WO2008100055A2

WO2008100055A2 - Microphone amplifier

Info

Publication number: WO2008100055A2
Application number: PCT/KR2008/000817
Authority: WO
Inventors: Jin Hyo Lee; Kyu Hong Lee; Hee Cheon Shin; Hun Ill Youn
Original assignee: Rfsemi Technologies, Inc.
Priority date: 2007-02-16
Filing date: 2008-02-12
Publication date: 2008-08-21
Also published as: WO2008100055A3; KR100733288B1; CN101675669B; CN101675669A

Abstract

The present invention relates to a microphone amplifier, including a bias unit for providing an input signal reference potential between a signal input terminal and a ground terminal, a voltage-to-current conversion unit for amplifying the input signal, a constant current source for supplying constant current to the voltage-to-current conversion unit, and a current amplification unit for amplifying variation in current between the constant current source and the voltage-to-current conversion unit. The above-described present invention provides a highly trustworthy microphone amplifier, in which a low-noise characteristic junction field effect transistor and a low- noise characteristic bipolar transistor are combined together, thereby realizing high input impedance, high amplification gain and low current consumption.

Description

[DESCRIPTION]

[invention Title]

MICROPHONE AMPLIFIER

[Technical Field] The present invention relates to microphones, and, more particularly, to a microphone amplifier that has high input impedance, high amplification gain and low current consumption using a Junction Field Effect Transistor (JFET) device and a bipolar transistor device. A microphone amplifier is an amplifier that is used for a microphone for converting voice signals into electrical signals .

[Background Art]

Fig. 1 is a circuit diagram of a prior art capacitor microphone amplifier, in which the microphone amplifier 10 includes a single JFET 15 and a bias unit 13 for providing an operation reference potential to the JFET 15.

The gate electrode of the JFET 15 of the amplifier 10, together with one end of an electrostatic charge capacitor Cmic for generating electrical signals through the vibration of voice signals, is connected to a node ni. The other end of the electrostatic charge capacitor Cmic, together with the source electrode of the JFET 15, is connected to a ground terminal GND. Accordingly, the node ni functions to transmit electrical signals, generated by the electrostatic charge capacitor Cmic, to the JFET 15, and thus it may be called the signal input terminal of the amplifier 10.

The drain electrode of the JFET 15 branches into two lines at a node no, one line is connected to external power VDD through a load resistor RL, and the other line is connected to an output terminal Vout, which is provided to enable the extraction and use of output signals.

Meanwhile, with regard to the bias unit 13 for providing a reference potential to the gate electrode of the JFET 15, one end thereof is connected to the node ni, at which the electrostatic charge capacitor Cmic and the JFET 15 are interconnected, and the other end thereof is connected to the ground terminal GND. The bias unit 13 is constructed by connecting a resistor R and a diode D in parallel.

The electrostatic charge capacitor Cmic is constructed by placing a conductive thin film and a fixed electrode plate opposite each other at a constant interval. Chargeable material is applied to one of the thin film and the fixed electrode plate, and the chargeable material is charged with electrostatic charge as high as tens of volts . When a voice signal is transmitted to the thin film, the thin film vibrates. The vibration of the thin film varies the capacity of the electrostatic charge capacitor Cmic, thereby generating a small electrical signal. When the electrical signal is input to the microphone amplifier 10, variation in current occurs at the output terminal Vout. After the variation in current is amplified by the JFET 15, it is divided into voltages by the load resistor RL and is then output.

The requirements for the amplifier 10 are described below. First, since the output impedance of the electrostatic charge capacitor Cmic is equal to or higher than hundreds of mega ohms, input loss must be minimized by setting the input impedance of the amplifier 10 to a value equal to or higher than hundreds of mega ohms. Second, since a small signal must be amplified, the amplification gain must be high, and the Signal to Noise Ratio (SNR) must be increased by setting noise to a value equal to or lower than -100 dB. Third, since the electrostatic charge capacitor is a part that is chiefly used for portable devices, low power consumption is required such that current consumption is equal to or lower than 500 μA and power supply voltage is equal to or lower than 3 volts . Accordingly, the use of a JFET device is suitable for meeting the above-described requirements .

