JP3057397B2 - Displacement proportional transducer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement proportional type converter in which the output voltage is proportional to the amount of displacement of a vibrator. More specifically, the converter unit of the displacement proportional type converter is the same as an input signal. The present invention relates to a displacement proportional converter that is excited by a phase signal to increase sensitivity.

[0002]

2. Description of the Related Art Displacement proportional type converters, such as condenser microphones and electret condenser microphones, which obtain an output voltage proportional to the displacement amplitude of a vibrator, can be reduced in size and weight, and are widely used from business use to consumer use. in use. For condenser microphones,
A DC voltage is applied between the electrodes of the microphone unit through a high resistance, and a change in the voltage generated at both ends of the high resistance due to a change in the capacity of the unit is taken out as an electric signal.The electret condenser microphone has a fixed pole of the unit or The diaphragm is made of a thin film having an electret phenomenon, and the electrode on the signal extraction side is connected with high impedance. The electric charge Q of the electrode does not move and remains constant. Since a voltage change appears in accordance with the relationship of C × V (V is the voltage between both ends of the capacitor), this voltage change is extracted as an electric signal.

A conventional displacement proportional type converter will be described in more detail with reference to FIG. 3 by taking an electret condenser microphone as an example. One of the fixed pole and the diaphragm of the microphone unit 1 changes its capacitance by changing the voltage. And a gate of an impedance-converting FET 3. That is, the capacitance of the microphone unit 1 is generally several tens.
Although the resistance value of the resistor 2 is required to be several hundred MΩ or more, the output impedance of the microphone needs to be as low as several hundred Ω. Therefore, the high input impedance is converted to a low output impedance by the resistor 2 and the FET 3. The drain of the FET 3 is connected to the power supply terminal 4 and
The source of ET3 is connected to the ground terminal 7 via the output terminal 5 and the resistor 6 which is the load resistance of the source of the FET3. The other end of the microphone unit 1 is also connected to the ground terminal 7. Reference numerals 8 and 9 denote resistors for setting the gate potential of the FET 3, and 10 denotes a capacitor for stabilizing the gate potential of the FET 3 with AC.

In such an electret condenser microphone, since the capacitance of the microphone unit 1 is small, the input signal is attenuated by the capacitance generated between the terminals of the FET 3, and the mutual conductance and load resistance of the FET 3 are reduced. Therefore, the input signal is attenuated also from the relationship. In order to improve the attenuation of the input signal, that is, the decrease in sensitivity, conventionally, the output of the microphone unit 1 is increased or the output terminal 5 is connected to an amplifier formed by an electronic circuit including a transistor or an operational amplifier to amplify the output or to connect the transformer. A method of connecting and boosting the output is adopted.

However, the increase in the output of the microphone unit 1 can only increase the sensitivity by a few dB at most due to the adsorption stability between the diaphragm and the fixed pole and the required frequency characteristics. If the step-up ratio is increased, the output impedance increases, which causes inconvenience in use. Therefore, an amplifier is generally connected.

[0006]

However, in order to use the amplifier, power for operating the amplifier is required, which not only increases the power consumption but also increases the S / N ratio of the microphone unit 1 and the resistor 2 in the preceding stage. FET3
And the S / N ratio of the impedance conversion circuit may be equal to or deteriorated.

SUMMARY OF THE INVENTION An object of the present invention is to provide a displacement proportional type converter which has improved sensitivity by a simple improvement of exciting the converter unit with an input signal and an in-phase signal.

[0008]

SUMMARY OF THE INVENTION The present invention comprises an impedance conversion circuit for extracting a capacitance change due to an audio input signal of a converter unit as a voltage change, and outputs a signal in phase with the audio input signal. The converter has a positive feedback loop that feeds back the output of the impedance conversion circuit to the converter unit.

[0009]

With this configuration, the converter unit superimposes the output signal from the impedance conversion circuit together with the audio input signal, so that the converter unit is excited with at least the output signal from the impedance conversion circuit added. That is, the sensitivity of the converter can be improved.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment shown in the drawings will be described below by taking as an example the case where the displacement proportional type converter of the present invention is applied to an electret condenser microphone as in the above-mentioned conventional example. In the above-described conventional example and each embodiment, substantially the same components are denoted by the same reference numerals, and detailed description thereof will be omitted.

