Microphone bias circuit

A bias circuit supplies a bias voltage VBIAS to a microphone. A variable gain amplifier amplifies a reference voltage VREF. A low-pass filter removes a high-frequency component from the output of the variable gain amplifier. A voltage follower amplifier receives the output voltage of the low-pass filter, and supplies the output voltage to the microphone.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. §119 to Japanese Application No. 2014-092053 filed Apr. 25, 2014, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a bias circuit that supplies a bias voltage to a microphone.

Description of the Related Art

Electronic devices having a recording function or a telephone calling function such as camcorders, digital still cameras, cellular phone terminals, tablet terminals, etc., each mount a microphone, a bias circuit that biases the microphone, and an amplifier that amplifies an electric signal output from the microphone.

FIG. 1is a circuit diagram showing a bias circuit for a microphone investigated by the present inventor. The bias circuit30includes a voltage source32and a variable gain amplifier34, and generates a bias voltage VBIASto be supplied to the microphone2. The output terminal of the bias circuit30is connected to a capacitor C1for phase compensation. The bias voltage VBIASthus generated is supplied to the microphone2via a resistor R1.

The voltage source32divides a stabilized voltage VREGso as to generate a reference voltage VREF. The variable gain amplifier34is configured as a non-inverting amplifier. The variable gain amplifier34amplifies the reference voltage VREFwith a given gain so as to output the bias voltage VBIAS. The suitable level of the bias voltage VBIASvaries according to the kind of microphone2. Thus, in order to provide the bias voltage VBIASsuitable for each microphone2, the gain of the variable gain amplifier34is adjusted.

The microphone2converts an input acoustic signal into an electric signal. The AC component, which is obtained by removing the DC component from the electric signal, is input to the microphone amplifier40via a capacitor C2for DC blocking. The microphone amplifier40amplifies such a weak electric signal. An A/D converter42converts the output of the microphone amplifier40into a digital signal. The bias circuit30shown inFIG. 1cannot be recognized as a known technique. The microphone amplifier40and the A/D converter42are integrated as an audio interface circuit4rtogether with the bias circuit30.

As a result obtained by investigating the bias circuit30shown inFIG. 1, the present inventor has come to recognize the following problem. An unshown digital circuit is mounted as a downstream stage of the A/D converter42. High-frequency noise that occurs due to a clock signal generated in the digital circuit contaminates the bias circuit30via the ground line. Specifically, as shown inFIG. 1, such noise contaminates the bias voltage VBIASfrom the ground via contamination paths (i) through (iii). The bias voltage VBIASnoise is input to the microphone amplifier40via the capacitor C2, leading to a problem of degraded sound quality.

In particular, the noise contamination via the paths (i) and (ii) is amplified by the variable gain amplifier34. Thus, such noise becomes a factor degrading the sound quality.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve such a problem. Accordingly, it is an exemplary purpose of an embodiment of the present invention to provide a bias circuit for a microphone with improved sound quality.

In order to solve the aforementioned problem, a bias circuit according to an embodiment of the present invention is configured as a bias circuit that supplies a bias voltage to a microphone. The bias circuit comprises: a variable gain amplifier that amplifies a reference voltage; a low-pass filter that removes a high-frequency component of an output of the variable gain amplifier; and a voltage follower amplifier that receives an output voltage of the low-pass filter, and that supplies the output voltage to the microphone.

With such an embodiment, the voltage follower amplifier is arranged as a downstream stage of the variable gain amplifier. Furthermore, the filter is interposed between the variable gain amplifier and the voltage follower amplifier. Thus, such an arrangement is capable of reducing a noise component included in the bias voltage while maintaining the driving capability of the microphone at a sufficient level, thereby providing improved sound quality.

Also, the low-pass filter may have a cutoff frequency that is lower than 50 Hz. Thus, such an arrangement is capable of appropriately removing the noise component in an audio band that has an effect on the sound quality.

Also, the low-pass filter may be configured as an RC filter.

Also, the variable gain amplifier may be configured as a non-inverting amplifier.

Also, the variable gain amplifier may comprise: an operational amplifier that receives the reference voltage via a non-inverting input terminal thereof; and a variable voltage-dividing circuit that divides an output voltage of the operational amplifier with a variable dividing ratio, and that inputs the voltage thus divided to an inverting input terminal of the operational amplifier.

Another embodiment of the present invention relates to an audio interface circuit. The audio interface circuit comprises: a bias circuit according to any one of the aforementioned embodiments; a microphone amplifier that amplifies an output signal of the microphone; an A/D converter that converts an output of the microphone amplifier into a digital signal; and a digital signal processing unit that performs signal processing on the digital signal.

Also, the audio interface circuit may monolithically be integrated on a single semiconductor substrate.

