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DESCRIPTION JP2011188419

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DESCRIPTION JP2011188419
PROBLEM TO BE SOLVED: To provide a condenser microphone which can increase the maximum
output level without using a transformer as compared with the conventional condenser
microphone using an emitter follower connection in an output circuit. SOLUTION: A transistor
with emitter follower connection is provided in an output circuit, and a transistor with emitter
follower connection is provided immediately after the FET constituting the impedance converter,
and a constant voltage circuit is provided at the emitter terminal of the transistor The condenser
microphone can operate the FET that constitutes the impedance converter by the [Selected
figure] Figure 1
コンデンサーマイクロホン
[0001]
The present invention relates to a condenser microphone using an emitter-follower connection in
an output circuit, and is characterized by an output circuit capable of increasing the maximum
output level without using a transformer.
[0002]
A power supply system such as a condenser microphone is specified in EIAJ standard (RC8162A).
The EIAJ standard relates to a phantom power supply, and three types (12 V, 24 V, 48 V) of
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1
supply voltages are defined. Therefore, when a phantom power source is used for the condenser
microphone, the maximum amplitude at the output of the condenser microphone is limited to 48
Vp-p.
[0003]
Then, in order to enlarge the output of a condenser microphone, a transformer may be used for
the output circuit. When it is inappropriate to use a transformer for the output circuit, a
transformerless output circuit in which the output circuit is configured without using a
transformer is adopted. In general, a transistorless emitter-follower circuit is used for the
transformerless output circuit. This is because the output impedance can be lowered.
[0004]
Here, an example of a conventional condenser microphone using a transformerless output circuit
will be described with reference to FIG. In FIG. 4, reference numeral 1 denotes a condenser
microphone unit, one output terminal is connected to the impedance converter 2, and the other
terminal is connected to the negative pole of the phantom power source 3.
[0005]
The output terminal of the condenser microphone unit 1 is connected to each of the bases of the
transistors Tr1 and Tr2 connected in an emitter-follower manner, which constitute an output
circuit, via a condenser. The phantom power supply 3 for supplying power to the condenser
microphone unit 1 has its positive pole connected to the emitters of the transistors Tr1 and Tr2
via the supply resistors R1 and R2. Therefore, the supply resistances R1 and R2 are also load
resistances of the transistors Tr1 and Tr2 connected in the emitter-follower connection.
[0006]
The emitters of the transistors Tr1 and Tr2 are connected to output terminals (not shown) so
that the audio signals converted by the microphone unit 1 are balancedly output from these
output terminals. Incidentally, when the voltage of the phantom power source 3 is 48 V, the
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resistance value of the supply resistors R1 and R2 is 6.8 kΩ, and the deviation thereof is
specified to be within 0.4%.
[0007]
In order to make the output of the condenser microphone unit 1 be a balanced output, the
collectors of the emitter follower circuits of the hot side (Tr1 side) and the cold side (Tr2 side)
are coupled. Since the collectors of the transistors Tr1 and Tr2 need to be AC grounded, they are
connected to the negative pole of the phantom power supply 3 via a capacitor (capacitor C3)
having a relatively high withstand voltage and a large electrostatic capacity.
[0008]
As described above, in the condenser microphone using the conventional transformerless output
circuit, the output circuit is constituted by the two transistors Tr1 and Tr2 connected in the
emitter-follower connection, and the collector of the coupled transistors Tr1 and Tr2 is the
phantom power supply 3 The voltage generated across the large capacitor C3 connected to the
negative pole is used as a power supply for operating the impedance converter 2.
[0009]
The impedance converter 2 shown in FIG. 4 is configured of a bias built-in FET which
incorporates a bias resistor and a diode.
[0010]
The voltage supplied from the phantom power supply 3 is a voltage necessary to operate the
emitter follower circuit which is an output circuit consisting of the transistors Tr1 and Tr2 and a
voltage necessary to operate the impedance converter 2 connected to the condenser microphone
unit. It is divided by the voltage.
[0011]
The maximum allowable input sound pressure level of a condenser microphone is mainly limited
by the signal amplitude at the output circuit.
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Therefore, it is desirable that both the voltage for operating the emitter follower circuit and the
voltage for operating the impedance converter 2 connected to the condenser microphone unit be
a high voltage.
In a condenser microphone provided with a transformerless output circuit for this purpose, the
two voltages are designed to have appropriate values.
[0012]
Here, the problem of the conventional transformerless output circuit will be described by taking
the case where the phantom power supply 3 supplies power at 48 V as an example.
