JP5414549B2 - Microphone output circuit - Google Patents

Description

  The present invention relates to an output circuit of a microphone using an emitter follower, and more particularly to adjustment of the balance level of a balanced output.

  The output of the microphone is normally output as a balanced signal so that noise is not generated in the audio signal even if an electric field or magnetic field causing noise is applied to the transmission path. The balanced output is required to have the same impedance on the hot side and the cold side. If the impedance is not the same, the balanced output becomes unbalanced and noise occurs in the audio signal.

  In the case of using a transformer in the output circuit, the output side winding is bifilar. That is, two parallel wires are wound as they are, a center tap is provided, and a current for operating the circuit is drawn from the end of the two wires and the center tap. According to the output circuit having such a configuration, there is an advantage that the balance of the balanced output can be prevented from being impaired by making the center tap have an alternating high impedance.

  On the other hand, there is a microphone output circuit that does not use a transformer as described above. In this output circuit, a circuit in which a PNP transistor is connected as an emitter follower is used to lower the output impedance. FIG. 2 shows an example of a microphone output circuit using a conventional emitter follower. In FIG. 2, the upper half shows the phantom power supply, and the lower half shows the output circuit of the microphone. Circled numbers 1, 2, and 3 indicate connectors that connect the phantom power supply side and the output circuit side of the microphone, respectively. The output circuit side includes two PNP transistors 13 and 14 connected in parallel.

  The bases of the transistors 13 and 14 are respectively connected to one output terminal 11 and the other output terminal 12 of a microphone unit (not shown), and base resistors 15 and 16 are connected between the bases and collectors of the transistors 13 and 14. Yes. The collectors of the transistors 13 and 14 are connected to the negative pole of the phantom power source 20 through a common resistor 17 and further through the first pin of the connector. In this way, the transistors 13 and 14 are connected to the emitter follower with their collectors grounded. The microphone unit in this circuit example is assumed to be a condenser microphone unit, and the audio signal taken out from the diaphragm and the counter electrode constituting the capacitor by facing each other is impedance-converted by the impedance conversion circuit, and the above output The signal is output from the terminals 11 and 12.

  The positive pole of the phantom power source 20 is connected to the emitters of the transistors 13 and 14 through the supply resistors 21 and 22 of the phantom power source and further through the second and third pins of the connector. Therefore, the supply resistors 21 and 22 are also load resistors 21 and 22 of the transistors 13 and 14 connected in the emitter follower. The emitters of the transistors 13 and 14 are connected to a hot output terminal 25 and a cold output terminal 26 for balanced output via electrolytic capacitors 23 and 24, and converted from these output terminals 25 and 26 by a microphone unit. The audio signal is output in a balanced manner. Incidentally, when the voltage of the phantom power supply 20 is 48V, the supply resistors 21 and 22 are 6.8 KΩ, and the deviation is defined to be within 0.4%.

  If the output impedance of the transistors 13 and 14 connected to the emitter follower varies, the balance of the balanced output is lost. If an electric field or magnetic field is applied to the output line, noise is generated in the audio signal. Therefore, as the pair of transistors 13 and 14 connected to the emitter follower, a dual transistor with uniform characteristics, that is, a transistor formed as a single chip is used. However, since individual transistors have individual differences and characteristics tend to vary from the beginning, it is difficult to obtain a high degree of balance without adjusting the balance of the balanced output. In particular, due to the spread of mobile phones in recent years, electromagnetic waves are often transmitted and received in the vicinity of microphones, and there are many problems that this electromagnetic wave enters the output line and causes noise in audio signals. ing. For this reason, it is required to further increase the balance of the balanced output so that noise is not generated.

  Prior art documents aimed at solving the technical problems as described above, that is, the problem of increasing the balance of the balanced output and preventing the generation of noise were not found. Patent Document 1 can be cited as an example of prior art documents. The invention described in Patent Document 1 relates to a condenser microphone in which an emitter follower transistor for current amplification is connected between an impedance converter provided in a condenser microphone unit and an output transformer.

JP 2006-352622 A

  The present invention solves the above-mentioned problems of the prior art, that is, by making it possible to adjust the balance of the balanced output in the output circuit of the microphone that employs the emitter follower in the output circuit, thereby increasing the balance. Thus, it is an object to prevent noise from being generated.

An output circuit of a microphone according to the present invention includes two transistors connected to an emitter follower at one output end and the other output end of the microphone unit, respectively, and each output end of the microphone unit is connected to a base of the two transistors. The collectors of the two transistors are grounded via a common resistor, and base resistances are connected to the bases of the two transistors, and the base currents of the two transistors are between the base resistances. The main feature is that a variable resistor for adjusting the voltage is connected in series, the variable terminal of the variable resistor is connected in common to the collectors of the two transistors, and the signals are balanced output from the emitters of the two transistors. And

  By adjusting the variable resistance connected between the bases of the two transistors, the base current of the two transistors can be adjusted and the emitter current can be adjusted, thereby changing the output impedance of each transistor. it can. Therefore, even if there are variations in the characteristics of the two transistors for balanced output of the microphone output signal, or even if there are variations in the values of the circuit elements, the balance of the balanced output can be accurately adjusted. And the degree of balance can be adjusted to a high degree.

It is a circuit diagram which shows the Example of the output circuit of the microphone which concerns on this invention. It is a circuit diagram which shows the example of the output circuit of the conventional microphone.

  An embodiment of a microphone output circuit according to the present invention will be described below with reference to FIG. This embodiment also uses a circuit in which a PNP transistor is connected as an emitter follower, as in the conventional example shown in FIG. Also, the same components as those of the conventional example shown in FIG.

