Switching Amplifier

The invention pertains to methods and apparatus for processing audio signals. More particularly, the invention pertains to a high efficiency switching amplifier useful in a portable battery operated device.

High efficiency switching amplifiers usable to amplify audio frequency signals are well known. One such amplifier system is disclosed in U.S. Pat. No. 3,185,768 to Cudahy entitled Amplifier Circuit.

Known switching amplifier systems generate a pulse width modulated switching signal in response to the applied audio input signal. The modulated signal includes components that replicate the audio input signal. In addition, it includes high frequency components associated with the frequency of the switching signal and its harmonics.

The modulated switching signal can be used to provide input signals to solid state switches. The solid state switches provide an output signal to a load.

Where the load has an inductive component, or there is an inductive filter in series with the load, the higher frequency components of the load current will be attenuated.

Because of limitations on the maximum value and the quality of the inductive components there can still be an appreciable current flow associated with the higher frequency components of the output signal. This current flow is especially apparent for small amplitudes of the audio input signal.

The use of an inductive filter is undesirable in that it increases amplifier size and weight. It also increases amplifier cost.

In addition, in the absence of a modulating audio input signal, the switching signal often appears at the load as a square wave. This square wave has an amplitude corresponding to the peak to peak output capability of the amplifier at the frequency of the switching waveform.

Such units tend to be used in a portable, battery powered environments. As a result, it is particularly desirable to minimize size and weight and losses of energy from the battery particularly when the input audio signal has substantially zero amplitude.

In accordance with the invention, a high efficiency switching amplifier provides an amplified representation of an input signal to a load. The amplifier includes circuitry for generating a selected high frequency clock waveform. The high frequency clock waveform is coupled to first and second pulse width modulators. The input signal to be amplified is also coupled to the two modulators. An input to one of the two modulators is inverted. Each modulator generates a pulse width modulated switching waveform in response to instantaneous values of the input signal.

A push-pull bridge output switching circuit is coupled to the output of each of the modulating circuits. The load is coupled across first and second terminals of the output bridge.

An amplified representation of the audio frequency input signal appears across the first and second output terminals of the push-pull bridge output stage and is delivered to the load. If the load is inductive, the current due to the high frequency components of the pulse width modulated switching signals will be attenuated providing high efficiency.

A particular advantage of the present amplifier is that if the input signal has a value substantially equal to zero, the amplified representation delivered to the load also has a value substantially equal to zero. No pulsed or switching signal is provided to the load in this condition.

The pulse width modulators in one embodiment each include a comparator for comparing the amplitude of the input signal to the amplitude of the high frequency clock waveform. The input to one modulator is an inverted representation of the input electrical signal.

The clock waveform can be a triangularly shaped waveform. Alternately, the clock waveform can be a saw-tooth. In an alternate embodiment of the invention, the high frequency clock input to one of the comparators could be inverted instead of inverting the input electrical signal.

The push-pull, bridge output stage of the amplifier includes four switching elements, such as field effect transistors. The load is coupled across the first and second output terminals of the bridge. As a result of alternately switching various pairs of the switching elements in the bridge, a tri-level output signal can be applied to the load even though only a single source of electrical energy is used to power the amplifier.

In yet another embodiment, the modulators can each include a voltage or current controlled pulse width generator. The high frequency clock signal can be coupled to a clock input to each generator. The input signal can be coupled to the pulse width input of one generator. An inverted input signal can be coupled to the pulse width input of the second generator. The pulse width modulated output sequence from each of the two generators can be coupled to a respective input of the bridge output stage.

In yet another embodiment of the invention, a system for processing an acoustical signal includes circuitry for sensing the acoustical signal and for generating an electrical signal in response thereto. The electrical signal is amplified in a switching amplifier having first and second modulator circuits interconnected as described above. Output from the switching amplifier is coupled to a transducer for converting the amplified electrical signals to an output acoustical signal.

Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings in which the details of the invention are fully and completely disclosed as a part of this specification.

FIG. 1 is a block diagram schematic illustrating a switching amplifier in accordance with the present invention;

FIGS. 2A, 2B and 2C are a set of graphs illustrating waveforms present in the switching amplifier of FIG. 1 as a function of time;

FIG. 3 is a detailed schematic of a switching amplifier in accordance with the present invention;

FIG. 4 is a block diagram of a hearing aid and audio processing unit in accordance with the present invention;

FIG. 5 is a partial block diagram of an alternate modulator configuration usable in a switching amplifier in accordance with the present invention;

FIG. 6 is a partial block diagram of yet another modulation system usable in a switching amplifier in accordance with the present invention; and

FIG. 7 is an alternate output stage usable with any of the previously noted modulators.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawing and will be described herein in detail a specific embodiment thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated.