The voltage across the supply resistances R1 and R2 (both have a resistance value of 6.8 kΩ) of
the phantom power supply 3 and the voltage between the emitter terminal and the collector
terminal of each transistor Tr1 and Tr2 of the emitter follower circuit which is an output circuit,
and the transistors Tr1 and Tr2 The voltage across the terminals of the capacitor C3 connected
to the collector (the operating voltage of the impedance converter 3) is substantially the same
voltage, so if the voltage of the phantom power supply 3 is 48V, the above respective voltages
are approximately 24V Become. Then, in the conventional output circuit shown in FIG. 4, the
maximum output level is 24 VP-P, and the maximum allowable input sound pressure level is 18.7
dBV.
[0013]
If the output circuit by the emitter follower connection shown in FIG. 5 can be used, the output
level can be maximized. FIG. 5 is a diagram in which the input side (condenser microphone unit
etc.) is omitted in the transformerless output circuit of the condenser microphone. The emitterfollower connection circuit of FIG. 5 has a configuration similar to that of the emitter-follower
connection circuit portion shown in FIG. According to the output circuit shown in FIG. 5, the
voltage (48 V) supplied from the phantom power supply 3 is divided by the voltage across the
supply resistors R1 and R2 and the voltage between the emitter terminal and the collector
terminal of the transistor Tr1 and transistor Tr2. Be done.
[0014]
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Therefore, the maximum output amplitude at each of the output points C and D to which the
output terminal (not shown) is connected in FIG. 5 can be taken up to about 24 V each.
Therefore, in the case of the output circuit as shown in FIG. 5, the maximum output level is
approximately 48 VP-P, and the maximum allowable input sound pressure level is approximately
24.76 BV.
[0015]
However, unlike the output circuit shown in FIG. 4, the output circuit shown in FIG. 5 can not
supply power to operate the impedance converter 2 (see FIG. 1) connected to the subsequent
stage of the microphone unit 1 . That is, the impedance converter can not be used in the output
circuit of FIG.
[0016]
Although no prior art document has been found for the purpose of solving the technical
problems as described above, that is, the problem of increasing the maximum output level while
securing the operating voltage of the impedance converter in the condenser microphone, Patent
document 1 can be mentioned as a prior art document relevant to invention. The invention
described in Patent Document 1 relates to a condenser microphone in which a transistor of an
emitter follower for current amplification is connected between an impedance converter included
in a condenser microphone unit and an output transformer.
[0017]
JP, 2006-352622, A
[0018]
The present invention has been made in view of the above problems, and in a condenser
microphone provided with a transformerless output circuit, a maximum output level can be
obtained by securing a voltage for operating an impedance converter without using a large-sized
condenser. It is an object of the present invention to provide a condenser microphone
characterized by an output circuit capable of raising
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5
[0019]
The present invention relates to a condenser microphone using an emitter-follower connection in
an output circuit, and a transistor in the emitter-follower connection is provided immediately
after the FET constituting the impedance converter, and a constant voltage circuit is provided on
the emitter terminal of the transistor. The present invention is mainly characterized in that the
FETs constituting the impedance converter are operated by the voltage supplied from the
constant voltage circuit.
[0020]
Further, according to the present invention, in the above-mentioned condenser microphone, the
output circuit is one emitter follower connection circuit connected to one output end of the
microphone unit and another emitter follower connection circuit connected to the other output
end of the microphone unit And the signal output from the microphone is balanced output from
the emitters of the transistors of the two emitter-follower connection circuits.
[0021]
Also, according to the present invention, in the above-mentioned condenser microphone, the
constant voltage circuit is configured by a diode, the cathode of the diode is connected to the
emitter terminal of the transistor, and the anode of the diode is connected to the supply
resistance of phantom power. It is characterized by
[0022]
According to the present invention, in the condenser microphone using the transformerless
output circuit, the constant voltage circuit is connected in series to the emitter terminal of the
output emitter follower circuit, and the operation power is supplied from the constant voltage
circuit to the impedance converter. The output level can be increased with a small output circuit
without using a large capacitor.
Further, according to the present invention, it is possible to obtain a condenser microphone
which is resistant to circuit noise and has a good frequency response in the low frequency range.
[0023]
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6
It is a circuit diagram showing an example of a condenser microphone concerning the present
invention.
It is a graph which shows the result of having measured the relationship between the input level
(dBV) of the said Example, and the distortion factor (%) of an output signal.
