Patent Application
20080101628
The present invention relates to amplifiers, and in particular, switching amplifier circuits and methods.
A switching amplifier, sometimes referred to as a class D amplifier, is an amplifier where the output transistors are operated as switches. One example of a transistor used in switching amplifiers is a MOSFET. When the transistor is off, the circuit behaves like an open circuit so the current is zero. When the transistor is on, the voltage across the transistor is ideally zero. In practice, the voltage is very small. Since the equation for power is P=V*I, the power dissipated by the amplifier is very low in both states. This increases the efficiency, thus requiring less power from the power supply and allowing smaller heat sinks for the amplifier. For example, the increased efficiency translates into benefits such as longer battery life. The decrease in the size of the heat sinks lowers the weight, cost and size of the amplifier. Example applications where these advantages would be useful are portable battery-powered equipment such as cellular technology or portable music players.
The modulated signal is then amplified in a switching output stage 102. Since the modulated signal is represented by a train of pulses, the output transistors operate like switches. This enables the transistors to have zero current when they are not switching and a low voltage drop across the transistors when they are switching.
The amplified signal generated by output stage 102 then enters a low pass filter 103 before entering a speaker 104. The low pass filter translates the modified amplified signal back into a continuous signal. A typical filter is an LC filter, for example. The resulting amplified continuous signal may be provided to a speaker and translated into sound. The benefits of low pass filters include minimizing electromagnetic interference (“EMI”) and power dissipation in the amplified signal.
However, one disadvantage of switching amplifiers is the cost and size of the LC filter. The components, especially the inductors, occupy board space and add to the overall cost. To address this, a separate inductor is sometimes eliminated to create what is referred to as an inductorless amplifier.
Thus, there is a need for an improved switching amplifier capable of low EMI and power dissipation in inductorless applications. The present invention solves these and other problems by providing improved switching amplifier circuits and methods.
Embodiments of the present invention include switching amplifier circuits and methods. In one embodiment, the present invention includes an audio amplification method comprising modulating an audio input signal to produce a first modulated signal and a second modulated signal, amplifying the first modulated signal to generate an amplified first modulated signal at a first output terminal of an amplifier, and amplifying the second modulated signal to generate an amplified second modulated signal at a second output terminal of the amplifier, wherein the first output terminal of the amplifier is constant when the second output terminal of the amplifier is switching, and wherein the second output terminal of the amplifier is constant when the first output terminal of the amplifier is switching.
In one embodiment, modulating comprises half-wave rectifying the audio signal and an inverse of the audio signal to produce first and second half-wave rectified signals, and pulse width modulating the first and second half-wave rectified signals to produce the first and second modulated signals.
In one embodiment, modulating comprises pulse width modulating the audio signal and an inverse of the audio signal to produce first and second pulse width modulated signals, and digitally half-wave rectifying the first and second pulse width modulated signals to produce the first and second modulated signals.
In one embodiment, the present invention further comprises coupling the amplified first modulated signal to a first integrated circuit package terminal and coupling the amplified second modulated signal to a second integrated circuit package terminal.
In one embodiment, the present invention further comprises coupling the first integrated circuit package terminal to a first terminal of a speaker and coupling the second integrated circuit package terminal to a second terminal of a speaker.
In one embodiment, the audio input signal is a single ended signal, the method further comprising generating an inverse of the audio input signal.
In one embodiment, the audio input signal is a differential signal.
In one embodiment, first and second modulated signals are pulse width modulated signals.
In one embodiment, the present invention includes an electronic circuit comprising a modulator having at least one input for receiving an input signal, the modulator generating a first modulated signal on a first modulator output terminal and a second modulated signal on a second modulator output terminal, and an amplifier having a first input coupled to receive the first modulated signal and a second input coupled to receive the second modulated signal, the amplifier generating a first amplified modulated signal on a first output terminal and the amplifier generating a second amplified modulated signal on a second output terminal, wherein the first output terminal of the amplifier is constant when the second output terminal of the amplifier is switching, and wherein the second output terminal of the amplifier is constant when the first output terminal of the amplifier is switching.
In one embodiment, the modulator comprises a first comparator coupled to receive the input signal, a second comparator coupled to receive an inverse of the input signal, a sawtooth wave generator coupled to the first and second comparators, and a digital half-wave rectifying circuit having a first input coupled to an output of the first comparator and a second input coupled to an output of the second comparator.
In one embodiment, the digital half-wave rectifying circuit comprises an XNOR circuit having a first input coupled to the output of the first comparator and a second input coupled to the output of the second comparator, a first NOR circuit having a first input coupled to the output of the first comparator and a second input coupled to the output of the XNOR circuit, and a second NOR circuit having a first input coupled to the output of the second comparator and a second input coupled to the output of the XNOR circuit.
