JP2005109842A - Amplifier circuit for condenser microphone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an amplifier circuit for a condenser microphone having little noise and little variations in voltage gain. <P>SOLUTION: This amplifier circuit 10 for a condenser microphone is connected to one terminal of an ECM (electret condenser microphone) 12 through a terminal A. A gate is connected to the terminal A, a drain is connected to the ground, one end of a constant current source is connected to a source and a p channel MOSFET 14 (metal oxide semiconductor film electric field-effect transistor) constituting a source follower is connected to the source. A source follower output by a p channel MOSFET 14 is amplified by an emitter follower of an NPN type transistor 20 and outputted from a terminal C. The constant current source 16 is connected to the source of the p channel MOSFET 14, an input of the constant current source 16 is connected to the collector of the NPN type transistor 20 to be a combination of a power source and an output terminal to the outside. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an amplifier for a condenser microphone, and more particularly to an electret condenser microphone amplifier using a MOSFET.

  A condenser microphone converts a sound pressure into an electric signal by using a change in capacitance. The capacitor is formed by storing static electricity between the fixed electrode and the conductive diaphragm by an external DC power source. When the diaphragm vibrates due to sound pressure, the distance from the fixed electrode changes, and The capacity changes. Condenser microphones are characterized by good followability to the original sound and wide dynamic range. An electret condenser microphone (ECM) applies an electret effect to an electrode of a condenser microphone. Here, the electret effect means an effect that when a high electric field is applied to a polymer material such as a synthetic resin, a film, or a plastic, a positive or negative charge remains in the material even if the electric field is removed. The ECM does not require an external DC power source, has a feature of being small, light, and low in cost, and is particularly widely used for mobile devices such as mobile phones.

  The capacitance of the ECM is generally 2 to 5 pF, and an input circuit with an input impedance of several GΩ is required to extract a sound pressure signal to a low audible band. As an amplifier circuit for transmitting a signal received with such a high input impedance to a subsequent circuit, a grounded source circuit that inputs an ECM output signal to the gate of a junction field effect transistor (FET) is used (for example, Patent Document 1).

JP 2000-260786 A

  In a common source circuit using a junction FET, [voltage gain] = gm × RD is determined (gm: mutual conductance, RD: drain resistance). In the production process of the junction type FET, it is difficult to control gm to be constant, and generally has a characteristic of varying from about 50% to 200%, and ranking is performed according to gm. There is a problem that the voltage gain of the common source circuit varies with the variation in gm.

  Furthermore, there are gate leakage currents in junction FETs, especially in high-temperature operating environments where gate leakage currents increase. In common source circuits using junction FETs, noise increases and reactive power consumption increases. There is.

  Therefore, if the junction FET is replaced with a MOS FET having a very small gate leakage current, the ECM outputs a zero bias signal, and the input circuit receiving the signal must adopt a source follower configuration of a p-channel MOSFET. I don't get it. The source follower configuration of this p-channel MOSFET has a voltage gain of about 1, and there is a problem that it cannot be amplified to a signal level for input to a subsequent circuit by simply replacing it.

  Accordingly, the present invention provides an amplifying circuit for a condenser microphone that performs amplification by inputting the output signal of the condenser microphone to the gate of a p-channel MOS-FET.

  The amplifier circuit for a condenser microphone of the present invention has a p-channel MOSFET that constitutes a source follower with a gate connected to one terminal of the condenser microphone, a drain grounded, and a source connected to the output of a constant current source. (Metal oxide field effect transistor), an NPN transistor having a base connected to the source of the p-channel MOSFET, an emitter grounded via an emitter resistor, and a collector connected to a signal output terminal and an external power supply terminal; A collector resistor connected between the collector of the NPN transistor and a power source, and the input of the constant current source has a three-terminal configuration connected to the collector of the NPN transistor.

  According to this configuration, since the output signal of the ECM is input to the gate of the p-channel MOSFET having a small gate leakage current, noise generated from the amplifier circuit can be suppressed. Further, since the voltage gain is determined by the resistance value of the resistor that can be produced with high accuracy, the influence of the gm variation of the FET can be eliminated.