Examples of such a “monolithically integrated” arrangement include: an arrangement in which all the circuit components are formed on a semiconductor substrate; and an arrangement in which principal circuit components are monolithically integrated. Also, a part of the circuit components such as resistors and capacitors may be arranged in the form of components external to such a semiconductor substrate in order to adjust the circuit constants. By monolithically integrating the circuit as a single IC, such an arrangement allows the circuit area to be reduced, and allows the circuit elements to have uniform characteristics.

Yet another embodiment of the present invention relates to an electronic device. The electronic device comprises a microphone; and the aforementioned audio interface circuit.

DETAILED DESCRIPTION OF THE INVENTION

In the present specification, the state represented by the phrase “the member A is connected to the member B” includes a state in which the member A is indirectly connected to the member B via another member that does not substantially affect the electric connection therebetween, or that does not damage the functions or effects of the connection therebetween, in addition to a state in which the member A is physically and directly connected to the member B. Similarly, the state represented by the phrase “the member C is provided between the member A and the member B” includes a state in which the member A is indirectly connected to the member C, or the member B is indirectly connected to the member C via another member that does not substantially affect the electric connection therebetween, or that does not damage the functions or effects of the connection therebetween, in addition to a state in which the member A is directly connected to the member C, or the member B is directly connected to the member C.

FIG. 2is an audio interface circuit (which will also be referred to as the “codec IC”)4including a bias circuit10according to an embodiment. The audio interface circuit4includes a microphone amplifier40and an A/D converter42in addition to the bias circuit10, and is configured as a function IC (Integrated Circuit) monolithically integrated on a single semiconductor substrate.

The power supply voltage VDDis supplied to a power supply (VDD) terminal of the audio interface circuit4. Each circuit component included within the audio interface circuit4operates receiving the power supply voltage VDD. A bias resistor R1is arranged between a microphone bias (MICBIAS) terminal and a microphone2. The MICBIAS terminal is connected to a phase compensation capacitor C1. Furthermore, the microphone2is connected to a microphone input (MICIN) terminal of the audio interface circuit4via a DC blocking capacitor C2.

The microphone amplifier40amplifies an audio electric signal input to the MICIN terminal. The A/D converter42converts the output of the microphone amplifier40into a digital audio signal.

The bias circuit10generates a bias voltage VBIAS, and supplies the bias voltage VBIASthus generated to the microphone2via the MICBIAS terminal and the resistor R1. The bias circuit10includes a voltage source102, a variable gain amplifier104, a low-pass filter106, and a voltage follower amplifier108.

The voltage source102generates a reference voltage VREF. For example, the voltage source102includes an LDO (Low Drop Output)110and resistors R11through R13. The LDO110receives the power supply voltage VDD, and generates an internal power supply voltage VREGstabilized to a predetermined level. The internal power supply voltage VREGis also supplied to an unshown digital circuit and the like, in addition to the voltage source102. For example, the power supply voltage VDDis set to 3.3 V, and the internal power supply voltage VREGis set to 1.8 V.

The internal power supply voltage VREGis divided by means of the resistors R11and R12so as to generate the reference voltage VREF. The reference voltage VREFis supplied via the resistor R13to the variable gain amplifier104configured as a downstream stage. In a case in which a first operational amplifier112has a sufficiently high input impedance, the resistor R13may be omitted.

The variable gain amplifier104amplifies the reference voltage VREF. The variable gain amplifier104is configured as a non-inverting amplifier, and includes the first operational amplifier112and a voltage-dividing circuit114. The reference voltage VREFis input to the non-inverting input terminal (+) of the first operational amplifier112. The voltage-dividing circuit114divides the output voltage VO1of the first operational amplifier112with a variable dividing ratio, and inputs the output voltage thus divided to the inverting input terminal (−) of the first operational amplifier112. The voltage-dividing circuit114includes resistors R21and R22. The dividing ratio K is represented by the following Expression (1).
K=R22/(R21+R22)  (1)

At least one of the resistors R21and R22is configured as a variable resistor. The variable gain amplifier104has a gain g represented by the following Expression (2).
g=1/K=(R21+R22)/R22  (2)

The low-pass filter106removes the high-frequency component of the output signal of the variable gain amplifier104. The low-pass filter106may preferably be configured to allow only a DC component having a frequency of 0 Hz to pass through. Thus, the low-pass filter106preferably has a cutoff frequency fc that is as low as possible, and specifically, preferably has a cutoff frequency of 50 Hz or less. More preferably, the cutoff frequency fc is preferably lower than the lower limit of the frequency band of the microphone2. The low-pass filter106is configured as an RC filter including a resistor R31and a capacitor C31configured as an external component.

The voltage follower amplifier108receives the output voltage VO2of the low-pass filter106. The output voltage of the voltage follower amplifier108is supplied to the microphone2as the bias voltage VBIAS. The voltage follower amplifier108includes a second operational amplifier116arranged such that its output terminal and its inverting input terminal are short-circuited.

The above is the configuration of the bias circuit10. Next, description will be made regarding the operation thereof.

