JPS58103291A - Capacitor type microphone - Google Patents

Abstract

PURPOSE:To attain high sensitivity, high S/N and high stability, by feeding back negatively and in terms of DC an output obtained on the basis of the change of the electrostatic capacitance between the vibrating film of a microphone cartridge and a fixed electrode between these film and electrode with superposition of the output on the reference DC bias voltage. CONSTITUTION:The absolute value of the electrostatic capacitance of a microphone cartridge 1'' is detected in real time by an electrostatic capacitance meter 2'' and then delivered to an output terminal 6'' from an output circuit 3'' via a capacitor 9. At the same time, this absolute value is fed to a bias control circuit 7 via a DC amplifier of the circuit 3'' to be superposed on the reference current bias voltage VREF and receives a DC amplification to be fed to a booster 8 and then applied as the bias of the cartridge 1''. This feedback system is formed so that the value is always set constant for the electrostatic capacity CM of the cartridge 1''. It is possible to extract the microphone output at any position of the feedback system and to attain the high sensitivity, high S/N and high stability.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a condenser microphone.1.
Especially when the boundary conditions of the vibrating membrane are kept constant, high sensitivity and high S/N
It is an object of the present invention to provide a condenser type microphone that can improve the ratio and achieve high stability.

In general, capacitor type microphones require a bias voltage to be applied to a diaphragm, and are broadly classified into a DC bias type and an AC bias type, depending on the method of applying a bias voltage.

Figure 1 shows the basic circuit configuration of a DC bias type capacitor microphone. In FIG. 1, 1 is a microphone cartridge, and CM is a microphone cartridge 1.
Eo is the DC bias voltage applied between the diaphragm of the microphone cartridge 1 and the fixed electrode from the bias supply terminal 6, 2
is a high input impedance head amplifier (impedance converter) using a field effect transistor or the like, to which the output of the microphone cartridge 1 is applied via a coupling capacitor 4. 3 is an output circuit, vo is a microphone output taken out to the output terminal 6, and R is a limiting resistor. The change in capacitance CM caused by the incidence of sound waves is the DC bias voltage E. is detected as a change in voltage via the impedance converter 2. Figure 2 shows a microphone cartridge used in a DC bias type condenser microphone.
In the figure, 20 is a vibrating membrane, which is fixed to a vibrating membrane ring 21. 22 is a fixed electrode, and a DC bias voltage E is applied between it and the vibrating membrane 20. is applied. Vibration membrane 2 due to electrostatic force FE generated by application of DC bias voltage E0
o is attracted to the fixed electrode 22 side and is displaced by ljx to a position balanced with the mechanical force F due to the tension of the vibrating membrane 2o. At this time, the electrostatic force FE2 mechanical force FM is: FM=80-ΔX (2) However,
ε0: Air 9 permittivity S: Effective area of capacitance formed by the vibrating membrane 20 and fixed electrode 22xo: Gap between the vibrating membrane 2o and fixed electrode 22 when E0=Ov So: Stiffness of the vibrating membrane Also, the displacement amount ΔX is x o> d x from FE-FM.
As, is given by. By the way, in order to improve the S/N of a DC bias type condenser microphone, since the noise component is limited by the hand amplifier 2,
Increase the DC bias voltage E0, or reduce the gap between the vibrating membrane and the fixed electrode! . There is no way to improve the sensitivity of the microphone cartridge by narrowing the diaphragm or loosening the tension of the diaphragm. However, as shown in Equation 1, the electrostatic attractive force FE increases rapidly by taking either of the above measures, and in extreme cases, FE>F.

As a result, the vibrating membrane is attracted to the fixed electrode. Further, even if FE<FM in a state where no sound waves are incident, FE>FM may occur due to the incidence of sound waves, and adsorption may occur. That is, in a DC bias type capacitor microphone, there is an inverse relationship between improved sensitivity and improved stability, and high sensitivity and high stability cannot be obtained. By the way, the sensitivity of the microphone cartridge is S. ns + stability M is given by the following equation.

In However,! 1: Effective displacement of the diaphragm against the reference sound pressure...
(6) Figure 3 shows the basic circuit configuration of an AC bias type condenser microphone. In FIG. 3, 1' is a microphone cartridge, 2' is a capacitance meter, and 3' is an output circuit. The capacitance meter 2' is a high frequency oscillator 10 with a high Q such as a crystal oscillator. It is composed of a phase shifter 11 determined by the capacitance of the microphone cartridge 1', a phase detector 12, and a low-pass filter 13. In this microphone, the high frequency generated by the high frequency oscillator 10 is divided into two parts, one of which is sent directly to the nine-phase detector 12, and the other is phase-modulated by the phase shifter 11 corresponding to the capacitance between the diaphragm and the identification electrode. Similarly, the signal is sent to the phase detector 12, the phases of the two are compared, and the difference is outputted and sent to the low-pass filter 13, where the high frequency component is converted into a dj wave.