However, of amplifiers using the same JFETs, an amplification circuit using a single JFET must minimize the parasitic capacitance of a signal input terminal and must have the resistor R of the bias unit 13 formed to have hundreds of mega ohms or more in order to improve input impedance in the audio frequency band of 100 Hz-10 kHz. However, in the case in which the size of the JFET 15 is increased in order to improve the voltage amplification gain of the amplifier 10, the parasitic capacitance is increased, and thus the input impedance is decreased, with the result that the voltage amplification gain is not considerably increased. Furthermore, in the case in which the voltage amplification gain is increased by increasing the operating current in a JFET having a certain size, a problem arises in that current consumption and operating voltage are increased. Even in the case in which the amplification gain is improved by increasing only input impedance, there is a problem in that noise is increased instead. The prior art capacitor microphone amplification circuit using a single JFET device is excellent from the viewpoint of low noise, but has limitations on the realization of high amplification gain, low current and high input impedance characteristics.

In another application, the prior art amplifier must also be used in a Micro-ElectroMechanical System (MEMS) microphone. Unlike the capacitor microphone, the MEMS microphone is configured such that, in Fig. 1, one end of the electrostatic charge capacitor Cmic is connected to an external constant voltage source, instead of the ground terminal GND, and the other end thereof is connected to the input node ni of the amplification circuit. Since, in the prior art capacitor microphone amplifier, the interior of the electrostatic charge capacitor Cmic is charged with electrostatic charge, one end of the electrostatic charge capacitor Cmic is connected to the ground terminal GND and the other end thereof is connected to the input node ni of the amplifier. However, in the case in which there is no electrostatic charge, as in the MEMS microphone, one end of the electrostatic charge capacitor Cmic is connected to an external constant voltage source, rather than the ground terminal GND, and the electrostatic charge capacitor Cmic is charged with applied charge, so that an electrical signal is generated through the vibration of a diaphragm, caused by a voice signal. In this case, since constant voltage, which can be applied from the outside, is as high as tens of volts, the output signal of the electrostatic charge capacitor Cmic is very weak, and thus an amplifier having a high amplification gain is required. Furthermore, in the MEMS microphone, the capacity of the electrostatic charge capacitor Cmic of the diaphragm is low, and thus the output impedance of the diaphragm is high, with the result that the amplifier requires high input impedance.

Although a differential amplifier method using a CMOS device (Korean Unexamined Patent Publication No. 10-2006- 0113925) is disclosed as another method, the CMOS device has higher noise than a JFET device. Furthermore, this method has a defect in that the resistance to external electrical impact is poor because an oxide film is as thin as hundreds of angstroms .

[Disclosure]

[Technical Problem]

Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a microphone amplifier in which a signal amplification circuit, designed to minimize input impedance in order to minimize voltage and current consumption and achieve high amplification gain, is formed on a semiconductor substrate or a chip.

[Technical Solution]

In order to accomplish the above object s, the present invention provides a microphone amplifier having a signal input terminal for providing an input signal, an output terminal for outputting an amplified signal and a ground terminal for providing reference to a circuit, including a bias unit for providing an input signal reference potential between the signal input terminal and the ground terminal; a voltage-to-current conversion unit for amplifying the input signal; a constant current source for supplying constant current to the voltage-to-current conversion unit; and a current amplification unit for amplifying variation in current between the constant current source and the voltage-to-current conversion unit. The microphone amplifier of the present invention further includes a third resistor for adjusting amplification gain of an input signal between the ground terminal, the current amplification unit and the voltage- to-current conversion unit; and a second resistor for adjusting an amount of bias current between the current amplification unit and the third resistor.

