifier whose output side is connected to a first input on a comparator. A second input on the comparator is supplied with a sawtooth-waveform signal. The output side of the comparator is connected to an output circuit which is pulse width modulated.  
         [0005]     Printed document EP 0503571 A1 likewise shows a pulse width modulated amplifier circuit whose signal input is connected to a comparator. A second input on the comparator is supplied with a sawtooth-waveform signal. The output of the comparator is connected to an output stage.  
         [0006]     However, such amplifiers have relatively poor properties in terms of the power supply rejection ratio (PSRR). If the low-level signal response of a circuit of this type is considered, the ratio of the output voltage to the input voltage is  
           Vout   Vin     =     d     1   +       s   2     ⁢   LC           ,       
 
 where L and C are the values of an LC filter at the output and d is the duty ratio of the pulse width modulation. The gain of the transfer function Vout/Vin is accordingly proportional to the duty ratio d, which may be between 0 and 1. However, this term determines the denominator of the formula for describing the power supply rejection ratio. Accordingly, fluctuations in the supply voltage or radio-frequency interference components in the supply voltage result in relatively severe unwanted effects on the output signal from the amplifier circuit. 
 
         [0007]     A further problem of the principle described is the unwanted convolution of signals. If the supply voltage for the output stage behaves like a relatively low-frequency sinusoidal oscillation but the useful signal is likewise a (higher-frequency) sinusoidal oscillation, then the low-level signal gain also varies sinusoidally. The resultant harmonics are at the summed frequency and the differential frequency between the frequencies of the two signals at an amplitude which corresponds to half of the product of the amplitudes of the two signals. The problem is of great significance particularly because the interference may be at frequencies below the cut-off frequency of the low-pass filter at the output and is therefore not filtered out.  
         [0008]     The relatively poor power supply rejection ratio is normally countered by increasing the signal gain. This increases the power consumption, however.  
         [0009]     The convolution problems described may be reduced by reducing the noise components and interference components on the supply voltage, for example by using a linear controller. This severely reduces the efficiency of the amplifier, however.  
       SUMMARY OF THE INVENTION  
       [0010]     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.  
         [0011]     The present invention is directed to an amplifier circuit which operates on the basis of the class D principle and has an improved power supply rejection ratio.  
         [0012]     In one embodiment of the invention, the amplifier circuit comprises an input for supplying a useful signal which is to be amplified, and an output for tapping off an amplified useful signal. The circuit further comprises a differential amplifier having a first input, which is connected to the input of the amplifier circuit, a second input, which is coupled to the output of the amplifier circuit, and an output. A comparator is also provided having a first input, which is connected to the output of the differential amplifier, and a second input, to which a periodic signal is supplied, along with an output stage having an input, which is connected to an output of the comparator, an output, and having a supply connection for supplying a supply voltage thereto. The circuit also comprises a signal generator which is connected to the second input of the comparator and provides the periodic signal at an amplitude which is proportional to the supply voltage for the output stage. The signal generator is configured in a phase locked loop to regulate the frequency of the periodic signal on the basis of a reference clock.  
         [0013]     In accordance with the present invention, the periodic signal used for pulse width modulation is provided such that its amplitude is always proportional to the supply voltage for the amplifier circuit, particularly for the output stage of the amplifier circuit.  
         [0014]     In accordance with another exemplary embodiment of the invention, the duty ratio is always proportional to the supply voltage.  
         [0015]     In accordance with yet another embodiment of the invention, the transfer function for the output voltage in relation to the supply voltage in consideration of the low-level signal response is ideally 0, in practical terms at least very low, which means that the power supply rejection ratio PSRR is greatly improved.  
         [0016]     The quotient of the supply voltage and the amplitude voltage of the periodic signal is always constant in one example of the present invention. However, this quotient also describes the low-level signal gain, in particular, of the circuit, and the circuit described accordingly always operates at constant gain such that an additional advantage is that no convolution with a harmonic signal component of the supply voltage may arise.  
         [0017]     If the periodic signal, which, in accordance with the present invention, is used for pulse width modulation, is a ramp signal, a triangular-waveform signal or a sawtooth signal, for example, then the proportionality of this signal to the supply voltage relates to the fact that the peak-to-peak voltage of the ramp signal is proportional to the supply voltage.  
         [0018]     The proportionality of the periodic signal to the supply voltage is produced, in one example, using an operational amplifier whose input side is connected to the supply voltage and whose output side is coupled to the signal generator in order to feed it.  
         [0019]     So that the frequency of the periodic signal always remains constant despite the amplitude of the signal being linked to a possibly fluctuating supply voltage, there is advantageously provided a phase locked loop which ensures that the signal generator&#39;s frequency is constant.  
         [0020]     The amplifier circuit described in one example is preferably of symmetrical design with two output stages which are each preferably in the form of inverters. The output nodes of the inverters are each preferably coupled to the output terminals of the output of the amplifier circuit via a series inductor. A stabilization capacitance is preferably connected between the two output terminals.  
         [0021]     In one alternative, a digital implementation of the present invention is contemplated, wherein provision is made for the duty ratio always to be set proportionally to the supply voltage.  
