JPH0139279B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a microphone device, and more particularly to a microphone device equipped with a capacitor type (bias type) or electret type electrostatic microphone unit.

FIG. 1 and FIG. 2 each show an impedance conversion circuit in a conventional microphone device. In the case of Figure 1, the output of the electret microphone _M0 is a field effect transistor.
The signal is supplied to a source follower circuit consisting of a Tr and a primary winding 1a of the transformer 1, and a balanced output corresponding to the microphone output is obtained from the secondary winding 1b of the transformer 1. Note that balanced output here means that, for example, when the signal level of one output terminal 2 of the secondary winding 1b increases, the signal level of the other output terminal 3 of the secondary winding 1b increases.
This is a differential signal whose signal level decreases.

For example, this balanced output is supplied to a transformer (not shown) of the receiving device via a microphone cable, and an audio signal is obtained from the secondary winding of this transformer.

In the case of FIG. 2, the output of the microphone _M0 is supplied to the primary winding 1a of the transformer 1 via a source follower circuit consisting of a field effect transistor Tr and a resistor R, and a capacitor Co, respectively.
A microphone output is obtained from the secondary winding 1b of this transformer 1.

In this way, in the past, the microphone unit
In order to obtain the output signal from M ₀ as a balanced output, an output transformer is used in the impedance conversion circuit, which increases the cost of the device and also makes it impossible to reduce the size and weight of the device. In other words, if the output transformer is made smaller in order to make the device smaller and lighter, good output characteristics cannot be obtained due to the deterioration of frequency characteristics and the effects of iron loss, etc., so the output transformer must be made relatively large. This is because it will not happen. Therefore, as a result, the device itself becomes large and has a considerable weight, which is disadvantageous.

The present invention was invented in view of the above-mentioned circumstances, and by using a differential amplifier circuit instead of a transformer, it is possible to reduce costs, reduce size and weight, and improve characteristics. The present invention provides a microphone device that is equipped with the following features.

Embodiments of a microphone device to which the present invention is applied will be described below with reference to FIGS. 3 to 5.

FIG. 3 shows a microphone circuit showing a first embodiment of the present invention, and this microphone circuit includes a pair of N-channel field effect transistors Tr ₁ and Tr ₂ .
and resistors _R1 to _R5 . That is, these pair of transistors
A resistor R ₁ having the same resistance value is connected between the drain electrodes of Tr ₁ and Tr ₂ and the power supply line +Vcc,
R ₂ are connected respectively. Resistors R ₃ and R ₄ having the same small resistance value are connected in series between the source electrodes of the transistors Tr ₁ and Tr ₂ .
One end of a resistor _R5 is connected between these resistors R ₃ and R ₄ , and large resistance values that are equal to each other are connected between the other end of the resistor R ₅ at point A and the gate electrodes of the transistors Tr ₁ and Tr ₂ . Resistors R ₆ and R ₇ having , respectively, are connected.

Furthermore, the drain electrode of an N-channel field effect transistor Tr ₃ that operates as a constant current source is connected to the above point A, and its gate electrode is grounded, and its source electrode is grounded via a variable resistor R ₀ . There is. As a result, the current value of the drain current _ID flowing into the drain electrode of the field effect transistor Tr ₃ is changed between the pair of transistors Tr ₁ and Tr 3 .
It is the sum of the drain currents I _D1 and I _D2 flowing into the drain electrode of T ₂ , respectively, and is always limited to a constant value (I _D = I _D1 + I _D2 = constant value). Note that the gain of the differential amplifier circuit 4 can be changed arbitrarily by adjusting the variable resistor R ₀ .
Furthermore, a pair of transistors Tr ₁ ,
Balanced outputs as described above can be obtained from the drain electrode of Tr ₂ to output terminals 5 and 6.

Further, between the gate electrode of one field effect transistor Tr ₁ and the ground line 7, there is an electret type microphone unit having a cardioid type directivity characteristic, that is, a unidirectional characteristic as shown by the curve _K1 in FIG. _M1 is connected.
That is, the semi-permanently charged fixed electrode of the microphone unit _M1 is connected to the gate electrode of the transistor _Tr1 , and the vibrating membrane of the microphone unit _M1 is connected to the earth line 7. In addition, between the gate electrode of the other field effect transistor Tr ₂ and the ground line 7, there is a connection between the gate electrode of the transistor Tr ₂ and the ground line 7 so that the gate electrode of the transistor Tr 2 is grounded for AC and floated from the ground line 7 for DC. A capacitor _C1 is connected to maintain the current.

