Self-oscillating amplifier system

Abstract

A new and improved self-oscillating amplifier system is presented, suitable for use in high fidelity audio applications. The self-oscillating amplifier system comprises a feedback path and a forward path including a pulse modulator, a switching power amplification stage and a demodulation filter. The forward path further includes a pair of parallel forward filters preceding the pulse modulators, a differentiating forward filter and an integrating forward filter. The differentiating forward filter is utilized for controlling a switching frequency of the system while the integrating forward filter is utilized for controlling the behavior of the amplifier system within an operating frequency band (e.g. audio band). The self-oscillating amplifier system exhibits improved performance in terms of open loop gain, reduced phase turn and improved robustness as compared to other previously known self-oscillating amplifier systems.

Claims

1. A self-oscillating amplifier system having an input for receiving an input signal and an output, said system comprising: a forward path from the input to the output, said forward path comprising: a pulse modulator configured to modulate the input signal and form a pulse modulated signal; a switching power amplification stage configured to receive the pulse modulated signal, amplify the pulse modulated signal, and form a switching output signal; a demodulation filter configured to demodulated the switching output signal and form a demodulated signal at the output; a first forward branch preceding the pulse modulator, the first forward branch comprising a differentiating forward filter for controlling a switching frequency; a feedback path from the output of the system to the input of the system, said feed path comprising a phase lead network for compensating at least a portion of a phase lag of the demodulation filter; and wherein the forward path further includes a second forward branch connected in parallel with the first forward branch, said second forward branch comprising an integrating forward filter.

2. The self-oscillating amplifier system according to claim 1, wherein the integrating forward filter is configured to form an integrated signal, and wherein the second forward branch further comprises an amplitude limiting circuitry configured to limit the amplitude of the integrated signal.

3. The self-oscillating amplifier system according to claim 2, wherein said amplitude limiting circuitry is configured to limit the amplitude of the integrated signal to be within a predefined amplitude range.

4. The self-oscillating amplifier system according to claim 2, wherein said amplitude limiting circuitry is connected to the input of the system and is configured to: determine a difference between the integrated signal and the input signal of the system; compare said difference to a predefined threshold value; and act on an input of the integrating forward filter in order to limit the amplitude of the integrated signal when said difference exceeds the predefined threshold value.

5. The self-oscillating amplifier system according to claim 4, wherein said integrating forward filter is an active integrating circuit comprising an operational amplifier, and wherein said amplitude limiting circuitry is configured to limit the integrated signal by generating a control signal at the inverting input of the operational amplifier.

6. The self-oscillating amplifier system according to claim 1, wherein the phase lead network of the feedback filter is a first phase lead network, and wherein said first forward branch further comprises a second phase lead network for compensating at least a portion of the phase lag of the demodulation filter.

7. The self-oscillating amplifier system according to claim 1, wherein the integrating forward filter is an active filter.

8. The self-oscillating amplifier system according to claim 1, wherein the integrating forward filter is a higher order filter.

9. The self-oscillating amplifier system according to claim 1, further comprising a low pass filter preceding the forward path, outside the feedback loop.

10. The self-oscillating amplifier system according to claim 1, wherein said pulse modulator is a comparator stage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For exemplifying purposes, the invention will be described in closer detail in the following with reference to embodiments thereof illustrated in the attached drawings, wherein:

(2) FIG. 1 is a schematic block diagram representation of a self-oscillating amplifier system in accordance with an embodiment of the present invention;

(3) FIG. 2 is a graph illustrating a comparison of the closed loop amplitude and phase characteristics between a self-oscillating amplifier system according to an embodiment of the present invention and of a conventional prior art self-oscillating amplifier system;

(4) FIG. 3 is a schematic block diagram representation of a self-oscillating amplifier system in accordance with another embodiment of the present invention;

(5) FIG. 4 is a schematic block diagram representation of a self-oscillating amplifier system in accordance with yet another embodiment of the present invention;

(6) FIG. 5 is a schematic block diagram representation of a self-oscillating amplifier system in accordance with yet another embodiment of the present invention;

(7) FIG. 6 is a schematic circuit representation of the self-oscillating amplifier system in accordance with an embodiment of the present invention; and

(8) FIG. 7 is a schematic circuit representation of the self-oscillating amplifier system in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION

(9) In the following detailed description, preferred embodiments of the present invention will be described. However, it is to be understood that features of the different embodiments are exchangeable between the embodiments and may be combined in different ways, unless anything else is specifically indicated. Even though in the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known constructions or functions are not described in detail, so as not to obscure the present invention.