FIG. 2shows the noise contamination paths (i) through (iii) in the bias circuit10via which noise contamination can occur from a ground line44. The noise contamination via such paths is removed or attenuated by means of the low-pass filter106.

Furthermore, the voltage follower amplifier108is arranged as a downstream stage of the low-pass filter106. The voltage follower amplifier108is configured such that only a lower-side power supply terminal118of the second operational amplifier116is connected to the ground line44. That is to say, there is no noise contamination path except for the line that connects the lower-side power supply terminal118and the ground line44.

The bias circuit10is capable of reducing noise that reaches the MICIN terminal from the ground line44via the bias circuit10, as compared with the bias circuit30shown inFIG. 1. Thus, such an arrangement is capable of improving the S/N ratio of the input signal input to the microphone amplifier40, thereby providing improved sound quality.

In order to confirm the effect of the bias circuit10shown inFIG. 2, the spectrum of the input signal input to the MICIN terminal was measured for each of the audio interface circuits shown inFIGS. 1 and 2.FIG. 3Ashows the spectrum of the output signal of the audio interface circuit shown inFIG. 2.FIG. 3Bshows the spectrum of the output signal of the audio interface circuit shown inFIG. 1. As shown inFIGS. 3A and 3B, directing attention to a frequency in the vicinity of 420 Hz, for example,FIG. 3Bshows a noise level of −70 dBFS (decibels relative to full scale). In contrast,FIG. 3Ashows a noise level of −110 dBFS. Thus, it has been confirmed that such an arrangement provides improvement of 40 dB. As shown inFIG. 3A, in other frequencies, the bias circuit10shown inFIG. 2provides a reduced noise floor level of −110 dBFS or less. Thus, it has been confirmed that such an arrangement provides an improved S/N ratio.

FIG. 4is a block diagram showing an electronic device500including an audio interface circuit4. The electronic device500is configured as a device having a recording function or a telephone calling function, such as a camcorder, digital still camera, cellular phone terminal, tablet terminal, or the like. The electronic device500includes the microphone2, the audio interface circuit4, and memory6.

The audio interface circuit4further includes a DSP (Digital Signal Processor)46arranged as a downstream stage of the A/D converter42. The DSP46performs various kinds of signal processing on an audio signal converted in the form of a digital signal by the A/D converter42. Examples of such signal processing provided by the DSP46include audio encoding, equalization, volume control, and the like. The audio signal thus encoded by the DSP46is stored in the memory6.

FIGS. 5A and 5Bare diagrams each showing the electronic device500. An electronic device500ashown inFIG. 5Ais configured as a camcorder, and includes an image acquisition element502, an image processing circuit504, a lens506, and the like, in addition to the microphone2, the audio interface circuit4, and the memory6. The audio interface circuit4and the image processing circuit504are mounted on a substrate510.

After light passes through the lens506, the light is converted by means of the image acquisition element502into an electric signal. The electric signal thus converted is subjected to image processing by means of the image processing circuit504. The electric signal thus subjected to image processing is encoded, and the signal thus encoded is stored in the memory6. In parallel with these steps, the microphone2and the audio interface circuit4operate together to record voice and sound.

The electronic device500bshown inFIG. 5Bis configured as a smartphone. In the electronic device500b, the microphone2and the audio interface circuit4are used to provide a voice call. The digital audio signal generated by the audio interface circuit4is input to a baseband circuit512. The baseband circuit512encodes the digital audio signal. An RF (Radio Frequency) circuit514converts the signal output from the baseband circuit512into a high-frequency signal, and transmits the high-frequency signal via an unshown antenna.

Description has been made regarding the present invention with reference to the embodiment. The above-described embodiment has been described for exemplary purposes only, and is by no means intended to be interpreted restrictively. Rather, it can be readily conceived by those skilled in this art that various modifications may be made by making various combinations of the aforementioned components or processes, which are also encompassed in the technical scope of the present invention. Description will be made below regarding such modifications.

The low-pass filter106shown inFIG. 2may be configured as an LC filter.

The low-pass filter106and the voltage follower amplifier108may be monolithically configured as an active filter.FIG. 6is a circuit diagram showing a bias circuit10aaccording to a second modification. A low-pass filter106aand a voltage follower amplifier108form a second-order active filter. It should be noted that the order of the low-pass filter is not restricted in particular. A capacitor of the low-pass filter106ais connected as an external component to the audio interface circuit4. Such a modification provides the same effects as those provided by the circuit shown inFIG. 2.

The variable gain amplifier104may be configured as an inverting amplifier. Also, the voltage source102may have a configuration that differs from that of a voltage-dividing circuit. For example, the voltage source102may be configured using a constant-voltage circuit including a Zener diode.

The usage of the present invention is not restricted to such audio signal processing, i.e., is not restricted to such a bias circuit for a microphone. Also, the present invention is applicable to other usages and fields that require such a low-noise bias circuit. That is to say, the present invention involves such usages and fields.