That is, in this case, if the capacitor C ₁ is not provided and the gate electrode of the transistor Tr ₂ is not connected to the ground line, the transistor Tr ₂ composed of a field effect transistor has a high input impedance, so for example, When an external noise component such as a bass sound is supplied to the conductor from the capacitor _C3 to the gate electrode of the transistor _Tr2 , this external noise component appears at the output electrode of the transistor _Tr2 , which is undesirable.

On the other hand, if a resistor or inductance is connected between the gate electrode of the transistor Tr ₂ and the ground line instead of the capacitor C ₁ , the above-mentioned drawbacks can be corrected. Since the gate leakage current of Tr ₂ flows from its gate electrode to the ground line via the above-mentioned resistance or inductance and is amplified and appears at its output electrode, the output due to the gate leakage current becomes a noise component, which is still preferable. do not have.

On the other hand, in the embodiment shown in FIG. 3, a capacitive element (capacitor C ₁ ) is connected between the transistor Tr ₂ and the ground line, so that the gate electrode of the transistor Tr ₂ is connected as described above. It is grounded in terms of AC and floating from the ground line in terms of DC. Therefore, external noise components such as the above-mentioned buzz can be dropped to the ground line, and the current path of the gate leakage current as mentioned above is blocked, so the transistor r ₂
These noises do not appear at the output electrode of the device. _{Moreover , a} capacitive element (capacitor C ₁ ) is connected to the gate electrode of the transistor _Tr2 , the symmetry of the differential amplifier circuit can be maintained, and therefore good circuit operation can be achieved.

Note that the capacitance of this capacitor _C1 may be the capacitance of the microphone unit _M1 (for example, about 10 pF), or may be any other capacitance. In short, it is sufficient that a capacitive element is connected between the gate electrode of the transistor Tr ₂ and the ground line 2 .

In Fig. 3, C ₂ and C ₃ are transistors.
This capacitor is used to adjust the gain of the balanced output obtained between the drain electrodes of Tr ₁ and Tr ₂ .

Next, the circuit operation of the microphone circuit configured as described above will be explained.

First, for example, in FIGS. 3 and 4, arrow a
When an audio signal enters the microphone unit _M1 from this direction, an output signal having an amplitude corresponding to the length on the cardioid pattern shown in FIG. 4 is supplied from the microphone unit _M1 to the gate electrode of the transformer _Tr1 . Along with this, the differential amplifier circuit 4
A pair of transistors are connected from output terminals 5 and 6 of
The difference between the signals supplied to the gate electrodes of Tr ₁ and Tr ₂ is obtained as an output, but the difference between the signals supplied to the gate electrodes of Tr 1 and Tr 2 is obtained as an output.
Since no signal is supplied to the gate electrode of Tr ₂ ,
From between the output terminals 5 and 6, an amplified output of the output signal from the microphone unit _M1 is directly obtained. Therefore, this balanced output has a cardioid unidirectional pattern as shown in FIG.

Note that the outputs obtained at the output terminals 5 and 6 are so-called balanced outputs (differential signals) in which, for example, when the signal level of one output terminal 5 increases, the signal level of the other output terminal 6 decreases. Therefore, only the transmitted signal components can be obtained from the secondary winding of the transformer (not shown) on the receiving side to which the balanced outputs of the output terminals 5 and 6 are supplied via the microphone cable. Even if in-phase noise components such as hum and buzz are superimposed on each of the two transmission lines of the cable, these noise components do not appear in the output of the transformer on the receiving side. Therefore, such external noise can be effectively removed.

In this way, in this embodiment, the above-mentioned balanced output is obtained by using the differential amplifier circuit 4 instead of using a transformer as in the conventional case, so that the size and weight can be reduced and the cost can be reduced. can be achieved.

Moreover, a capacitor _C1 is connected between the gate electrode of the transistor _Tr2 on the side to which the microphone unit M1 is not connected and _the earth line 7,
The gate electrode is grounded in an alternating current manner and floated from the earth line 7 in a direct current manner. Therefore, the noise appearing on the output electrode side of the transistor Tr ₂ can be effectively reduced, and
The symmetry of the differential amplifier circuit can be maintained and good circuit operation can be achieved. In addition, as mentioned above, it is less susceptible to the effects of external noise superimposed on each of the two transmission lines of the microphone cable.
Even if the balanced output is transmitted over a microphone cable of about 100 m, for example, there will be no problem with hearing, and the characteristics can be improved.