  In another aspect of the present invention, a p-channel MOSFET (source channel follower) is configured such that the gate is connected to one terminal of the condenser microphone, the drain is grounded, the source is connected to the output of the constant current source. A metal oxide field effect transistor), an NPN transistor having a base connected to the source of the p-channel MOSFET, an emitter grounded via an emitter resistor, and a collector connected to the signal output and the first external power supply terminal And a collector resistor connected between the collector of the NPN transistor and a power source, and the input of the constant current source has a four-terminal configuration connected to a second external power source.

  According to this configuration, since the source potential of the p-channel MOSFET is not affected by the voltage drop of the collector resistance connected between the collector of the NPN transistor and the power supply, the dynamic range of the output can be widened. The power reduction characteristics can be improved.

  According to another aspect of the present invention, a gate is connected to one terminal of the condenser microphone, a drain is grounded, a source is connected to one end of a constant current source, and a p-channel MOSFET constituting a source follower is provided. , An n-channel MOSFET whose gate is connected to the source of the p-channel MOSFET, whose source is grounded via an emitter resistor, and whose drain is connected to a signal output terminal / external power supply terminal; A drain resistor connected in between, and the input of the constant current source has a three-terminal configuration connected to the drain of the n-channel MOSFET.

  According to another aspect of the present invention, a gate is connected to one terminal of the condenser microphone, a drain is grounded, a source is connected to one end of a constant current source, and a p-channel MOSFET constituting a source follower is provided. An n-channel MOSFET having a gate connected to the source of the p-channel MOSFET, a source grounded via an emitter resistor, and a drain connected to the signal output and first external power supply terminal; and a drain of the n-channel MOSFET; And a drain resistor connected between power supplies, and the input of the constant current source has a four-terminal configuration connected to a second external power supply.

  In another aspect of the invention, the constant current source is a resistor.

  In another aspect of the present invention, a diode connected in the forward direction is provided between the gate and drain of the p-channel MOSFET.

  According to this configuration, it is possible to prevent an excessive voltage from being applied to the gate of the p-channel MOSFET when an excessive voltage is applied to the input.

  In another aspect of the present invention, an input resistor connected between the gate and drain of the p-channel MOSFET is provided.

  According to this configuration, the capacitor / microphone amplifier circuit can be reduced in size, weight, and cost by reducing the number of circuit elements.

  As described above, in the present invention, the output of the capacitor microphone is input to the gate of the p-channel MOSFET to perform amplification. Therefore, it is possible to suppress the noise due to the gate leakage current, the reactive power consumption, and the variation in voltage gain due to the variation in gm, which are problems in the configuration of inputting to the gate of the junction FET.

  In addition, since it can be a three-terminal configuration like an amplifier circuit using a conventional junction FET, it is not necessary to change peripheral circuits, production facilities, etc. by replacing it as it is, and the circuit of the present invention can be widely applied. can do.

  Further, according to the amplifier circuit of the present invention, the voltage gain is determined by the resistance value that can be manufactured with high accuracy, so that it is not necessary to rank the transistor due to manufacturing variations of transistors, and the voltage gain can be produced at a low cost.

  Further, as the ECM unit, the power supply terminal and the output terminal are made common and the external terminal has a two-terminal configuration, which is advantageous for downsizing.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
FIG. 1 is a circuit diagram of a condenser microphone amplifying circuit 10 according to Embodiment 1 of the present invention.

  The condenser microphone amplifying circuit 10 has a three-terminal configuration including terminals A, B and C.

  One end of an electret condenser microphone (ECM) 12 is electrically connected to a terminal A. The terminal A is connected to the gate of the p-channel MOSFET 14, and the capacitance change generated in the ECM is caused by the drain of the p-channel MOSFET 14. It is converted into a change in current. Here, the air vibration input to the ECM 12 is indicated as a microphone input signal 1 as an equivalent circuit.