It is a graph which shows the result of having measured the relationship between the frequency
(Hz) of the said Example, and an input level (dBV).
It is a circuit diagram showing an example of a conventional condenser microphone.
It is a circuit diagram showing an example of an output circuit of a condenser microphone.
[0024]
An embodiment of a condenser microphone according to the present invention will be described
with reference to FIG. In FIG. 1, reference numeral 1 denotes an electret condenser microphone
unit. One end of the microphone unit 1 is connected to the impedance converter 2, and the other
end is connected to the negative pole of the phantom power supply 3.
[0025]
The positive terminal of the phantom power source 3 is connected to the emitter of the transistor
Q1 through the supply resistor R1 and the diode D1, and to the emitter of the transistor Q2
through the supply resistor R2 and the diode D2. Therefore, the supply resistors R1 and R2 are
also load resistors of the emitter-follower connected transistors Q1 and Q2. Output terminals (not
shown) are connected to the emitters of the transistors Q1 and Q2 at points indicated by dotted
circles A and B, and the audio signals converted by the microphone unit 1 are balancedly output
from these output terminals. . Incidentally, when the voltage of the phantom power source 3 is
48 V, the resistance value of the supply resistors R1 and R2 is 6.8 kΩ, and the deviation thereof
is specified to be within 0.4%.
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[0026]
A diode D1 connected between the emitter terminal of the transistor Q1 and the supply resistor
R1 constitutes a constant voltage circuit. Since the resistance value of the diode D1 is
approximately 1 kΩ, the voltage drop across the both ends is approximately 0.7 V. The
impedance converter 3 can operate by applying this voltage between the anode and the cathode
of the FET that constitutes the impedance converter 3 via the resistor R3.
[0027]
The impedance converter 2 shown in FIG. 1 is configured by a bias built-in FET which
incorporates a bias resistor and a diode.
[0028]
As described above, the diode D1 is connected in series between the emitter terminal of the
output circuit by the emitter follower connection and the supply resistor R1 of the phantom
power supply 3, and the impedance converter 2 is connected in parallel to the diode D1. The
impedance converter 2 can be operated with a voltage related to a slight voltage drop generated
in the diode D1.
[0029]
Further, since the voltage drop in the diode D1 is about 0.7 V even if the supply voltage of the
phantom power source 2 is 48 V, the maximum amplitude at the output end A can be made
approximately 24 V.
[0030]
Further, since the condenser microphone shown in FIG. 1 is a balanced output, an emitterfollower connection configured with the transistor Q1 and an emitter-follower connection circuit
to be targeted are configured by the transistor Q2, and The other output end B is provided
between them.
[0031]
As described above, according to the condenser microphone of the present invention, an output
circuit substantially equivalent to the output circuit shown in FIG. 6 can be configured, so that
the maximum output width can be made larger than in the prior art. become.
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[0032]
According to the embodiment shown in FIG. 1, while using the transformerless output circuit as
described above, the maximum output level can be increased compared to the prior art, and the
maximum allowable input sound pressure level can also be increased.
[0033]
FIG. 2 shows the result of measuring the relationship between the input level (dBV) and the
distortion factor (%) of the output signal in the embodiment shown in FIG.
The graph G1 is an example of the case where the condenser microphone 10 according to the
present embodiment is operated at the power supply voltage 48V.
As shown in FIG. 2, when “the distortion rate is 1%” is the allowable upper limit of the speech
quality, the maximum output level is approximately 21.3 dbV.
Therefore, the maximum output level can be made larger in the condenser microphone according
to the present embodiment than the maximum output level (about 18.7 dBV) of the condenser
microphone (see FIG. 5) provided with the conventional output circuit.
[0034]
FIG. 3 shows the result of measuring the relationship between the frequency (Hz) and the input
level (dBV) in the embodiment shown in FIG.
The graph G3 in FIG. 3 is the measurement result of the open circuit voltage sensitivity using a
measuring instrument with an input impedance of 100 kΩ.
When the input level is 30 dBV and the capacitance of the condenser microphone unit is 18 pF,
when the frequency is shifted from 20 Hz to 200 kHz, the variation of the capacitance is about
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0.4 pF.
The graph G4 is an example measured by applying a load of 600 ohms, and according to the
graph G4, the output impedance of the condenser microphone according to the present
embodiment is 44 ohms, which is sufficiently small. Therefore, according to the condenser
microphone of the present embodiment, a practical condenser microphone can be obtained.
[0035]
1 Condenser microphone unit 2 Impedance converter 3 Phantom power supply
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