In the microphone amplifier of the present invention, the third resistor or the fourth resistor is connected through a conductor. In the microphone amplifier of the present invention, the constant current source is formed of a Junction Field Effect Transistor (JFET), or is formed such that the gate and source electrodes of a first JFET are connected to each other. In the microphone amplifier of the present invention, the bias unit is formed of a first resistor, is formed by connecting a first diode to the first resistor in parallel, and clamps an input signal, or is formed by connecting a second diode to a first diode, connected to the first resistor in parallel, in parallel in opposite directions.

In the microphone amplifier of the present invention, the voltage-to-current conversion unit is formed of a second JFET, is formed by connecting a third JFET to the second JFET in parallel, or is formed by connecting a fourth JFET to the second JFET in series . In the microphone amplifier of the present invention, the current amplification unit is formed of a PNP-type first bipolar transistor, the current amplification unit is a Darlington transistor that includes the PNP-type first bipolar transistor and a PNP-type second bipolar transistor, or is a PNP/NPN Darlington transistor that includes the PNP-type first bipolar transistor and an NPN- type third bipolar transistor.

In the microphone amplifier of the present invention, one or more JFET devices are used in the voltage-to-current conversion unit connected to the signal input terminal of the amplification circuit. Since the sizes of the JFET devices can be freely adjusted regardless of circuit characteristics, adjustment can be easily performed such that high input impedance can be realized based on low noise characteristics. Furthermore, since the amplification circuit has a separate resistor, current gain and power consumption can be adjusted.

[Advantageous Effects]

As described above, the present invention is configured such that low-noise characteristic JFET and bipolar transistors are combined together and are constructed in a minimal space, and thus the present invention has an advantage of being able to more easily adjust the amplification gain, current consumption and input capacitance of a microphone amplification circuit, and an advantage of being able to be manufactured using a small-sized semiconductor chip.

[Description of Drawings]

Fig. 1 is a circuit diagram of a prior art capacitor microphone amplifier;

Fig. 2 is a circuit diagram of a capacitor microphone amplifier according to the present invention;

Fig. 3 is a circuit diagram showing the results of application of respective embodiments in the bias unit, voltage-to-current conversion unit and current amplification unit of Fig. 2;

Figs. 4 and 5 are circuit diagrams showing bias units according to embodiments of the microphone amplifier of Fig. 2; Figs. 6 and 7 are circuit diagrams showing voltage- to-current conversion units according to embodiments of the microphone amplifier of Fig. 2; and

Figs . 8 and 9 are circuit diagrams showing current amplification units according to embodiments of the microphone amplifier of Fig. 2. <Description of reference numerals of principal elements in the drawings>

20: microphone amplifier 21: constant current source 23, 23a, 23b: bias unit 25, 25a, 25b: voltage-to- current conversion unit

27, 27a, 27b: current amplification unit

[Best Mode]

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings .

Fig. 2 is a circuit diagram of a capacitor microphone amplifier according to the present invention.

Referring to Fig. 2, the signal input terminal of a microphone amplifier 20 according to the present invention is a node ni at which a voice signal, detected and converted by an electrostatic charge capacitor Cmic, is provided as an electrical signal. Since the intensity of the electrical signal entering the signal input terminal is weak, the current is converted and amplified by a voltage- to-current conversion unit 25.

One end a of a bias unit 23 for stabilizing the bias of the electrical signal, entering from the electrostatic charge capacitor Cmic, by adjusting it to a constant value is connected to the node ni, and the other end b thereof is connected to a ground terminal GND. The output side of the amplifier 20 branches into two lines at a node no, one line is connected to external power VDD through a load resistor RL, and the other line is connected to an output terminal Vout, which is provided to enable the extraction and use of an output signal. The node no, which coincides with or is short-circuited to the output terminal Vout through a conductor, supplies current to the amplifier 20. Furthermore, with regard to the output terminal Vout, voltage division is performed on the electrical signal, which is input to the signal input terminal, converted into current, amplified and output as current, using the load resistor RL, thus causing voltage to be output through the output terminal Vout.