(10) FIG. 1 illustrates a block diagram representation of a self-oscillating amplifier system 1 according to an embodiment of the invention. The amplifier system 1 has an input 2 for receiving an input signal (V.sub.in) and an output 3. A load in the form of a speaker is connected to the output 3 of the amplifier system 1. The amplifier system has a forward path 4, extending from the input 2 of the amplifier system 1 to the output 3 of the amplifier system 1. The forward path 4 has at least two parallel forward branches 31, 32 preceding the pulse modulator 5 which in turn is configured to modulate the (filtered) input signal in order to generate or form a pulse modulated signal (e.g. PWM signal). The first forward branch 31 includes a differentiating forward filter 8, while the second forward branch 32 includes an integrating forward filter 11. Thus, the first forward branch 31 may to some extent be construed as a differentiating forward branch/path while the second forward branch 32 may be construed as an integrating forward branch/path. The pulse modulator 5 may for example be realized as a comparator stage. The pulse modulated signal is fed to a switching power stage 6 which is configured to receive the pulse modulated signal, amplify the pulse modulated signal, and form a switching output signal. The pulse modulator 5 and switching power stage 6 may however be construed as a single power switch and comparator stage, as readily understood by the skilled artisan.

(11) Further, the forward path has a demodulation filter 7, preferably realized as a passive higher order filter, such as a second order LC filter, configured to receive the switching output signal and to demodulate the switching output signal in order to generate or form a demodulated signal at the output 3 of the amplifier system 1. The amplifier system 1 further has a feedback path 9 comprising a phase lead network 10 which is utilized for compensating at least a portion of a phase lag introduced by the demodulation filter 7. The feedback path 9 extends from the system output 3 to the system input 2, where the feedback signal is combined with the system input signal (may also be referred to as a reference signal) in a summing box/junction 15. The summing box 15 is preferably realized as an error amplifier which subtracts the feedback signal from the input signal and amplifies the difference, as known in the art. The feedback path 9 (may also be referred to as a global feedback) can be voltage controlled, as described in WO 98/19391 or current controlled as described in WO 02/25357, both documents hereby incorporated by reference. The feedback path may further include an optional feedback filter 12 in order to suppress errors in the switching power stage 6 and the demodulation filter 7. However, the feedback filter 12 may also be reduced to unity gain.

(12) In this embodiment illustrated in FIG. 1, the parallel forward filters 8, 11 consists of a first forward filter 8, which is a differentiating forward filter 8 used for controlling the switching frequency of the amplifier system, and a second forward filter 11 which is an integrating forward filter 11. The differentiating forward filter 8 may be such that implementation is possible without operational amplifiers. However, other realizations where the differentiating forward filter 8 comprises a compensation block with a more complex transfer function, are feasible. The integrating forward filter 11 may for example be an active filter comprising active components. Also, the integrating forward filter 11 may be a higher order filter, such as a second order filter. By providing a higher order integrating forward filter one can achieve higher gain within the operational frequency band and larger attenuation outside the operational frequency band, as compared to first order integrating forward filters.

(13) Advantageously, the first forward branch 31 further comprises a second phase lead network (see e.g. ref. 14 in FIG. 5 or 6) which together with the first phase lead network 10 compensates at least a portion of the phase lag of the demodulation filter 7. Stated differently, the two phase lead networks are used to control the switching frequency of the amplifier system by introducing a lead-zero in both the feedback path 9 and in the forward path 4 in order to compensate for the poles of the demodulation filter 7. Conventionally, only the feedback path 9 would comprise a phase lead network, however, by splitting the phase lead network into two, the overall performance of the amplifier system 1 is improved. More specifically, it provides for improved closed loop characteristics with reduced phase turn and relatively constant gain across the operational frequency band (e.g. audio band), i.e. larger small signal bandwidth.