FIG. 1 illustrates a block diagram of a high efficiency switching amplifier system 10 in accordance with the present invention. The system 10 includes a source 12 of high frequency clock or comparison signals.

The source 12 includes an oscillator 14 which provides a pulse train of electrical signals to a triangular waveform generator 16. The generator 16, in combination with the clock oscillator 14, generate a triangularly shaped waveform of a frequency on the order of 100 kilohertz.

The system 10 includes first and second comparators 20 and 22. Output from the triangular wave generator 16 on a line 24 is supplied to a positive input to each of the comparators 20 and 22.

The system 10 also includes an inverter circuit 28. The inverter circuit 28 can be implemented as an operational amplifier configured to have a gain of minus one.

An electrical input signal to be amplified is coupled to the system 10 at an input port 30. The input port 30 is in turn coupled to a negative input of the comparator 20 and the input of the inverter 28. Output from the inverter 28 is coupled to a negative input of the comparator 22.

As is apparent from the schematic diagram of FIG. 1, comparators 20 and 22 receive the same comparison waveform from the generator 16. However, as a result of the inverter 28, the modulating signals applied to the comparators 20 and 22, respectively, are out of phase.

Output from the comparator 20, a pulse width modulated pulse train S1, on a line 34, is in turn coupled to a first switching element 36. Output from the comparator 22, a pulse width modulated pulse train S2, on a line 38, is coupled to a second switching element 40.

The elements 36, 40 form a four element push-pull output bridge. Since a bridge output stage is incorporated into the amplifier 10, only a single source of electrical energy (B+) is needed to produce a tri-level output signal.

The switching element 36 includes an inverter 42 with an output line coupled to a first transistor switch 44. Output from the comparator 34 is coupled to a second transistor switch 46 in the switching element 36. The switching element 40 is identical to the switching element 36.

Output conducting members 50 and 52 are each coupled to the transistor switches of a respective one of the switching elements 36 and 40. For example, conducting member 50 is coupled to semiconductor switches 44 and 46. The same voltage source, which could be a low voltage mercury battery, on the order of 1.3 volts, or a higher voltage 3 volt battery is coupled to each of the switching elements 36 and 40.

An output signal is generated across the conducting members 50 and 52. A load L can be connected therebetween. As is described subsequently, an amplified representation of the electrical signal applied to the input port 30 appears across the conducting members 50, 52 and hence is applied to the load L.

The switching amplifier system 10 of FIG. 1 is particularly advantageous in that the circuitry thereof substantially reduces the amplitude of high frequency switching components in the output signal appearing across the conducting members 50, 52. As a result, no separate inductive filter is needed if the load L inherently has an inductive impedance. This is usually the case for hearing aid receivers intended to provide an acoustic frequency response within the normal hearing range on the order of 100 hertz to 10,000 hertz.

FIG. 2 illustrates some of the waveforms associated with the system 10 on lines 50, 52 for different modulation conditions. The switching elements 36 and 40 as configured in FIG. 1 and driven by comparators 20 and 22 combine the electrical signals on the line 34 and the line 38 by subtracting one from the other at the conducting members 50 and 52.

FIG. 2A illustrates the outputs on the lines 50, 52 where the input electrical signal on the input port 30 produces a 60% positive modulation condition. The output graph of lines 50-52 is the push-pull output signal applied to the Load L. It corresponds to the difference of the signals on the lines 50, 52.

FIG. 2B illustrates the outputs on the lines 50, 52 where the input electrical signal on the input port 30 has a zero instantaneous value thereby producing an unmodulated condition at the Load L. As can be seen from FIG. 2B, the outputs on the lines 50 and 52 are identical.

Hence, the difference between the lines 50 and 52 results in an essentially zero value amplitude signal delivered to the Load L. This zero amplitude signal is particularly advantageous in that no switching signal is applied to the load when the input voltage has a value of zero volts.

The waveforms of FIG. 2C illustrate the outputs on the lines 50, 52 where the input electrical signal produces a 60% negative modulation condition. The output graph of the lines 50-52 is the push-pull output signal applied to the Load L.