Here, all of the input signals are based on the ground terminal GND. The voltage-to-current conversion unit 25 for amplifying and converting the input signal into a current signal is formed of an N channel JFET having characteristics superior to those of other semiconductor devices . A constant current source 21 for supplying constant current to the voltage-to-current conversion unit 25 is formed of a first JFET Tl . The gate and source electrodes of the first JFET Tl are short-circuited, and the drain electrode thereof is short-circuited to the node no on the output side of the amplifier 20.

Since the constant current source 21 causes constant current to flow, varied current flows through a terminal g of the current amplification unit 27 when the current flowing through the voltage-to-current conversion unit 25 varies in response to variation in the input signal. The voltage-to-current conversion unit 25 is formed of one or more N channel JFETs, and has three terminals c, d and e. One terminal c is short-circuited to the source and gate of the constant current source 21 and the node nl. Another terminal d is connected to the node ni, which is short-circuited to the terminal a of a bias unit 23 as described above, and is supplied with an electrical signal. The remaining terminal e is connected to one end of a second resistor R2. The other end of the second resistor R2 branches into two lines at a node n2, one line is short- circuited to the ground terminal GND through a third resistor R3, and the other line is short-circuited to one terminal h of the current amplification unit 27.

The current amplification unit 27 is formed of one or more bipolar transistors, and has three terminals f, g and h. One terminal f is connected to the node no, which is short-circuited to the output terminal Vout. Another terminal g is connected to the source and gate of the constant current source 21 at the node nl. Furthermore, the remaining terminal h is connected to the node n2, at which the second and third resistors R2 and R3 are short- circuited to each other, as described above. The second resistor R2 adjusts the reference voltage of the terminal e in response to variation in current and variation in input potential based on the device characteristics of the voltage-to-current conversion unit 25, thereby adjusting the power consumption of the amplifier 20. Furthermore, a conductor, rather than the second resistor R2, performs connection depending on the characteristics of the JFET device of the voltage-to- current conversion unit 25. Furthermore, the third resistor R3 adjusts current gain by varying the potential difference between the terminals e and d of the voltage-to-current conversion unit 25 depending on the intensity of current flowing through the third resistor R3. Furthermore, the case in which the third resistor R3 is replaced with the conductor has an advantage in that the amplification gain of the current amplification unit 27 is maximized.

Fig. 3 is a circuit diagram showing the results of application of respective embodiments in the bias unit, voltage-to-current conversion unit and current amplification unit of Fig. 2.

Referring to Fig. 3, a bias unit 23a is formed of a first resistor Rl, and a voltage-to-current conversion unit 25a is formed of a second JFET T2. The gate, drain and source electrodes of the second JFET T2 are short-circuited to one terminal d, another terminal c and the remaining terminal e of the voltage-to-current conversion unit 25a, respectively. The structure of the voltage-to-current conversion unit 25a enables the characteristics of high input impedance and low input capacitance, required at the signal input terminal of the amplifier 20, to be maximally utilized. Furthermore, the current amplification unit 27a is formed of a PNP-type first bipolar transistor Ql according to an embodiment of the present invention. The base, emitter and collector electrodes of the first bipolar transistor Ql are short-circuited to one terminal g, another terminal f and the remaining terminal h of the current amplification unit 27a, respectively.

Fig. 4 is a circuit diagram showing the construction of a bias unit according to another embodiment in the amplifier of Fig. 2, and Fig. 5 is a circuit diagram showing the construction of a bias unit according to still another embodiment in the amplifier of Fig. 2.

Referring to Fig. 4, the bias terminal 23b of the amplifier 20 has a function of clamping the input signal by connecting a first diode Dl in parallel with the first resistor Rl of the bias terminal 23a.

Referring to Fig. 5, the bias terminal 23c of the amplifier 20 is configured to connect the first and second diodes Dl and D2 in parallel with each other in opposite directions when the bias terminal 23c is constructed using a first resistor Rl and the first and second diodes Dl and D2. This further increases the symmetry of the input signal compared to the structure of the bias terminal 23b of Fig. 4a.

