JP2000236383A - Power supply system for electrostatic microphone, its amplifier circuit and telephone terminal using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply system for an electrostatic microphone that is not susceptible to an effect of a power supply or ground and causes less noise, SOLUTION: A couple of complementary bipolar transistors(TRs) 10, 11 generate a positive voltage and a negative voltage with respect to a reference voltage from a reference voltage generating circuit 1 of a microphone amplifier 7 that detects and amplifies a voice output from a condenser microphone 5 and the positive voltage and the negative voltage generated by the TRs are given to both terminals of the condenser microphone 5. Since a base current of the TRs is based on a voltage from the reference voltage generating circuit 1, a voice signal outputted from the condenser microphone 5 includes very less superimposition of a noise from a power supply VCC and ground, resulting that the voice output to a differential amplifier 20 being a component of the microphone amplifier 7 includes less noise.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply system for a condenser microphone, an amplifier circuit therefor, and a telephone terminal device using the same, and more particularly to a power supply system for a condenser microphone requiring a DC bias current.

[0002]

2. Description of the Related Art Such a peripheral circuit of a condenser microphone is generally used for a portable telephone, a PHS (Personal Handy Phone).
e System) and transmission circuits of wired telephones, etc., and it is required that this transmission voice suppress noise as much as possible. In order to respond to such a demand, for example, as disclosed in Japanese Patent Application Laid-Open No. Hei 10-200625, a current supplied to a microphone is a reference voltage for generating a reference voltage of an amplifier circuit for detecting and amplifying an audio output of the microphone. There has been proposed a technique for generating a voltage based on the potential of a generating circuit.

FIG. 2 is a circuit diagram for explaining this technique. The output of the condenser microphone 5 is input to the microphone amplifier 7 via the DC cut capacitor 6 and amplified. The microphone amplifier 7 is constituted by a differential amplifier, and a voltage of the reference voltage generating circuit 1 is used as a reference voltage of the microphone amplifier 7. This voltage is also used as a bias for the condenser microphone 5.

That is, the output potential of the reference voltage generating circuit 1 is supplied to the base of an NPN transistor 2 using the device power supply VCC as a collector power supply.
Is supplied to one end of a condenser microphone 5 via a load resistor 4. The capacitor 3
Is a stabilizing capacitor.

In such a configuration, the reference voltage generating circuit 1
The current output from is supplied to the condenser microphone 5 via the load resistor 4 after the noise component is removed by the transistor 2 and the stabilizing capacitor 3. As a result, a power supply system with less noise is realized.

[0006]

However, in this technique, the ground is connected to the negative side of the condenser microphone 5. Therefore, in a circuit such as a telephone having a high ground impedance, the reference voltage generation circuit 1
In some cases, an AC potential may be generated between the output potential of the microphone amplifier 7 and the ground, and as a result, the difference may occur in the output of the microphone amplifier 7 as noise.

Further, the transistor 2 of this circuit is an NPN
As a result, the emitter output potential is lower than the output potential of the reference potential generating circuit 1 by about 0.7 V (due to a voltage effect between the base and the emitter of the transistor). Therefore, in recent years, the power supply voltage that has become general is 3
In a circuit of about V, the output of the reference voltage generation circuit is V
Assuming CC / 2, the bias voltage applied to the condenser microphone 5 is 3V / 2-0.7 = 0.8V, which is lower than the general lower potential lower limit of 1.5V of the condenser microphone. There is also a problem that the use of a simple microphone results in high cost.

SUMMARY OF THE INVENTION An object of the present invention is to provide a power supply system for a condenser microphone which is not affected by power supply and ground and has low noise, an amplifier circuit thereof, and a telephone terminal device using the same.

Another object of the present invention is to provide a power supply system for a condenser microphone operable with a low voltage power supply, employing a power supply system with a small voltage drop, an amplifier circuit thereof, and a telephone terminal device using the same. It is to be.

[0010]

According to the present invention, there is provided a power supply system for a condenser microphone which requires a DC bias current, wherein an output of a reference voltage generation circuit of an amplifier circuit for amplifying an audio output of the condenser microphone is provided. There is provided a power supply method including bias generation means for generating a positive potential and a negative potential based on the potential, and applying the positive potential and the negative potential to both ends of the condenser microphone.

The bias generating means includes a stabilizing circuit for stabilizing an output of the reference potential generating circuit.
Complementary conductivity type transistors each having the output of the stabilization circuit as a base input, and collector outputs of these transistors are applied to both ends of the condenser microphone. Characterized by having a conversion capacitor.

According to the present invention, there is provided an amplifier circuit for a condenser microphone employing each of the above power supply methods,
An amplifier circuit having a differential amplifier having audio outputs from both ends of the condenser microphone as differential inputs is obtained. The audio output from both ends of the condenser microphone is supplied to the differential input via a DC cut capacitor.

According to the present invention, there is provided a telephone terminal device characterized by employing the above-described power supply system and amplifier circuit.