FIG. 5 shows a second embodiment of the microphone device of the present invention. In this embodiment, the gate electrodes of a pair of field effect transistors Tr ₁ and Tr ₂ constituting the differential amplifier circuit 4 are A pair of unidirectional electret microphone units M ₁ and M ₂ which are a type of capacitive element and have the same characteristics are connected to the line 7, respectively.

In this case, these pairs of microphone units
M ₁ and M ₂ are arranged and fixed in the microphone device in a state in which they face each other, that is, in a state in which their respective sound collection surfaces (vibrating membrane sides) face outward in opposite directions. Note that the other configurations may be completely the same as in the case of FIG. 3.

According to the microphone device configured in this way, when audio input is received from the direction a in FIGS. 4 and 5 to one microphone unit _M1 , the audio input from the microphone unit _M1 follows the cardioid pattern shown in FIG. An output with an amplitude corresponding to the length OE of the microphone unit is obtained, and the output of the other microphone unit is
An output with an amplitude corresponding to the length OF is obtained from _M2 . And the difference between these outputs is the transistor
This is the output of the differential amplifier 4 consisting of Tr ₁ and Tr ₂ .
The differential output obtained from the differential amplifier 4 has an amplitude corresponding to the length OG in FIG. 5, and the locus of this point G exhibits bidirectionality as shown by the dotted line in FIG. Therefore, the difference output has bidirectional characteristics.

In this example, a pair of unidirectional microphone units M ₁ and M ₂ are used, but unidirectional, bidirectional, and omnidirectional microphone units may be combined in various ways (however, (excluding combinations of a pair of omnidirectional microphone units), for example, if these microphone units are configured so that they can be arbitrarily connected to the microphone circuit described above using a predetermined connection switching means, various directional characteristics can be adjusted as necessary. can be obtained. Further, in each of the embodiments described above, an electret type microphone unit is used, but a bias type capacitor type microphone unit can also be used. Further, the method of biasing the differential amplifier circuit 4 and the method of feeding back the signal may be changed in various ways.

As described above, the present invention configures a differential amplifier circuit by a pair of active elements, and connects an electrostatic microphone unit between the input electrode of at least one of the pair of active elements and the earth line. In addition, a capacitive element is connected between the input electrode of the other active element and the ground line, and a microphone output converted to a balanced output is obtained between the output electrodes of the pair of active elements. Therefore, according to the microphone device of the present invention, since the transformer is replaced with a differential amplifier circuit to obtain the above-mentioned balanced output, it is possible to reduce the size and weight of the device, as well as to reduce costs. Moreover, compared to the case where a truss is used, there is an advantage that characteristics can be improved because there is no influence of iron loss, output loss, etc. In addition, since a capacitive element is connected between the input electrode of the other active element to which the electrostatic microphone unit is not connected and the ground line, various noises appearing in this other active element can be effectively reduced. At the same time, the symmetry of the differential amplifier circuit can be maintained and good circuit operation can be achieved.

[Brief explanation of drawings]

1 and 2 are circuit diagrams showing impedance conversion circuits in a conventional microphone device, and FIGS. 3 to 5 are for explaining an embodiment of the present invention. A circuit diagram of a microphone circuit showing a first embodiment of the invention, FIG. 4 is a pattern diagram showing the directivity characteristics of the differential output obtained from the microphone circuit of FIG. 3, and FIG. 5 is a circuit diagram of a microphone circuit according to a second embodiment of the invention. FIG. 2 is a circuit diagram of a microphone circuit illustrating. In addition, in the symbols used in the drawings, 5,
6...output terminal, _M1 , _M2 ...electret type microphone unit, _Tr1 , _Tr2 ...field effect transistor, _C1 ...capacitor.

Claims (1)

[Claims] 1 A differential amplifier circuit is configured by a pair of active elements, and an electrostatic microphone unit is connected between the input electrode of at least one of the active elements and the earth line, and the A microphone device characterized in that a capacitive element is connected between the input electrode of the active element and the ground line, and a microphone output converted into a balanced output is obtained from between the output electrodes of the pair of active elements.