  The output of the constant current source 16 is connected to the source of the p-channel MOSFET 14, and a constant current of 5 μA, for example, is supplied. The constant current amount is desirably smaller in view of power consumption, noise reduction, and voltage reduction characteristics. Further, the drain is connected to the terminal B, and the terminal B is grounded. The source is connected to the base of the NPN transistor 20, and a difference current between the current from the constant current source 16 and the drain current of the p-channel MOSFET 14 flows to the base of the NPN transistor 20.

  The NPN transistor 20 constitutes a grounded emitter circuit, and the collector is connected to the input of the constant current source 16 and the power supply / output terminal C, and the output signal of the capacitor / microphone amplifier circuit 10 is output from the terminal C to the outside. To do. The terminal C and the external power source are connected to the power source through the collector resistor 24, and the output signal is obtained as a voltage drop generated at the collector resistor 24. The emitter is connected to the emitter resistor 22. For example, if the emitter resistor 22 is 2 kΩ, an emitter current of 50 μA causes a voltage drop of 100 mV, and the output signal is output from the terminal C as this voltage drop. The

  The voltage gain Av of the capacitor / microphone amplifying circuit 10 is given by Av = Rc / Re by the resistance value Rc of the collector resistor 24 connected between the terminal C and the external power supply and the resistance value 22 of the emitter resistor 22. Determined.

  Therefore, in the capacitor / microphone amplifying circuit 10 according to the first embodiment, the voltage gain Av includes the resistance value Rc of the collector resistor 24 and the resistance value 22 of the emitter resistor 22 that can be produced with higher accuracy than the mutual conductance of the FET. Therefore, the voltage gain Av can be made uniform with high accuracy, and there is no need for ranking due to manufacturing variations of the transistors, so that the production can be performed at low cost.

  It is not necessary to connect anything between the gate and drain of the p-channel MOSFET 14, but an input resistor 18 may be connected as shown in FIG. With this input resistance, the DC component of the gate of the p-channel MOSFET 14 can be stabilized to the ground potential via the input resistance 18. The input resistor 18 to be connected determines the input impedance of the capacitor / microphone amplifier circuit 10 and is several GΩ in order to secure the characteristic to the low range of the audible band with respect to the ECM capacitance of 2 to 5 pF. Resistance value.

  Here, a diode may be used instead of the input resistor 18. By connecting the diode in the forward direction between the gate of the p-channel MOSFET 14 and the ground, a resistance value of about 40 GΩ was obtained in the vicinity of the zero bias of the gate potential, and an excessive voltage was applied to the terminal A. In some cases, the current can be bypassed with a forward voltage to protect the p-channel MOSFET 14 from breakdown.

In addition, a diode is preferably forward-connected in parallel with the input resistor 18. According to this configuration, the input impedance can be set to a desired value by the resistance value of the resistor, and the diode is prevented from applying a voltage higher than the diode V _F (about 0.7 V) to the p-channel MOSFET 14. Can do.

  Further, the NPN transistor 20 can be replaced with an n-channel MOSFET. In this case, a source common circuit is formed, the source of the p-channel MOSFET 14 is connected to the gate, the input is connected to the input terminal of the constant current source 16 and the power supply / output terminal C, and the source is connected to the resistor 22. Then, the source potential of the p-channel MOSFET 14 is converted into the drain current of the n-channel MOSFET. The voltage gain Av is represented by Av = −Rc / (, where gm is the mutual inductance of the n-channel MOSFET, Re is the resistance value of the resistor 22, and Rc is the resistance value of the resistor 24 connected between the terminal C and the external power supply. (1 / gm) + Re).

  The constant current source 16 can be replaced with a resistor. Since the external power supply is generally a constant voltage power supply, the resistor operates as a constant current source. According to this configuration, the capacitor / microphone amplifier circuit 10 according to the first embodiment can be reduced in size, weight, and cost.