Fig. 6 is a circuit diagram showing the construction of a voltage-to-current conversion unit according to another embodiment in the amplifier of Fig. 2, and Fig. 7 is a circuit diagram showing the construction of a voltage- to-current conversion unit according to still another embodiment in the amplifier of Fig. 2. Referring to Fig. 6, in the voltage-to-current conversion unit 25b of the amplifier 20, a second JFET T2 and a third JFET T3 are connected in parallel with each other on the basis of the drain and source electrodes thereof. In this case, the gate electrode of the third JFET T3 is short-circuited to the third JFET T3's own source electrode .

The structure of the voltage-to-current conversion unit 25b decreases input capacitance by dividing the gate electrode of the second JFET T2, through which the signal is input, into two parts.

Referring to Fig. 7, the voltage-to-current conversion unit 25c of the amplifier 20 may be formed of second and fourth JFETs T2 and T4. Here, the second and fourth JFETs are connected in parallel with each other on the basis of the drain and source electrodes thereof.

The drain electrode of the second JFET T2 is connected to the source electrode of a fourth JFET T4. The gate electrode of the second JFET T2 is short-circuited to one terminal d of the voltage-to-current conversion unit 25c. The drain electrode of the fourth JFET T4 is short- circuited to another terminal c of the voltage-to-current conversion unit 25c. Furthermore, the source electrode of the second JFET T2 and the gate electrode of the fourth JFET T4 are short-circuited to the remaining terminal e of the voltage-to-current conversion unit 25c. The structure of the voltage-to-current conversion unit 25c has an advantage in that the second and fourth JFETs T2 and T4 have divided voltages, and thus the voltage between the source and drain of the second JFET T2 is reduced, thereby minimizing the size of the second JFET T2. In the structures of Figs. 6 and 7, the geometrical size of the second JFET T2 can be reduced, thereby being advantageous in the reduction in input capacitance and the size of the device.

Fig. 8 is a circuit diagram showing the construction of a current amplification unit according to another embodiment in the amplifier of Fig. 2, and Fig. 9 is a circuit diagram showing the construction of a current amplification unit according to still another embodiment in the amplifier of Fig. 2. Referring to Fig. 8, the current amplification unit 27b of the amplifier 20 may be formed of PNP-type first and second bipolar transistors Ql and Q2. The base electrode of the first bipolar transistor Ql is short-circuited to one terminal g of the current amplification unit 27b, and the base electrode of the second bipolar transistor Q2 is short-circuited to the emitter electrode of the first bipolar transistor Ql . The collector electrodes of the first and second bipolar transistors Ql and Q2 are connected to each other, and are short-circuited to another terminal h of the current amplification unit 27b. Furthermore, the emitter electrode of the second bipolar transistor Q2 is short-circuited to the remaining terminal f of the current amplification unit 27b.

The structure of the current amplification unit 27b adopts a PNP Darlington bipolar transistor having the above-described construction, thereby being able to improve the current gain of the current amplification unit 27.

Referring to Fig. 9, the current amplification unit 27c of the amplifier 20 may be formed of first and third bipolar transistors Ql and Q3. In this case, the third bipolar transistor is an NPN type. The base electrode of the first bipolar transistor Ql is short-circuited to one terminal g of the current amplification unit 27c, and the base electrode of the third bipolar transistor Q3 is short- circuited to the collector electrode of the first bipolar transistor Ql. The emitter electrode of the first bipolar transistor Ql and the collector electrode of the third bipolar transistor Q3 are connected to each other, and are short-circuited to another terminal f of the current amplification unit 27c. Furthermore, the emitter electrode of the third bipolar transistor Q3 of the current amplification unit 27c is short-circuited to the remaining terminal h.