In one embodiment, the modulator comprises first means for comparing the received the input signal to a sawtooth waveform, second means for comparing the received an inverse of the input signal to a sawtooth waveform, and means for digitally half-wave rectifying coupled to the first and second means for comparing.
In one embodiment, the modulator comprises a first half-wave rectifier coupled to receive the input signal, a second half-wave rectifier coupled to receive an inverse of the input signal, a first comparator coupled to the output of the first half-wave rectifier, a second comparator coupled to the output of the second half-wave rectifier, and a sawtooth wave generator coupled to the first and second comparators.
In one embodiment, the modulator comprises first means for half-wave rectifying coupled to receive the input signal, second means for half-wave rectifying coupled to an inverse of the input signals, first means for comparing the half-wave rectified input signal to a sawtooth waveform, and second means for comparing the half-wave rectified inverse of the input signal to a sawtooth waveform.
In one embodiment, the first output terminal is coupled to a first integrated circuit package terminal and the second output terminal is coupled to a second integrated circuit package terminal.
In one embodiment, the present invention includes an audio amplifier comprising means for modulating an audio signal to produce a first modulated signal and a second modulated signal, means for amplifying the first modulated signal to generate a first amplified modulated signal and a second amplified modulated signal, wherein the first amplified modulated signal is constant when the second amplified modulated signal is switching, and wherein the second amplified modulated signal is constant when the first amplified modulated signal is switching.
In one embodiment, modulating comprises means for half-wave rectifying the audio signal and an inverse of the audio signal to produce the first and second half-wave rectified signals, and means for pulse width modulating the first and second half-wave rectified signals to produce the first and second modulated signals.
In one embodiment, modulating comprises means for pulse width modulating the audio signal and an inverse of the audio signal to produce first and second pulse width modulated signals, and means for digitally half-wave rectifying the pulse width modulated signals to produce the first and second modulated signals.
In one embodiment, the present invention includes an electronic circuit comprising a modulator having at least one input for receiving an input signal, the modulator generating a first modulated signal on a first modulator output terminal and a second modulated signal on a second modulator output terminal, and an amplifier having a first input coupled to receive the first modulated signal and a second input coupled to receive the second modulated signal, the amplifier generating a first amplified modulated signal on a first output terminal during a first time period and the amplifier generating a second amplified modulated signal on a second output terminal during a second time period, wherein the first amplified modulated signal is constant during the first time period when the second amplified modulated signal is switching, and wherein the second amplified modulated signal is constant during the second time period when the first amplified modulated signal is switching.
In one embodiment, the present invention further comprises an inverter circuit for generating an inverse of the first input signal.
In one embodiment, the modulator comprises a sawtooth wave generator.
In one embodiment, the modulator comprises a plurality of comparators.
In one embodiment, the modulator comprises a digital half-wave rectifier. In one embodiment, the digital half-wave rectifier comprises a XNOR gate, a first NOR gate and a second NOR gate.
In one embodiment, the modulator comprises a first half-wave rectifier coupled to receive the input signal and a second half-wave rectifier coupled to receive an inverse of the input signal.
In one embodiment, the amplifier comprises a first transistor having a first terminal coupled to a first reference voltage, a second terminal coupled to the first output terminal, and a control terminal coupled to the first amplifier input, a second transistor having a first terminal coupled to a second reference voltage, a second terminal coupled to the first output terminal, and a control terminal coupled to the first amplifier input, a third transistor having a first terminal coupled to the first reference voltage, a second terminal coupled to the second output terminal, and a control terminal coupled to the second amplifier input, and a fourth transistor having a first terminal coupled to the second reference voltage, a second terminal coupled to the second output terminal, and a control terminal coupled to the second amplifier input.
In one embodiment, the present invention includes a method of driving a speaker comprising generating a first pulse width modulated half-wave rectified signal on a first output terminal of a first amplifier, and generating a second pulse width modulated half-wave rectified signal on a second output terminal of a second amplifier; wherein the first signal is constant when the second signal is switching, and wherein the second signal is constant when the first signal is switching.
In one embodiment, the present invention further comprises modulating an audio input signal to produce a first modulated signal and a second modulated signal.
In one embodiment, the present invention further comprises amplifying the first modulated signal and amplifying the second modulated signal.
Additional embodiments will be evident from the following detailed description and accompanying drawings, which provide a better understanding of the nature and advantages of the present invention.