(14) Further, by arranging the two forward filters 8, 11 in parallel it is possible to shut down or stop the integrating forward filter 11 during abnormal operating conditions of the amplifier system 1 (e.g. large voltage variations at the input 2 or a state after current limit) without stopping the self-oscillation of the whole amplifier system 1. Stated differently, one can reduce the negative impacts of these abnormal operating conditions by stopping the integrating forward filter 11, and still have the amplifier system 1 running, a feature which is not possible with conventional switching amplifier systems. The trade-off is that the amplifier system 1 may be subjected to more non-linearity while the integrating forward filter 11 is stopped. Moreover, the integrating forward filter 11 provides means for the error amplifier (represented by the summing box 15) to more accurately perform its job. In more detail, the feedback factor of a system is its ability to correct for errors. It is defined as the open loop gain minus the closed loop gain at a certain frequency. If for example the open loop gain at 1 kHz is 60 dB and the closed loop gain is set to 20 dB the feedback factor is 40 dB. Thereby the system's ability to correct for errors when generating 1 kHz is 40 dB. Higher order harmonics in the error will be corrected with the feedback factor at that certain frequency and in many systems the open loop gain decreases at higher frequencies in order to maintain phase margin meaning the ability to correct errors is decreased the higher the frequency. Thus, by increasing the open loop gain, the feedback factor is increased and error correcting ability of the system is improved.

(15) FIG. 2 shows a schematic plot of the closed frequency response loop frequency response for a self-oscillating amplifier system 20a, 20b according to an embodiment of the invention (broken lines) and for a self-oscillating amplifier system according to the prior art 21a, 21b (solid lines). The schematic plot is based on a simulation of an amplifier system comprising a pair of parallel forward filters (differentiating and integrating) and a phase lead network in the same branch as the differentiating forward filter. The advantages of the inventive system are immediately apparent as the skilled person readily appreciates the flat, i.e. relatively constant, gain characteristics 20a across the whole operational frequency band (the audio band in this illustration) of the inventive system, while a conventional system has a slightly sloped gain 21a at the upper range of the operational frequency band. The 3 dB point is substantially higher in frequency for the inventive system, approx. 242 kHz vs approx. 60 kHz.

(16) Moreover, the phase characteristics are improved since the phase shift at 20 kHz is approx. 5 degrees for the inventive system, while it is approx. 16 degrees for the conventional prior art system. Thus, the inventive self-oscillating amplifier system exhibits larger small signal bandwidth and therefore allows for more accurate reproduction of the input signal, as compared to conventional prior art systems.

(17) Further, as illustrated in FIG. 3, the second forward branch 32 may further include an amplitude limiting circuitry 13a which is configured to limit an output of an of the integrating forward filter 11. The integrating forward filter 11 is accordingly configured to generate an output in the form of an integrated signal, which signal's amplitude is then kept within certain limits by means of the amplitude limiting circuitry 13a.

(18) However, the amplitude limiting circuitry 13a, 13b may be realized in various different ways. As for example, illustrated in FIG. 4, instead of limiting the integrated signal formed by the integrating forward filter 11 within predefined limits (e.g. by means of transistors and fixed biasing levels), the amplitude limiting circuitry 13b may be connected to the input 2 of the system 1 in order to retrieve a reference signal and compare it to the integrated signal and from this comparison control the amplitude range of the integrated signal. This allows for more dynamic amplitude control, and is advantageous in that the risk of over-modulation in the subsequent modulating step is reduced.