  In FIG. 1, about half on the upper side shows the phantom power supply side, and about half on the lower side shows the output circuit side of the microphone. Circled numbers 1, 2, and 3 indicate connectors that connect the phantom power supply side and the output circuit side of the microphone, respectively. The output circuit side includes two PNP transistors 13 and 14 connected in parallel. The transistors 13 and 14 are respectively connected in an emitter follower as will be described below.

  The bases of the two transistors 13 and 14 are respectively connected to one output terminal 11 and the other output terminal 12 of a microphone unit (not shown). One ends of base resistors 15 and 16 are connected to the bases of the two transistors 13 and 14, respectively, and fixed terminals at both ends of the variable resistor 18 are connected to the other ends of the base resistors 15 and 16, respectively. The variable terminal of the resistor 18 is commonly connected to the collectors of the two transistors 13 and 14. In other words, a base resistor is connected to each base of the two transistors 13 and 14, and a variable resistor is connected in series between these base resistors. The resistance value between the bases of the two transistors 13 and 14 is a value fixed by the base resistor 15, the variable resistor 18, and the base resistor 16 connected in series.

  The collectors of the transistors 13 and 14 are both connected to the variable terminal of the variable resistor 18 and are connected to the negative pole of the phantom power source 20 through the common resistor 17 and the first pin of the connector. In this way, the transistors 13 and 14 are connected to the emitter follower with their collectors grounded. The microphone unit type in this circuit example is assumed to be a capacitor type microphone unit, and the audio signal taken out from the diaphragm and the counter electrode constituting the capacitor by opposing each other is impedance-converted by the impedance conversion circuit And output from the output terminals 11 and 12.

  The positive pole of the phantom power source 20 is connected to the emitters of the transistors 13 and 14 through the supply resistors 21 and 22 of the phantom power source and further through the second and third pins of the connector. Therefore, the supply resistors 21 and 22 also serve as the load resistors 21 and 22 of the transistors 13 and 14 connected in the emitter follower. The emitters of the transistors 13 and 14 are connected to a hot output terminal 25 and a cold output terminal 26 for balanced output via electrolytic capacitors 23 and 24, and the two transistors 13 and 14 are connected through these output terminals 25 and 26. The sound signal converted by the microphone unit is output in a balanced manner from the emitter. Incidentally, when the voltage of the phantom power supply 20 is 48V, the values of the supply resistors 21 and 22 are both the same resistance value of 6.8 KΩ, and the deviation is set to be within 0.4%.

  As described in the conventional example shown in FIG. 2, if the output impedance of the transistors 13 and 14 connected to the emitter follower varies, the balance of the balanced output is lost. If an electric field or magnetic field is applied to the output line, an audio signal is generated. Noise is generated. Therefore, as the pair of transistors 13 and 14 connected as the emitter follower, it is preferable to use a dual transistor whose characteristics are easily uniform, that is, a transistor formed as a single chip. However, there are individual differences in characteristics among individual transistors, and the characteristics tend to vary. Due to the variations in characteristics, the balance of the balanced output shifts and it is difficult to obtain a high balance without adjustment.

  Therefore, in the embodiment shown in FIG. 1, the balance of the balanced output can be adjusted with high accuracy by adjusting the variable resistor 18. That is, the base resistors 15 and 16 and the variable resistor 18 are connected so that when the value of the base resistance of one transistor 13 increases by adjusting the variable resistor 18, the value of the base resistance of the other transistor 14 decreases. Yes. Therefore, when the base current Ib1 of one transistor 13 decreases due to the adjustment of the variable resistor 18, the base current Ib2 of the other transistor 14 increases. Conversely, when the base current Ib1 of one transistor 13 increases, The base current Ib2 of the transistor 14 decreases.

  According to the embodiment shown in FIG. 1, as described above, by adjusting the variable resistor 18, it is possible to adjust the output impedances of the two transistors 13 and 14 to be equal, and the output terminals 25 and 26. It is now possible to adjust the balance of the signal output from the balance with high accuracy.

Each of the two transistors 13 and 14 may be a single transistor, and need not be a dual transistor formed on one chip as described in the illustrated embodiment.
The variable resistor 18 may be a semi-fixed resistor that maintains its adjustment position once adjusted, but may be a variable resistor that can be arbitrarily adjusted at any time.

  Although the illustrated embodiment is configured to use a condenser microphone unit that uses a phantom power source, the microphone output circuit according to the present invention requires that a phantom power source be used. In addition, the microphone unit may be, for example, a dynamic microphone unit other than the condenser microphone unit.

DESCRIPTION OF SYMBOLS 11 Output terminal of microphone unit 12 Output terminal of microphone unit 13 Transistor 14 Transistor 15 Base resistance 16 Base resistance 18 Variable resistance 20 Phantom power supply 21 Load resistance 22 Load resistance

Claims (6)

Equipped with two transistors connected to the emitter follower respectively at one output end and the other output end of the microphone unit,
Each output terminal of the microphone unit is connected to the bases of the two transistors,
The collectors of the two transistors are grounded through a common resistor,
A base resistor is connected to each base of the two transistors, and a variable resistor for adjusting a base current of the two transistors is connected in series between the base resistors .
The variable terminal of the variable resistor is commonly connected to the collectors of the two transistors,
A microphone output circuit in which signals are balancedly output from the emitters of the two transistors.   2. The microphone output circuit according to claim 1, wherein the two transistors are dual transistors formed on one chip.   3. The microphone output circuit according to claim 1, wherein the two transistors are both PNP transistors. 4. The microphone output circuit according to claim 1 , wherein each emitter of the two transistors is connected to be supplied with phantom power via a supply resistor of phantom power . 5. The microphone output circuit according to claim 1 , wherein a supply resistance of the phantom power supply also serves as a load resistance of two transistors . 6. The microphone output circuit according to claim 5 , wherein the load resistances of the two transistors have the same resistance value .

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