The operation of the present invention will be described. A pair of complementary bipolar transistors generates a positive potential and a negative potential with reference to the potential of a reference voltage generating circuit of a circuit that detects and amplifies the sound output of the condenser microphone, and the positive potential and the negative potential by these transistors are used as capacitors. It is provided to both ends of the microphone. Since the base current of these transistors is based on the voltage of the reference voltage generation circuit, the superimposition of noise from the power supply VCC and ground is extremely small on the audio signal output from the condenser microphone, and as a result, the microphone amplifier is not used. The noise is reduced in the audio output to the differential amplifier.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention, and the same parts as those in FIG. 2 are denoted by the same reference numerals. In FIG. 1, the output voltage of the reference voltage generating circuit 1 of the microphone amplifier 7 is supplied to the base of the PNP transistor 10 via a parallel circuit of a stabilizing capacitor 15 and a resistor 14. The stabilizing capacitor 17 and the resistor 16
Is also supplied to the base of the NPN transistor 11 through the parallel circuit of.

The emitter of the transistor 10 is connected to the device power supply V
The collector output is supplied to one end of the condenser microphone 5 via the load resistor 12.
The emitter of the transistor 11 is grounded, and the collector output is supplied to the other end of the condenser microphone 5 via the load resistor 13.

The audio output of the condenser microphone 5 is used as a differential input of a differential amplifier 20 of the microphone amplifier 7 via DC cut capacitors 18 and 19. Incidentally, 21
Reference numerals 24 denote resistors. The output voltage of the reference voltage generating circuit 1 is applied as a reference voltage of the differential amplifier 20 to a negative-phase input via a resistor 21.

The resistors 14 and 16 are connected to the transistors 10 and 1 respectively.
1 functions as a base current limiting resistor. Further, the stabilizing capacitors 15 and 17 have a function of smoothing an alternating current flowing through the bases of both transistors and removing a fluctuation component of VCC and ground potential.

The PNP transistor 10 is connected to VCC and outputs a current from the emitter. Since the base current flowing through the PNP transistor 10 is stabilized at the output voltage of the reference voltage generating circuit 1 as described above, Similarly, the emitter output current supplied to the condenser microphone 5 through the load resistor 12 becomes hfe times as large as the transistor, and is very stable.

For the same reason, the base current of the NPN transistor 6 is stabilized at the output voltage of the reference voltage generating circuit 1, so that the collector output current flowing from the condenser microphone 5 through the load resistor 13 is also very stable. I have. Since the voltage drop between the collector and the emitter in the circuit configuration of these transistors is about 0.2 V,
The drop of the bias voltage applied to the condenser microphone 5 can be minimized. Therefore, it is possible to operate a general-purpose condenser microphone even with a VCC of about 3 V which has recently become general.

The differential amplifier 20 is connected to the condenser microphone 5 via the DC cut capacitors 18 and 19.
Are differentially amplified with reference to the voltage of the reference voltage generating circuit 1. Therefore, the condenser microphone 5
The audio signal detected by the differential amplifier 20 and output from the differential amplifier 20 can operate at the voltage of the reference voltage generating circuit 1 on all paths, and is not affected by fluctuations of VCC and the ground potential. Will be output.

The detailed configuration and operation of the differential amplifier 20 and the reference voltage generating circuit 1 shown in FIG. 1 are well known to those skilled in the art, and thus description thereof will be omitted. Further, the above-described circuit configuration is merely an example, and it is needless to say that various modifications are possible.

[0023]

As described above, according to the present invention, a basic technique of supplying a positive potential and a negative potential to a microphone with reference to the potential of a reference voltage generating circuit of an audio circuit for detecting and amplifying the sound of the microphone. Based on the configuration, there is provided a power supply method for a condenser microphone which is hardly affected by fluctuations of the power supply and the ground and has little noise. In particular, if the present invention is applied to a power supply circuit of a microphone of a device such as a telephone set whose VCC and ground are not so stable, it is possible to realize an audio output with very little noise superposition from VCC and ground.

Further, since the present circuit does not have a large voltage drop in the transistor as in the prior art, a general condenser microphone can be connected, and the circuit can be realized at low cost.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional example.

[Description of Signs] 1 Reference voltage generation circuit 5 Capacitor microphone 7 Microphone amplifier 10 PNP transistor 11 NPN transistor 12, 13 Collector load resistance 14, 16 Base current limiting resistance 15, 17 Stabilizing capacitor 18, 19 DC cut capacitor 20 Differential amplifier

Claims (7)

[Claims] 1. A power supply system for a condenser microphone requiring a DC bias current, wherein a positive potential and a negative potential are referenced to an output potential of a reference voltage generating circuit of an amplifier circuit for amplifying an audio output of the condenser microphone. And a bias generation means for generating the positive and negative potentials at both ends of the condenser microphone. 2. The bias generation means includes a stabilization circuit for stabilizing an output of the reference potential generation circuit, and transistors of complementary conductivity type each having the output of the stabilization circuit as a base input. 2. A power supply system according to claim 1, wherein collector outputs of said transistors are applied to both ends of said condenser microphone. 3. The power supply system according to claim 2, wherein said stabilizing circuit has a stabilizing capacitor. 4. An amplifier circuit for a condenser microphone adopting the power supply system according to claim 1, further comprising a differential amplifier that receives audio output from both ends of the condenser microphone as a differential input. An amplifier circuit characterized by the above-mentioned. 5. The amplifier circuit according to claim 4, wherein audio outputs from both ends of said condenser microphone are supplied to said differential inputs via respective DC cut capacitors. 6. A telephone terminal device adopting the power supply system according to claim 1. 7. A telephone terminal device employing the amplifier circuit according to claim 4.