(19) The first forward branch 31 is designed to only have 180 degrees of phase shift (which is the required condition for oscillation) at a frequency far above the operational frequency range. The criteria for obtaining self-oscillating conditions are discussed in for example WO 2004/47286 and EP1994638, incorporated herein by reference. The second forward branch 32 however, will have more than 180 degrees close to the operational frequency range meaning that it can cause sub-harmonic oscillation. The control signal to the modulator 5 is the sum of the two paths 31, 32. However, sub-harmonic oscillation in the system requires that the excursion of the integrated signal is much larger than normal. Accordingly, by limiting the integrated signal generated in the integrating forward filter 11, one will also prevent it from locking the system 1 onto a too low switching frequency. Stated differently, by limiting the output of the integrator 11, the differentiating path (i.e. first forward branch) will be in control.

(20) FIG. 5 illustrates another block diagram representation of a self-oscillating amplifier system 1 according to another embodiment of the present invention. More specifically, the system 1 comprises a low pass filter 16 at the input of the amplifier system 1, i.e. preceding the forward path 4, outside the feedback loop 9. This is advantageous in order to further flatten the closed loop frequency response and to compensate for any potential peaking in the closed loop frequency response introduced by the forward filter(s) 8, 11.

(21) FIG. 6 illustrates a schematic circuit realization of a self-oscillating amplifier system 1 having an integrating forward filter 11 with an amplitude limiting circuitry 13a configured to limit the output of the active integrating forward filter 11 to be within a predefined amplitude range. Various blocks (same reference numerals are used for the corresponding blocks in the appended figures) and their corresponding functions have been discussed in the foregoing, and will for the sake of brevity not be repeated, however, this illustration in FIG. 6 serves to show how an example of how a circuit implementation may be realized.

(22) The predefined amplitude range can for example be controlled by setting appropriate biasing levels at the gates/bases of the two transistors 51a, 51b. The biases may for example be set to be 3.0V, 2.0V, 1.5V, 1.25V or 1.0V. Stated differently, the transistors 51a, 51b are configured to control the magnitude (i.e. absolute value) of the output signal of the integrating branch so that it is less than or equal to 3.0V, 2.0V, 1.5V, 1.25V, 1.0V. In operation, an input signal (e.g. audio signal) is received at the input 2 and fed to an error amplifier 15 which is configured to amplify the difference between the output signal (provided via the feedback loop 9) and the input signal to produce an error signal which is fed via the filters of the two parallel forward branches 31, 32 to a modulating stage 5 (e.g. a comparator or zero cross detector). The modulated signal is subsequently amplified in the switching power stage 6 and then demodulated before reaching the output 3.

(23) The output of the integrating forward filter 11 is further connected to an inverter for inverting the integrating signal before being fed to the modulating stage 5. This is however readily understood by the skilled artisan to be a specific implementation, and the circuit may be realized by other equivalent components or arrangements as known in the art. Further, the switching power stage 6 is here represented by a pair of schematic switches being connected by a positive and negative supply rail. The output 3 of the amplifier system 1 is connected to a load, here in the form of a resistor.

(24) FIG. 7 illustrates another schematic circuit realization of a self-oscillating amplifier system 1 having a different realization of an amplitude limiting circuitry 13b for limiting the output of the integrating forward filter 11 of the second forward branch 32. More specifically, the amplitude limiting circuitry 13b is here connected to the input 2 of the system 1 and configured to compare (subtract) the input signal fed to the system input 2 with the output (integrated signal) of the integrating filter 11 and generate a control signal at the negative (inverting) input of the operational amplifier in the integrating filter 11, and thereby control and limit the amplitude of the integrating signal. However, the illustrated embodiment of FIG. 6 is just an example of how the comparing amplitude limiting circuitry 13b may be realized. The skilled person readily understands that there are other possible circuit configurations which would provide an equivalent comparing function.

(25) The invention has now been described with reference to specific embodiments. However, several variations of the self-oscillating amplifier system are feasible. For example, the (global) feedback loop may be current controlled or voltage controlled, the differentiating (first) forward filter may comprise a cascade of active and/or passive filters, the amplifier system may comprise additional local feedback loops, etc. Further, the pulse modulator may be configured to modulate the signal in numerous ways and with interchangeable components providing the same functionality. Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word comprising does not exclude the presence of other elements or steps than those listed in the claim. The word a or an preceding an element does not exclude the presence of a plurality of such elements.