[Embodiment 2]
FIG. 2 is a circuit diagram of the condenser microphone amplifying circuit 11 according to the second embodiment of the present invention. The capacitor / microphone amplifier circuit 11 has a four-terminal configuration in which the power source of the constant current source 16 is independent of the collector terminal C of the NPN transistor 20 and is a terminal D, as compared with the capacitor / microphone amplifier circuit 10. .

  The dynamic range of the condenser microphone amplifying circuit 10 is proportional to the amplitude of the source potential of the p-channel MOSFET 14. Therefore, in the case of the capacitor / microphone amplifier circuit 10 according to the first embodiment, the source potential of the p-channel MOSFET 14 is equal to or lower than the potential obtained by subtracting the voltage drop at the collector resistor 24 from the external power supply voltage. The collector resistor 24 determined from the voltage gain requirement limits the dynamic range.

  In order to generate a high voltage gain in the capacitor / microphone amplifier circuit 10, it is necessary to increase the resistance value of the collector resistor 24. However, when the resistance value of the collector resistor 24 is increased, there is a problem that noise generated from the resistor is increased, a voltage drop generated in the collector resistor 24 is increased, and a power reduction characteristic is deteriorated.

  On the other hand, since the capacitor / microphone amplifier circuit 11 has a configuration in which the power source of the constant current source 16 is independent from the collector terminal C of the NPN transistor 20, the source potential of the p-channel MOSFET 14 directly takes an amplitude up to the external power source voltage. Thus, the influence of the voltage drop at the collector resistor 24 is eliminated, and amplification having a wide dynamic range that effectively uses the external power supply voltage can be performed without increasing noise.

  In addition, since the power source of the constant current source 16 is independent from the collector terminal C, it is possible to eliminate the influence of noise due to the bias current of the constant current source 16 on the output.

[Embodiment 3]
FIG. 3 is a diagram showing a configuration of the capacitor / microphone amplifier circuit 30 according to the third embodiment of the present invention. The condenser microphone amplifying circuit 30 is configured such that the constant current source 16 is composed of two current mirror circuits with respect to the condenser microphone amplifying circuit 10.

  Current flowing in the circuit from the external power supply voltage via the collector resistor 24 connected to the outside, the resistor 46, the base-emitter of the NPN transistor 42 whose collector and base are short-circuited, and the diode 40 connected in the forward direction. When the external power supply voltage and the collector resistor 24 connected to the outside are determined, the capacitor / microphone amplifier circuit 30 can determine the constant current I0 only by the resistor 46.

  The NPN transistors 42 and 44, the diode 40, and the resistor 48 constitute a first current mirror circuit, and a constant current flowing through the resistor 48 is obtained by dividing the voltage drop of the diode 40 at the constant current I0 by the resistor 48. I1. If the base current of the NPN transistor 44 is small and ignored, the emitter current of the PNP transistor 50 becomes a constant current I1.

  The PNP transistor 50 and the PNP transistor 52 whose collector and base are short-circuited constitute a second current mirror circuit. The emitter current of the PNP transistor 52 becomes a constant current I1 and is input to the source of the p-channel MOSFET 14. Constant current.

  Here, as described in the second embodiment, it is preferable that the power supply terminal C of the NPN transistor 20 and the power supply terminal D of the constant current source are independent and have a four-terminal configuration.

  FIG. 4 shows a case where the capacitor / microphone amplifier circuit 30 has a three-terminal configuration in which the power supply terminal of the NPN transistor 20 and the power supply terminal of the constant current source are common, and the power supply terminal of the NPN transistor 20 and the power supply of the constant current source. It is the figure which plotted the relationship between the external power supply voltage at the time of making a terminal independent and having a 4 terminal structure, and a voltage gain.

  The capacitor / microphone amplifier circuit 30 has a certain voltage gain characteristic when the external power supply voltage is equal to or higher than a predetermined external power supply voltage. The amplitude of the potential decreases, and the voltage gain decreases rapidly.