Described herein are techniques for switching amplifiers. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include obvious modifications and equivalents of the features and concepts described herein.
The modulator output terminals 308 and 309 are coupled to the input terminals of a switching amplifier output stage 302. Amplifier 302 may receive the modulated signals and amplify the signals (e.g., the current) to drive a speaker, for example. Amplifier 302 may include output terminals for provided amplified modulated signals corresponding to the modulated signals received from modulator 301. If the circuits are implemented on an integrated circuit, the amplified modulated signals may be coupled from output terminals of the amplifier to integrated circuit package terminals 310 and 311 for example. The dashed line represents the boundary between an integrated circuit and a printed circuit board, for example. The package terminals may, in turn, be coupled to a speaker. Therefore, if an audio input signals increases (e.g., above zero for an audio signal with no DC offset or above half-supply for an audio signal that is operating around half-supply), one terminal of the speaker (e.g., coupled to terminal 310) may be held constant while the other terminal of the speaker (e.g., coupled to terminal 311) receives the amplified modulated signal. Similarly, if the audio input signal decrease, the other terminal of the speaker (e.g., coupled to terminal 311) may be held constant while the opposite terminal of the speaker (e.g., coupled to terminal 310) receives the modulated signal. Based on the modulation techniques describe above, it can be seen that the first output terminal of the amplifier is constant (e.g., zero volts) when the second output terminal of the amplifier is switching, and the second output terminal of the amplifier is constant (e.g., zero volts) when the first output terminal of the amplifier is switching. When no input signal is received a very short pulse may appear on both positive and negative outputs (i.e. current through load is negligible), for example.
An input analog signal 401 is received by circuit 400 and is transmitted to comparator 406 and inverter circuit 414. In this example, the input signal 401 is a single ended signal, and inverter circuit 414 may be used to generate an inverse of the signal. Other embodiments of the invention, the input signal may be a fully differential signal, in which case an inverter circuit may be not be used. In this example, the means for modulating the inputs signal include two comparators 406 and 407, sawtooth wave generator 405, and a digital circuit for generating the desired output signals—here XNOR 408, NOR 409 and 410 are digitally half-wave rectifying the pulse width modulated signals (e.g., using digital subtraction) to create a modulated representation of a half cycle of the audio signal. A modulated representation of a half cycle of the input audio signal, positive or negative, is referred to herein as a half-wave rectified modulated signal, or in the case of PWM a half-wave rectified pulse width modulated signal.
Comparator 406 receives the input signal on the positive input terminal and a sawtooth waveform on the negative input terminal at node 415 and generates a pulse width modulated signal at node 417. Plot 500 in
Similarly, comparator 407 receives inverse input analog signal on the positive input terminal at node 416 and sawtooth waveform on the negative input terminal at node 415. Plot 510 in
The combinational logic consisting of XNOR gate 408, NOR gate 409, and NOR gate 410 processes the signal at nodes 417 and 418 into half-wave rectified, pulse width modulated representations of the inverted and non-inverted input analog signal. The signal at node 419 represents the half-wave rectified, pulse width modulated representation of the inverted input analog signal, while the signal at node 420 represents the half-wave rectified pulse width modulated representation of the input analog signal. Plot 540 in
One advantage to using the switching amplifier architecture shown in
Another advantage to using the switching amplifier architecture shown in
Input analog signal 701 is received by circuit 700 and transmitted to half-wave rectifier 705 and inverter circuit 713. The half-wave rectifier transmits only the positive portions of the incoming signal to the output. Comparator 708 receives the half-wave rectified signal at node 715 along with the sawtooth waveform at node 717, which is generated by sawtooth wave generator 706. Plot 800 in
This embodiment contains the same advantages as shown in circuit 400 in
In one embodiment, another advantage of the present invention may include reductions in the size of the devices needed for the high side (between the output and supply) of the switching output because the average of the waveform is typically on the low side rather than the high side. For example, if the modulating output transitions between zero volts (0v) on the low side and another voltage, Vhi, on the high side, the average over 1 cycle of a sinewave input results in the high side output driver device (e.g., transistor 904 or 909) being on only about 25% of the time and the low side output driver device (e.g., transistor 905 or 910) being on about 75% of the time. Thus, smaller devices may be used on the high side. For example, if transistors 904 and 909 are P-channel devices, such devices may be decreased in size by about 30% using the techniques described above. If transistors 904 and 909 are N-channel devices, such devices may be decreased in size by about 15-20% using the techniques described above, for example.
The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. For example, switching amplifier circuits and methods according to the present invention may include some or all of the innovative features described above. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as defined by the claims.