         [0022]     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The invention is explained below using an exemplary embodiment with reference to the figure.  
         [0024]      FIG. 1  is a block diagram illustrating an amplifier circuit according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 1  shows an amplifier circuit based on the present invention using a block diagram. This circuit comprises an input  1  for supplying a useful signal to be amplified which is in the form of a symmetrical input with a pair of differential input terminals. The signal amplified using the present amplifier circuit, which signal is derived from the signal applied to the input  1 , can be tapped off at the output  2 , which is likewise symmetrically in the form of a pair of output terminals.  
         [0026]     The input  1  has an amplifier  3  connected to it which forms a differential signal from the signal difference between the signal applied to the input  1  and the signal provided at the output  2 . This differential signal is provided at the symmetrical input of the operational amplifier  3 . For this, the pair of output terminals  2  is connected to the input of the amplifier  3  via a respective resistor  4 ,  5 . In addition, the two output terminals of the operational amplifier  3  are connected to the two inputs of the amplifier  3  in an inverting feedback loop via a respective capacitor  6 ,  7 . In addition to the differential input and the differential output, the amplifier  3  also has a common-mode input for supplying a common-mode level V cm . A common-mode signal may be supplied to this common-mode input.  
         [0027]     Connected to one of the two output terminals of the differential amplifier  3  is the positive input of a comparator  8 , whose negative input is connected to a signal generator  9 . The output of the comparator  8  designed for pulse width modulation PWM is connected to a first output driver  10 l directly and to a second output driver  11  via an inverter  12 . The output drivers  10 ,  11  are respectively connected to the control inputs of a CMOS inverter  13 ,  14 . The CMOS inverters  13 ,  14  each comprise a load-side series circuit comprising a p-channel MOS transistor and an n-channel MOS transistor which are connected between a supply potential connection  15  and a reference potential connection  16 , as is usual for CMOS inverters. The outputs of the CMOS inverters  13 ,  14 , which form the output stages of the present amplifier, each have their output node at the connecting node for the MOS transistors. These output nodes are connected to the output terminals  2  via a respective series inductor  17 ,  18 . Connected between the output terminals  2  is a stabilization capacitance  19  which forms an LC filter together with the series inductors  17 ,  18 .  
         [0028]     The signal generator  9  is in the form of a triangular-waveform signal generator which provides a periodic signal having a peak-to-peak voltage V max -V min  at an output  20 , said voltage being obtained from the difference between an upper peak value V max  and a lower peak valve V min .  
         [0029]     The frequency (conditioned as a digital clock signal) of the periodic signal provided by the signal generator is provided at a digital clock output  21  which is connected to a first input on a phase detector  22 . A further input on the phase detector  22  is connected to a reference clock source  23 . The output of the phase detector  22 , which provides any phase error between the two input signals, is routed via a voltage/current converter  24  to the quiescent current input  25  of the signal generator  9 .  
         [0030]     The amplitude V max -V min  is set at a symmetrically designed amplitude control input  26  on the signal generator  9 . Connected to this input is the differential output of a further operational amplifier  27 . The negative input of the operational amplifier  27  is connected to reference potential  16  via a resistor  28 . The positive input of the operational amplifier  27  is connected to supply potential connection  15  via a further resistor  29 . In this case, the supply and reference potential connections  15 ,  16  match the supply voltage connections  15 ,  16  of the output stages  13 ,  14  in the amplifier. The operational amplifier  27  likewise has a common-mode input which is connected to the common-mode input of the operational amplifier  3  for the purpose of supplying the common-mode signal V cm . The operational amplifier  27  has a negative feedback loop from the differential output to the differential input via a respective resistor  30 ,  31 .  
         [0031]     It can clearly be seen that the peak-to-peak voltage V max -V min  of the ramp signal provided by the signal generator  9  is proportional to the supply voltage for the amplifier. The bias current of the signal generator  9  is controlled by a phase locked loop in order to keep the frequency of the periodic signal constant while the peak-to-peak voltage V max -V min  changes. When designing the circuit, it is important to remember that the bandwidths of the two control loops, namely that of the amplifier and that of the phase controller, are greater than the cut-off frequency of the low-pass filter.  
         [0032]     The present amplifier circuit significantly improves the linearity of the output stage. A further improvement could be attained, by way of example, by increasing the bandwidth of the open loop gain. With the feed-forward technology presented, there is no need for error compensation for any interference components on the supply voltage.  
         [0033]     As already explained, the principle described, namely the setting of the peak-to-peak voltage of the periodic signal proportionally to the supply voltage, allows a significant improvement in the power supply rejection ratio and at the same time makes it possible to avoid unwanted convolution effects to a large extent. In this case, the principle described can be implemented using particularly simple circuit means and with little complexity.  
         [0034]     Instead of the illustrated analog actuation of the reference signal generator  9  with the aim of keeping the duty ratio constant regardless of the supply voltage, a digital implementation may also be provided. For this, the supply voltage would be converted into a digital voltage signal using an analog/digital converter, and the duty ratio of the pulse width modulation would be tracked proportionally to the supply voltage.  
         [0035]     While the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”; “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.