Self-oscillating switching amplifier

• US 7,221,216 B2
• Filed: 05/18/2005
• Issued: 05/22/2007
• Est. Priority Date: 05/18/2004
• Status: Expired due to Fees

• Current Assignee: Optoma Corporation
• Original Assignee: Nphysics Incorporated
• Inventors: Nguyen, Tranh T.
• Primary Examiner(s): MOTTOLA, STEVEN J
• Application Number:
  US11/132,810
• Publication Number:
  US 20060006933A1
• Time in Patent Office: 734 Days
• Field of Search: 330/10, 330/207.A, 330/251
• US Class Current: 330/10
• CPC Class Codes: H03F 1/32 Modifications of amplifiers to reduce non-linear distortion by negative feedback H03F1/34
• CPC Class Codes: H03F 2200/03 the amplifier being designed for audio applications
• CPC Class Codes: H03F 3/181 Low-frequency amplifiers, e.g. audio preamplifiers
• CPC Class Codes: H03F 3/217 Class D power amplifiers; Switching amplifiers
• CPC Class Codes: H03F 3/3022 CMOS common source output SEPP amplifiers H03F3/3008 takes precedence

Abstract

A self-oscillating switching amplifier having an error amplifier output combined with the output of an output signal differentiator according predetermined weighing factors to force a small oscillation around the averaged output signal at a high frequency. The feedback voltage is sensed at the output of the switching amplifier. Additional feedback can be derived from a switching node of the power switch. The switching of the power switch can be dynamically changed from binary switching when the amplitude of the audio signal is low, to ternary switching when the amplitude of the input signal is high to minimize distortion of the output signal. The amplifier can be supplied with a pulsing voltage. In certain embodiments the output signal differentiator is simply a capacitor or a LC resonance circuit coupled directly to an appropriate speaker terminal for the highest possible self-oscillating frequency of the switching amplifier.

Claims

1. A self-oscillating switching amplifier, the amplifier comprising:

a power switch providing a high-power output signal,

an error amplifier providing an error voltage representative of differences between an input signal and the high-power output signal,

an output differentiator for receiving a feedback signal representative of the output voltage and generating a differentiated signal representative of high frequency gain and phase shift in the high-power output signal;

a pulse width modulation (“

PWM”

) signal generator for providing a PWM signal representative of a linear combination of the error voltage with the differentiated signal, and

a reconstruction filter for receiving the high-power output signal and providing a reconstructed output signal.

2. The self-oscillating switching amplifier of claim 1 wherein the output differentiator includes a low-frequency blocking filter.

3. The self-oscillating switching amplifier of claim 1, wherein the feedback signal is provided by a feedback circuit configured to cause the high-power signal to oscillate.

4. The self-oscillating switching amplifier of claim 3 wherein the feedback circuit includes a resonance circuit.

5. The self-oscillating switching amplifier of claim 3 wherein the feedback circuit includes a low-pass filter for controlling the switching frequency of the high-power signal.

6. The self-oscillating switching amplifier of claim 1 wherein the power switch has one or more modes of operation including a binary mode and a ternary mode, and wherein a current mode of operation is selected based on the amplitude of the input signal.

7. The self-oscillating switching amplifier of claim 6 wherein the power switch is coupled to a DC voltage when the binary mode is selected, and coupled to a pulsing supply voltage when the ternary mode is selected.

8. A method for controlling a self-oscillating switching amplifier, the self-oscillating switching amplifier receiving an input signal and providing an output signal, comprising the steps of:

amplifying and integrating the difference between the input signal and the output signal

to obtain an integrated error signal;

differentiating the output signal to obtain a differentiated signal;

linearly summing with predetermined weighting factors the integrated error signal with the differentiated signal to obtain a sum signal;

comparing the sum signal with a predetermined voltage to obtain a PWM signal, wherein the PWM signal controls an output of a power switch; and

sensing an output of a reconstruction filter to provide a feedback signal to the error amplifier and the differentiator, wherein the reconstruction filter receives the output of the power switch.

9. The method of claim 8, wherein the output of the reconstruction filter is coupled to the output signal.

10. The method of claim 8 wherein the step of differentiating is implemented using a differentiator comprising a resonance circuit with a predetermined quality factor for maintaining the self-oscillation frequency in a narrow frequency range.

11. The method of claim 8, and further comprising selecting a mode of operation of the self-oscillating amplifier, wherein the mode is one of a plurality of modes including a binary mode and a ternary mode, and wherein mode selection is based on instantaneous amplitude of the audio input signal.

12. A method for controlling the high frequency switching of a self-oscillating switching amplifier having a power switch, a reconstruction filter, an input signal and an output signal, the method comprising the steps of amplifying and integrating the difference between the input signal and the output signal with an error amplifier to obtain an error signal;

extracting a carrier frequency from the output signal with a frequency-selective filter;

combining the error signal with the carrier frequency via predetermined impedances to obtain a sum voltage;

comparing the sum voltage with a predetermined voltage to obtain a PWM signal to drive the power switch;

wherein the frequency-selective filter in combination with the reconstruction filter determine the switching frequency of the amplifier.

13. The method of claim 12 wherein the output signal is combined with the error signal via a resonance circuit and a resistor, resulting in a sum signal having a high frequency carrier of small amplitude, that when compared to a predetermined voltage, generates the PWM signal.

14. The method of claim 12 which further comprises the step of injecting via a linear network a switching voltage of the power switch to the error integrator.

15. A self-oscillating switching amplifier amplifying an input signal into a high power output signal driving a load via its two output terminals, the amplifier comprising:

an oscillator comprising a power switch, a comparator, and a feedback signal taken from an output terminal and coupled to an input of the comparator via a frequency-selective network for producing a natural oscillation,

an error amplifier for amplifying the difference between the output signal and the input signal to produce an error voltage, which is in turn fed to the comparator of the oscillator to modulate the pulse width of the oscillation pulses,

wherein the oscillator known to oscillate consistently also forms with the error amplifier a self-oscillating switching amplifier capable of very low distortion, very high bandwidth and very high efficiency, and wherein the frequency-selective network is a resonance circuit.