The greatest feature of this AC bias type capacitor type microphone is that by using a high frequency oscillator with a very high Q, a microphone with a high S/N ratio can be produced. Furthermore, since an output proportional to the capacitance CM of the microphone cartridge 1' can be obtained, the output can also include a DC component. or.

Unlike the DC bias method, the adhesion phenomenon between the fixed electrodes of the vibrating membrane does not occur. The disadvantage is that, because the sensitivity is high, variations in the initial value of the capacitance CM and the tension of the diaphragm greatly affect the sensitivity of the microphone.

Problems common to such conventional DC bias type and AC bias type capacitor microphones will be described below. Figure 4 shows what happens when a sound wave is incident on a completely rigid wall. The boundary condition in this case is
When the sound pressure is P2 and the particle velocity is U, the following holds true: P-maximum U=0. In the case of a normal object, the light is not completely reflected as described above, but is partially transmitted. For something that is extremely thin and moves lightly, such as a vibrating membrane, the boundary conditions are complex and not constant. In other words, by changing the boundary conditions,
This has a negative effect on subtle aspects such as sound quality.

The present invention solves these conventional drawbacks,
A reference DC bias voltage is applied to the capacitance formed by the diaphragm and fixed electrode of the microphone cartridge, and changes in the capacitance are detected and the output is superimposed on the reference DC bias voltage to remove the diaphragm from the diaphragm. It is configured to be applied between the fixed electrode and the fixed electrode. Depending on such configuration,
Since negative feedback can be applied so that the capacitance between the diaphragm and the fixed electrode remains constant even when a sound wave is incident, high sensitivity and high S/
This has the advantage of being able to achieve nitrogen conversion and high stability.

Examples of the condenser microphone of the present invention will be described below. FIG. 6 shows an embodiment of the present invention, and in FIG. 6, 1" is a microphone cartridge;
2" is a capacitance meter, '7:' is an output circuit, 7 is a bias control circuit, and 8 is a booster.

The absolute value of the capacitance of the microphone cartridge 1'' is detected in real time by a capacitance meter 2'', and is output from an output circuit 3'' to an output terminal 6'' via a capacitor 9. on the other hand,
It is sent to the bias control circuit 7 via the DC@rtl device of the output circuit 3'', where the reference DC bias voltage vR
The DC current is increased by being superimposed on EF, sent to the booster 8, and applied as a bias to the microphone cartridge 1''.This feedback system is designed to keep the value of the capacitance Cb4 of the microphone cartridge 1'' constant at all times. It is configured. The microphone output may be taken out from any position in this feedback system. The bias voltage for the capacitance CM when there is no sound wave is adjusted by the reference DC bias voltage vREF so that it is at a position of % of the maximum voltage of the booster 8, as shown in FIG.

As described above, according to the present invention, the output based on the change in capacitance between the diaphragm and the fixed electrode is superimposed on the reference DC bias voltage between the diaphragm and the fixed electrode constituting the microphone cartridge. Since the structure is configured to provide negative feedback, the DC operating point of the capacitance can be kept constant at all times, and the boundary conditions of the diaphragm can be kept constant to achieve high sensitivity.
This has the advantage of achieving high S/N and high stability.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of a DC bias type condenser microphone, Figure 2 is an enlarged cross-sectional view of its main parts, Figure 3 is a basic configuration diagram of an AC/IAS type condenser type microphone, and Figure 4 is a diagram of the basic configuration of a DC bias type condenser type microphone. A diagram showing what happens when a sound wave is incident on an ideal perfectly rigid body. Figure 5 is a configuration diagram showing an embodiment of the condenser type microphone of the present invention. Figure 6 is a diagram showing the sound pressure vs. 75 ias of the condenser type microphone. Shows the relationship between voltages.
Figure 1. 1"...Microphone cartridge, 2"...
...Capacitance meter, 3" ...Output circuit,
7...Bias control circuit, 8...Elevator, 9...Coupling capacitor, R...
・・・／；Ias resistance. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 3 Figure 2' Figure 2 Figure 4

Claims (1)

[Claims] The structure is configured to detect changes in capacitance formed by a diaphragm and a fixed electrode that constitute a microphone cartridge, and extract it as a microphone output. A capacitor-type microphone configured to provide negative feedback between the fixed electrode and the fixed electrode.