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JPS56191
DESCRIPTION JPS56191
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an example of the structure of a
microphone unit of an electret condenser microphone, FIGS. 2a and 2b show an example of an
output circuit of a conventional electret condenser microphone, and FIG. FIG. 1 is a diagram
showing an output circuit of an embodiment of an electret condenser microphone according to
the present invention. Explanation of symbols of the main parts, 1 · · · · · · · · · · · · · · · · · · · · · · · · · · · ·
························································································
· · · ... FET 0
DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrostatic
microphone or electret condenser microphone using an electret material as a diaphragm. The
electrostatic microphone is configured to obtain an output by a capacitance change of a capacitor
formed by a predetermined fixed electrode and a voice detection diaphragm disposed opposite to
the fixed electrode. Although this electrostatic microphone is characterized by its simple
structure and excellent sound quality, it is necessary to apply a predetermined DC bias voltage
between the fixed electrode and the diaphragm, so that the output voltage can be extracted. Was
not easy. In order to improve such a defect, there is an electret condenser microphone using an
electleft material, such as polyester or teflon, whose charges are fixed in advance as a diaphragm.
An example of the structure of the electret condenser microphone is shown in FIG. 1, but in this
figure, the case 1, the diaphragm 2 electrically connected to the case 1, and vibration in the case
1-2 A microphone unit is configured by the fixed electrode 3 provided so as to face the plate 2. A
capacitor is formed by the diaphragm 2 and the fixed electrode 3, and the diaphragm 2 is
displaced by air vibration transmitted from a predetermined sound source, and the distance
between the diaphragm 2 and the fixed electrode 3 is increased or decreased. A change occurs,
which is a structure for obtaining the output voltage of the microphone unit. In this case, the
electret material used as the diaphragm 2 is usually negatively charged, and since it is necessary
to eliminate inductive noise from the outside, the diaphragm 2 and the case 1 are used as the
ground to the fixed electrode 3. Generates a positive output voltage. The sound detection
function of the microphone unit will be described in more detail. When the compression wave of
the sound wave, that is, the air reaches the diaphragm 2, the diaphragm at the time when the air
pressure outside the diaphragm 2 rises 2 moves to the side of the fixed electrode 3 and the
capacitance of the capacitor formed by the two increases. Further, since the amount of electric
charge existing on the diaphragm 2 and the fixed electrode 3 is constant, the potential difference
between the diaphragm 2 and the fixed electrode 3 is reduced due to the increase of the
capacitance. Similarly, when the low pressure portion of the compression wave from the
predetermined sound source reaches the diaphragm 2, the distance between the diaphragm and
the fixed electrode increases, so the capacitance of the capacitor decreases and the vibration
decreases. The potential difference between the plate 2 and the fixed electrode 3 is increased. As
described above, in the microphone unit of an ordinary electret condenser microphone, the
positive terminal voltage generated at the fixed electrode 3 fluctuates in the opposite phase to
the phase of the sound wave to be detected.
For this reason, in the conventional electret condenser microphone, it is necessary to provide an
output circuit for performing the impedance inversion and the phase inversion on the output
voltage from the microphone unit. FIGS. 2a and 2h show an example of an output circuit of a
conventional electret condenser microphone. First, in the example shown in FIG. 2a, the output
voltage of the microphone unit 4 is input to a field effect transistor (FET) 5 which is a high input
impedance element. Next, the signal input to the gate G of the FET 5 is inverted in phase by the
original FET 5 and output from the drain D. Since the output signal from the drain D has a
relatively high impedance, it is configured to use a transformer 6 for impedance conversion to
send out an output with a low impedance of, for example, about 600 Ω. Next, in the case of the
output circuit shown in FIG. 2h, the output voltage of the microphone unit 4 is inputted to the
gate G of the FET 5 as described above, but in this case, the FET 5 is used for impedance
conversion. 9, source Si1 of FET5. The signal having the same phase as that of the input signal to
the gate G is output with low i ···· ′ ′ ′ ′ denpidance. Next, in order to invert the phase of the
output signal from the FET5, the phase inversion transformer 7 is used, and the output of the
microphone is obtained through this transformer 7. As described above, in the conventional
electret condenser microphone, since the phase of the output voltage of the microphone unit is in
the opposite phase to the phase of the sound wave to be detected, the FET and transformer are
used for impedance conversion and phase inversion. The present invention improves the abovementioned drawbacks of the conventional electret condenser microphone and has an in-phase
condition against air vibration. It is an object of the present invention to provide an electret
condenser microphone capable of configuring an output circuit without using a transformer by
using a microphone unit that generates an output signal of. yへ(r;、。 Next, an embodiment
of an electret note microphone according to the present invention will be described in detail with
reference to the drawings. An embodiment of the present invention is shown in FIG. 3, and the
microphone unit 4 used for voice detection in this figure has the same structure as the
conventional one, but is positively charged as an electret material used as a diaphragm. It is
characterized by using materials.
For this reason, the case of the microphone unit and the diaphragm have a positive potential
contrary to the conventional case, and the fixed electrode has a negative potential. In the case of
a predetermined sound pressure being applied to the microphone unit 4 in the case 6-, the
distance between the plates of the capacitor formed by the diaphragm and the fixed electrode
changes, and the vibration according to the resulting change in capacitance The potential
difference between the plate and the fixed electrode changes. The above process is the same as
that of the conventional microphone unit, but since the fixed electrode is held at a negative
potential with respect to the diaphragm, the phase of the alternating signal component is
opposite to that of the conventional one. As a result, in the microphone unit 4, an output signal in
phase with the acoustic vibration transmitted to the diaphragm can be obtained. For this reason,
it is not necessary to provide a phase inverting circuit in the output circuit for the microphone
unit 4, and only the impedance conversion circuit can be configured. ← In Fig. 3, Z, the output
voltage from the microphone unit 4 is input to the gate G of the FET 5 that constitutes the
impedance conversion means, and the impedance is converted by the FET 5 after the source S
from the source S It has been output. At this time, although the output voltage from the
microphone unit 4 has a minus potential in terms of direct current, it differs from the
conventional seven, but there is no need to reverse the phase. There is an advantage that the
output circuit is simplified and at the same time the deterioration of the sound quality due to the
transformer is prevented. The electret condenser microphone according to the present invention
is structurally the same as the conventional one except that the polarity of the electrification
charge of the electret material used as the diaphragm is different, and it can be easily
implemented in various electret condenser microphones. is there.


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