Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2001-10-05
2003-04-22
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S251000
Reexamination Certificate
active
06552606
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an amplifier circuit for amplifying electric signals, comprising controllable switching means for generating a block wave signal whose amplitude varies between first and second supply voltage values during operation, filter means for filtering said block wave signal so as to produce an output signal, which filter means comprise a self-inductance and a capacitance, means for providing a filter capacitance current proportional to the current through the filter capacitance, modulating means for pulse width modulation of the block wave signal by driving the switching means in response to an input signal to be amplified, and correction means for providing from a reference value derived from the input signal and an output signal value proportional to the output signal a correction signal for controlling the modulating means.
An amplifier circuit of this kind is disclosed in U.S. Pat. No. 5,606,289, in practice also known as a so-called class D amplifier.
In a class D amplifier circuit a block wave signal is generated which has a frequency which is much higher than the highest frequency of the input signal to be amplified. The pulse width ratio of this signal is modulated so that the average value of the block wave signal is proportional to the input signal. By applying the block wave signal to a low-pass filter or resonator circuit, with a cut-off frequency ranging between the highest signal frequency and the frequency of the block wave signal, an output signal is produced from which the switching frequency or block wave frequency and higher frequencies of the block wave signal have been removed. The output signal represents the average value of the block wave signal, and consequently of, the input signal, which is amplified, however, by an amplification factor which is determined by the electrical characteristics of the modulator, the correction signal, the supply source and the switching means. Usually, switching transistors such as MOSFET's (Metallic Oxide Semiconductor Field Effect Transistors) are used as the switching means.
Linear amplifiers, such as class A and class AB amplifiers, for example, whose amplifier stage is essentially operated as a controllable series resistor, have a very low energetic efficiency, since a high heat dissipation occurs in the output stage when the amplifier is not driven to full load. An amplifier circuit comprising a switched output stage, on the other hand, such as the amplifier circuit according to the invention, only exhibits a small degree of heat dissipation, since the current through the output stage is zero when the switching means are off, and the voltage across the output stage is practically zero when the switching means are on. Switching amplifiers, or class D amplifiers, have a very high energetic efficiency, >90% in practice.
In practice a number of undesirable effects occur in switching amplifiers, which cause interferences of the ideal output signal. The interferences can be subdivided into internal errors and external errors.
The output impedance of the amplifier is mainly determined by the filter means for filtering out the block wave signal. This impedance is frequency dependent, and is for practical reasons approximately equal to the nominal load resistance at the end of the frequency band. Accordingly, interferences in the output signal due to external causes are hardly suppressed in the signal that is applied to the load. Furthermore, a load impedance-dependent frequency transfer will occur.
Furthermore, switching transistors, for example, have a limited response time, which is mainly caused by parasitic capacitances. Transistors connected in a so-called half-bridge circuit, wherein two switching transistors are arranged in series and the block wave signal is generated at the junction of the transistors, must never be “on” simultaneously, since a current path from the positive supple terminal to the negative supply terminal will be formed in that case. The current that occurs during such a short circuit will undoubtedly cause damage to the switching transistors. For this reason a so-called “dead time” is maintained during the switching of the transistors, in order to ensure that at least one of the transistors will be “off”. Especially at small amplitudes this dead time causes a strong non-linearity in the signal transfer of a loaded amplifier circuit.
Finally, depending on the type of modulating means, interferences which are present on the supply voltage can be transferred to the output signal.
Both the output impedance caused by the output filter and the non-linearity resulting from the dead time and interferences on the supply voltage can be reduced as much as possible with the prior art switching amplifier circuits, provided they are adequately designed, by means of the correction signal in a closed loop feedback to the modulating means. It has been found, however, that the maximum suppression that can be realised is not adequate due to the output filter impedance/phase shift. Moreover, the stability condition may depend on the load and the supply voltage.
Accordingly, it is a first object of the invention to provide enhanced suppression of interferences in the output signal of a class D amplifier circuit caused by internal and external error sources by eliminating the influence of the output filter on the signal transfer characteristics of the amplifier circuit.
SUMMARY OF THE INVENTION
In accordance with the invention, this objective is accomplished by providing means for deriving a reference current from the input signal, wherein the correction means are arranged for providing the correction signal as a current correction signal from the reference current and the filter capacitance current.
The invention is based on the insight that an adequate differential correction is necessary in order to effect a quick correction of interferences in the output signal, to which end the invention advantageously makes use of the filter capacitance current, which is proportional to the derivative of the output signal voltage of the amplifier circuit, without the drawbacks of high-frequency noise and other interferences that occur when separate differentiating means are used for providing a D correction signal.
By placing the output filter within the control loop in accordance with the invention, the impedance of the output filter of an amplifier which is controlled in this manner will hardly result in an output impedance of the amplifier, if at all. A change in the load of the amplifier circuit will directly be detected and corrected, since the change in the filter capacitance current caused by said change in the load will immediately result in the generation of a current correction signal.
In a preferred embodiment of the invention the object is to minimise such interferences as quickly as possible, preferably within one switching period of the switching means. To this end the means for providing the filter capacitance current are wideband means, that is, comprising on average five times the signal bandwidth of the amplifier or higher.
Basically, there are two possibilities for measuring the filter capacitance current. In the case of direct measurement, a sensor or other electric component, such as a resistor, is connected in series with the filter capacitor. In the case of indirect measurement, the current is provided by a capacitor which is connected in parallel with the filter capacitance. The advantage of direct measurement is that the current through the filter capacitor can be represented with maximum accuracy and without any appreciable phase shift.
In a further embodiment of the invention, in order to provide a filter capacitance current which is maximally proportional to the current through the filter capacitance, the means for providing the filter capacitance current comprise a current transformer which is connected in series with the filter capacitance or with part thereof, which current transformer is built up of a core having a coaxial cable wound thereon, o
Domensino Hendrikus Johannes Jacobus
Veltman André
Finnegan Henderson Farabow Garret & Dunner, L.L.P.
Nguyen Khanh Van
Pascal Robert
Technische Universiteit Eindhoven
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