The structure of the current amplification unit 27c adopts a PNP/NPN Darlington bipolar transistor, in which the first bipolar transistor Ql and the third bipolar transistor Q3 are constructed in two stages, thereby improving the current gain of the current amplification unit 27.

In still another embodiment, all of the JFETs used in the constant current source 21 and the voltage-to-current conversion unit 25 may be replaced with Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). Such a microphone amplifier can realize the above-described circuit characteristics .

The microphone amplifier of the present invention is implemented on a semiconductor substrate or a chip.

The rights of the present invention are not limited to the above-described embodiments, but are defined by the description of the claims. It will be apparent that those having ordinary skill in the field of the present invention can make various modifications and variations within the scope of rights set forth in the claims. [industrial Applicability]

The present invention relates to a microphone amplifier implemented on a semiconductor substrate or a chip. The microphone amplifier is designed to minimize voltage and current consumption and to minimize input capacitance in order to realize high amplification gain, thus being able to be universally and widely used in digital electronic devices, which are decreasing in size.

Claims

[CLAIMS]

[Claim l]

A microphone amplifier having a signal input terminal for providing an input signal, an output terminal for outputting an amplified signal and a ground terminal for providing reference to a circuit, comprising: a bias unit for providing an input signal reference potential between the signal input terminal and the ground terminal; a voltage-to-current conversion unit for amplifying the input signal; a constant current source for supplying constant current to the voltage-to-current conversion unit; and a current amplification unit for amplifying variation in current between the constant current source and the voltage-to-current conversion unit.

[Claim 2]

The microphone amplifier as set forth in claim 1, further comprising: a third resistor for adjusting amplification gain of an input signal between the ground terminal, the current amplification unit and the voltage-to-current conversion unit; and a second resistor for adjusting an amount of bias current between the current amplification unit and the third resistor .

[Claim 3]

The microphone amplifier as set forth in claim 2, wherein the third resistor is connected through a conductor.

[Claim 4]

The microphone amplifier as set forth in claim 2, wherein the second resistor is connected through a conductor.

[Claim 5]

The microphone amplifier as set forth in claim 1, wherein the constant current source is formed of a Junction Field Effect Transistor (JFET) .

[Claim 6] The microphone amplifier as set forth in claim 1, wherein the constant current source is formed such that gate and source electrodes of a first JFET are connected to each other.

[Claim 7] The microphone amplifier as set forth in claim 1, wherein the bias unit is formed of a first resistor. [Claim 8]

The microphone amplifier as set forth in claim 7, wherein the bias unit is formed by connecting a first diode to the first resistor in parallel, and clamps an input signal.

[Claim 9]

The microphone amplifier as set forth in claim 7, wherein the bias unit is formed by connecting a second diode to a first diode, connected to the first resistor in parallel, in parallel in opposite directions.

[Claim lθ]

The microphone amplifier as set forth in claim 1, wherein the voltage-to-current conversion unit is formed of a second JFET.

[Claim ll]

The microphone amplifier as set forth in claim 10, wherein the voltage-to-current conversion unit is formed by connecting a third JFET to the second JFET in parallel.

[Claim 12] The microphone amplifier as set forth in claim 10, wherein the voltage-to-current conversion unit is formed by connecting a fourth JFET to the second JFET in series .

[Claim 13]

The microphone amplifier as set forth in claim 1, wherein the current amplification unit is formed of a PNP- type first bipolar transistor.

[Claim 14]

The microphone amplifier as set forth in claim 13, wherein the current amplification unit is a Darlington transistor that comprises the PNP-type first bipolar transistor and a PNP-type second bipolar transistor.

[Claim 15]

The microphone amplifier as set forth in claim 13, wherein the current amplification unit is a PNP/NPN

Darlington transistor that comprises the PNP-type first bipolar transistor and an NPN-type third bipolar transistor.

[Claim 16]

The microphone amplifier as set forth in claim 1, wherein a signal entering the signal input terminal is an electrical signal into which a voice signal recognized by the electrostatic charge capacitor is converted.