  On the other hand, in the case of the four-terminal configuration, the source potential of the p-channel MOSFET 14 is not affected by the voltage drop in the collector resistor 24, so that a constant voltage gain characteristic can be maintained until the external power supply voltage is 1.5V.

  Therefore, the voltage reduction characteristic can be improved when the four-terminal configuration is used compared to the three-terminal configuration.

1 is a circuit diagram of a capacitor / microphone amplifier circuit 10 according to a first embodiment of the present invention. FIG. It is a circuit diagram of the amplifier circuit 11 for capacitor | condenser microphones which concerns on Embodiment 2 of this invention. It is a circuit diagram of the amplifier circuit 30 for capacitor | condenser microphones which concerns on Embodiment 3 of this invention. In the circuit diagram of the capacitor / microphone amplifier circuit 30 according to the third embodiment of the present invention, the relationship between the external power supply voltage and the voltage gain when the three-terminal configuration is used and when the four-terminal configuration is used is plotted.

Explanation of symbols

  1 Microphone input signal 10, 11, 30 Microphone amplifier circuit, 14 p-channel MOSFET, 16 constant current source, 18 input resistance, 20, 42, 44, 50, 52 transistor, 22, 24, 46, 48 resistance, 40 diode.

Claims (7)

An amplifier circuit for a condenser microphone,
A p-channel MOSFET (metal oxide field effect transistor) constituting a source follower with a gate connected to one terminal of the capacitor microphone, a drain grounded, and a source connected to the output of a constant current source;
An NPN transistor having a base connected to the source of the p-channel MOSFET, an emitter grounded via an emitter resistor, and a collector connected to a signal output terminal / external power supply terminal;
A collector resistor connected between the collector of the NPN transistor and a power source;
With
An input circuit for the constant current source has a three-terminal configuration connected to the collector of the NPN transistor, and is an amplifier circuit for a condenser microphone. An amplifier circuit for a condenser microphone,
A p-channel MOSFET (metal oxide field effect transistor) constituting a source follower with a gate connected to one terminal of the capacitor microphone, a drain grounded, a source connected to the output of a constant current source,
An NPN transistor having a base connected to the source of the p-channel MOSFET, an emitter grounded via an emitter resistor, and a collector connected to the signal output and the first external power supply terminal;
A collector resistor connected between the collector of the NPN transistor and a power source;
With
An input circuit for the constant current source has a four-terminal configuration connected to a second external power source. An amplifier circuit for a condenser microphone,
A p-channel MOSFET having a gate connected to one terminal of the capacitor microphone, a drain grounded, a source connected to one end of a constant current source, and constituting a source follower;
An n-channel MOSFET having a gate connected to the source of the p-channel MOSFET, a source grounded via an emitter resistor, and a drain connected to a signal output terminal / external power supply terminal;
A drain resistor connected between the drain of the n-channel MOSFET and a power source;
With
An input circuit for the constant current source has a three-terminal configuration connected to the drain of the n-channel MOSFET. An amplifier circuit for a condenser microphone,
A p-channel MOSFET having a gate connected to one terminal of the capacitor microphone, a drain grounded, a source connected to one end of a constant current source, and constituting a source follower;
An n-channel MOSFET having a gate connected to the source of the p-channel MOSFET, a source grounded via an emitter resistor, and a drain connected to the signal output and first external power supply terminal;
A drain resistor connected between the drain of the n-channel MOSFET and a power source;
With
An input circuit for the constant current source has a four-terminal configuration connected to a second external power supply. An amplifier circuit for a condenser microphone according to any one of claims 1 to 4,
2. The capacitor / microphone amplifier circuit according to claim 1, wherein the constant current source is a resistor. An amplifying circuit for a condenser microphone according to any one of claims 1 to 5,
A capacitor microphone amplifying circuit comprising a diode connected in a forward direction between a gate and a drain of the p-channel MOSFET. An amplifying circuit for a condenser microphone according to any one of claims 1 to 5,
An amplifying circuit for a condenser microphone comprising an input resistor connected between the gate and drain of the p-channel MOSFET.