FIG. 3 is a schematic diagram of a circuit in accordance with the system 10 of FIG. 1. Elements of the amplifier system 10 of FIG. 3 which correspond to elements of FIG. 1 have been assigned the same identification numeral. In the schematic of FIG. 3, the clock oscillator 14 has been implemented with a series coupled pair of inverters. The triangle generator 16 has been implemented with a D flip-flop in combination with a resistor and a capacitor.

The output bridge, composed of switching elements 36 and 40 has been illustrated in FIG. 3 implemented with a commercially available CMOS element type 74HCU04. The comparator elements 20, 22 illustrated in FIG. 3 are open drain output devices which require output load resistors 21 and 23. Alternately, comparators could be used having complementary pull-up and pull-down outputs for lower power dissipation.

FIG. 4 is an overall block diagram of a hearing aid or an acoustic processing system 60. The acoustic processing system 60 includes an input microphone 62. The input microphone 62 converts an acoustic input P.sub.IN to a corresponding electrical signal.

The electrical signal from the microphone 62 can be amplified in a preamplifier 64. Output from the preamplifier 64 on the line 30 corresponds to the input to the switching amplifier 10.

Output from the switching amplifier 10 on the lines 50, 52 can be applied to a receiver 66. The receiver 66 converts electrical signals on the lines 50, 52 to an acoustical output P.sub.OUT.

The amplifier system 10 has several significant advantages when compared to a standard Class D type switching amplifier. One of the advantages of the system 10 is that the high frequency components of the switching signal and its harmonics which are present in the load output voltage on the lines 50, 52 are substantially less than for the case of a standard Class D type switching amplifier. Further, when the input voltage on the input terminal 30 to the system 10 is on the order of zero volts there is substantially no switching signal applied across the output conductors 50, 52 as would be the case of standard Class D switching amplifiers. As a result, for low-level inputs, the system 10 will draw less current from the B+ supply through its output switching elements than will conventional class D-type amplifiers.

Finally, with respect to the detailed implementation of the system 10 illustrated on FIG. 3, variations on the reference voltage coupled to a line 28a of the inverter 28 do not cause a net DC current to flow through the Load L. Variations in the reference voltage V.sub.R appear as only a common mode offset in the output on each of the lines 50, 52. In a standard Class D amplifier, such variations in the value of the quiescent operating point would result in a DC current flowing through the Load L.

An alternate modulator system 70 usable with the amplifier 10 is illustrated in FIG. 5. The modulator system 70 includes the comparators 20 and 22 both of which are coupled to the input port 30. The modulator system 70 also includes the triangular waveform generator 16 and inverting operational amplifier 28. However, in the modulator system 70, the output of the triangular wave generator 16 is inverted before it is coupled to the comparator 22.

Outputs from the comparators 20, 22 the signals S1, S2 on the lines 34 and 38 can be coupled to the push-pull bridge output stage 36, 40 illustrated in FIG. 1.

FIG. 6 illustrates another modulator system 74. The modulator system 74 includes a source of high frequency clock pulses 76. Output pulses from the source 76 are coupled to clock inputs of variable pulse width generator circuits 78 and 80. The variable pulse width circuits 78 and 80 could be implemented, for example, as mono-stable multivibrators.

As illustrated in FIG. 6, the width input to each of the generators 78 and 80 is coupled respectively to the input signal and the inverted input signal. The two pulse width modulated waveforms S1, S2 generated on the lines 82 and 84 can in turn be coupled to the push-pull bridge output stage 36, 40 of FIG. 1 to drive the load.

FIG. 7 illustrates an alternate, single ended, output stage 90. The output stage 90 utilizes a positive and a negative source in combination with three non-inverting semiconductor switching elements 92, 94 and 96.

Inputs to the switching elements 92, 94 and 96 are provide via coded input signals on the lines 92a, 94a, and 96a. The coded inputs are derived from pulse width modulated output waveforms S1 and S2 and can be generated using any of the previously discussed modulator systems in combination with conventional digital logic.

The outputs from the switching elements 92-96 are coupled together at an output node 98 and in turn coupled to the load. As was the case with the push-pull output stage 36, 40 previously discussed, the output stage 90 provides a tri-level output to the load. When the instantaneous value of the input electrical signal is substantially zero volts, the output stage 90 delivers substantially zero volts to the load.

It will be understood that the present amplifier system can be used not only for the purpose of amplifying audio frequency signals but for the purpose of amplifying other signals. Other examples include control signals such as could be used to control the velocity of a rotating shaft of a motor. In addition, it will be understood that the present amplifier is useful wherever high efficiency, portable, battery-powered operation is required.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the novel concept of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.