First, the first embodiment shown in FIG. 1 will be described. In this electret condenser microphone, the output terminal 5 side is connected to the other of the fixed pole of the microphone unit 1 or the other of the diaphragm, and the source-follower output of the FET 3 is output. Is the same as the above-mentioned conventional example, except that the other unit of the microphone unit 1 is excited. As a result, the microphone unit 1 is also superimposed with the audio input signal and the output signal from the impedance conversion circuit, so that the microphone unit 1 is excited with at least the output signal from the impedance conversion circuit plus, and the sensitivity is reduced. Can be improved. Even if such positive feedback is applied to the microphone unit 1, F
The source / follower output of the ET3 is in phase with the audio input signal, and when the level of the audio input signal is 1, if the feedback is less than 1 to the microphone unit 1, no oscillation due to sound will occur. And the microphone unit 1 has several p
Since the capacitance is as small as F to several tens of pF, the input signal is attenuated by the capacitance generated between the terminals of the FET 3 and the FE
Attenuation is also caused by the mutual conductance of T3 and the resistance 6 which is the load resistance of FET3.
In the source follower described above, the feedback amount can be made smaller than 1.

Therefore, the ratio of the internal impedance and the resistance 6 as viewed from the source side, which is the transconductance of the FET 3, is set to be large so that the attenuation by the FET 3 is minimized. In this case, if the resistance 6 is simply increased, the current of the FET 3 decreases and the transconductance decreases.

With such a simple structure of providing one positive feedback loop, the sensitivity of the microphone unit 1 can be increased, and a high-sensitivity converter can be obtained. Here, the amplification degree and the noise level when the electret condenser microphone unit having the input capacitance of 27 pF was driven by the circuit of the conventional example shown in FIG. 3 and the circuit of the first embodiment shown in FIG. 1 were measured. The amplification and the noise level of the conventional example are -0.8 dB, respectively.
In contrast to -122.3 dB (A), in the first embodiment, it becomes +32.9 dB and -90.9 dB (A), and S / S
The degree of improvement of the N ratio was also 2.3 dB.

The second embodiment shown in FIG. 2 shows a case where the sensitivity is lowered when the sensitivity becomes higher than necessary at the time of setting according to the first embodiment. That is, two resistors 11 and 12 connected in series are connected between the output terminal 5 and the ground terminal 7 of the above-described first embodiment, and the intermediate point is connected to the fixed pole of the microphone unit 1 or the other of the diaphragm. Connected. Other than the above, it is the same as the first embodiment. The resistors 11 and 12 are called bleeder resistors, and an output is obtained from the intermediate point in which the output is reduced by a ratio obtained by dividing the resistance value of the resistor 12 by the total resistance value of the resistors 11 and 12, so that the signal to be fed back is determined by this ratio. , And as a result, the overall amplification degree, that is, the sensitivity, can be reduced.

[0015]

As described above, the displacement proportional type converter according to the present invention has the positive feedback loop for feeding back the output of the impedance conversion circuit to the converter unit. Since the output signal from the impedance conversion circuit is also superimposed, the converter unit is excited at least in a form in which the output signal from the impedance conversion circuit is added, and the sensitivity of the converter can be improved. Further, as compared with the conventional one using an amplifier, the configuration is much simpler, space saving, low cost, low power consumption can be achieved, and the S / N ratio can be improved.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing an example of a conventional converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Microphone unit 2, 11, 12 Resistance 3 FET 5 Output terminal 7 Ground terminal

Claims (1)

(57) [Claims] 1. A displacement proportional converter, comprising: an impedance conversion circuit for extracting a change in capacitance due to an audio input signal of a converter unit as a change in voltage; and outputting a signal in phase with the audio input signal. A displacement proportional converter having a positive feedback loop for feeding an output of a circuit back to the converter unit.