JPH0465566B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a microphone output circuit that amplifies and extracts an output signal from an electret condenser microphone.

Conventionally, in the output circuit of a microphone using an electret condenser microphone as a signal source, the preamplifier directly connected to the microphone and its negative feedback circuit (NFB) system are in an independent loop. As a result, the circuit configuration becomes complicated and expensive, and harmonic distortion (THD) deteriorates, resulting in deterioration of frequency characteristics.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a microphone output circuit which integrates an electret condenser microphone and an amplification system to simplify the circuit configuration and reduce harmonic distortion.

That is, the output circuit of the microphone of this invention is
A first transistor 18, which is an output circuit that amplifies and extracts the output signal of the electret condenser microphone 2, receives the output signal of the electret condenser microphone at its base.
and a second transistor 26 forming an emitter follower circuit for amplifying the collector output of the first transistor.
A first capacitor 34, a first resistor 36 and a second
A voltage divider circuit for voltage dividing the output is installed on the emitter side of the second transistor using a series circuit of resistors 16, and a voltage dividing point between the first and second resistors of this voltage divider circuit and the first transistor are connected. By connecting the electret condenser microphone to the base side thereof via a second capacitor (coupling capacitor 4), the emitter of the second transistor and the base input section of the first transistor A feedback circuit 8 for feeding back the output on the emitter side of the second transistor to the base input side of the first transistor via the electret condenser microphone is formed between the electret condenser microphone and the electret condenser microphone. It is characterized in that a third resistor 12 is connected between the connection point with the second capacitor and the power supply.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.

The figure shows an embodiment of the microphone output circuit of the present invention. In the figure, an electret condenser microphone (hereinafter simply referred to as microphone) 2 as a signal source is connected to an amplifier 6 that receives its output signal via a coupling capacitor 4 as a second capacitor. A feedback circuit 8 is formed between the output section and the output section. That is, one output terminal of the microphone 2 is connected to the power line to which the drive voltage Vcc is applied from the power supply terminal 10 via the third resistor 12,
A bias voltage is applied, and the other output terminal is connected via a resistor 16 of the feedback circuit 8 to a reference potential line on which a reference potential point terminal 14 connected to a reference potential point (GND) is formed.

At the input of the amplifier 6 is installed a first transistor 18 whose base receives the output of the microphone 2 via the coupling capacitor 4. Resistors 20 and 22 are connected between the base of this transistor 18 and a power supply line or a reference potential line, respectively, and the collector of this transistor 18 is connected to the power supply line via a resistor 24, and a second transistor The emitter of the transistor 18 and the collector of the transistor 26 are both connected to a reference potential line. Further, a resistor 28 is connected between the emitter of the transistor 26 and the power supply line.
The transistor 26 constitutes an emitter follower circuit, and its output is taken out from the emitter. That is,
A capacitor 30 is connected to the emitter of the transistor 26.
An output terminal 32 is formed through the amplified output terminal 32 to take out the amplified output.

A first capacitor 34 of the feedback circuit 8 and a first resistor 3 are connected between the emitter of the transistor 26 and the reference potential point terminal of the microphone 2.
6 is connected. That is, the feedback circuit 8 is provided with a voltage dividing circuit composed of a resistor 36 and a resistor 16 as a second resistor, and a voltage is connected between the voltage dividing point of this voltage dividing circuit and the input section of the amplifier 6. Microphone 2 is connected.

The operation will be explained based on the above configuration. An audio signal input from microphone 2 is provided to the base of transistor 18 via coupling capacitor 4 . The operating current of the transistor 18 is excited by the audio current, and the collector outputs an output corresponding to the amplification factor.This output is further amplified by the transistor 26 and taken out from the output terminal 32 via the capacitor 30. At the same time, the signal is fed back from the emitter of the transistor 26 to the microphone 2 via the feedback circuit 8. That is, the amplified output generated at the emitter of the transistor 26 is transmitted to the resistor 3.
A feedback amount is given by the divided voltages 6 and 16, and this feedback amount becomes the amplification factor of the amplifier 6. That is, if the amplification factor of the amplifier 6 is A _V , the resistance value of the resistor 36 is R _NF1 , and the resistance value of the resistor 16 is R _NF2 , then the amplification factor of the entire system is
A _V0 is A _V0 = A _V (R _NF1 + R _NF2 )/R _NF2 . However, A _V ≫ _{A V0} .

By configuring the microphone 2 as a part of the feedback circuit 8 of the amplifier 6 that amplifies its output, the amplifier 6 and the feedback circuit 8, which are the amplification system, are integrated and configured with a semiconductor integrated circuit. As a result, an amplifier circuit with an extremely simple configuration can be obtained, and harmonic distortion (THD) can be reduced and a flat frequency characteristic can be obtained. In particular, since the distortion rate of the microphone can be improved, excellent acoustic characteristics can be obtained with an inexpensive microphone.

Further, by setting the resistor 16 as a variable resistor and adjusting its resistance value, the sensitivity of the microphone 2 can be finely adjusted.

Furthermore, in such an amplifier circuit, the amplifier 6 and the feedback circuit 8 can be mounted within the package of the microphone 2, and a microphone with high gain and low harmonic distortion can be constructed.

Although this embodiment has been described using an electret condenser microphone as an example of a signal source, the present invention can also be applied to other microphones.

As explained above, according to the present invention, an amplifier is installed which includes a first transistor that receives an output signal of an electret condenser microphone at its base, and a second transistor forming an emitter follower circuit on its output side, By forming a feedback circuit between the input and output sections of this amplifier via an electret condenser microphone, the electret condenser microphone and amplifier are integrated, so that even if the electret condenser microphone and amplifier are configured separately, In comparison, the circuit configuration can be simplified by sharing the bias circuit, etc., and the level of the output signal of the electret condenser microphone can be adjusted by automatic gain control performed by the feedback circuit. , the impedance consisting of the first capacitor, the second capacitor, and the first and second resistors installed in the feedback circuit eliminates distortion even in response to minute inputs, and further reduces harmonic distortion to provide a flat signal. It is possible to amplify and extract the output signal of the electret condenser microphone by obtaining excellent frequency characteristics, and it also achieves DC stability by sharing the bias circuit between the amplifier and the electret condenser microphone and by sharing the DC potential between the two. In addition, it is possible to prevent alternating current oscillation in the low frequency range and stabilize the operation, and it is also possible to reduce manufacturing costs by simplifying the circuit configuration.

[Brief explanation of the drawing]

The figure is a circuit diagram showing an embodiment of the microphone output circuit of the present invention. 2...electret condenser microphone,
4...Coupling capacitor (second capacitor), 6...
Amplifier, 8... Feedback circuit, 12... Third resistor, 16
...Second resistor, 18...First transistor, 26
...Second transistor, 34...First capacitor, 36...First resistor.

Claims (1)

[Scope of Claims] 1. An output circuit for amplifying and extracting an output signal of an electret condenser microphone, comprising: a first transistor whose base receives the output signal of the electret condenser microphone; a second transistor constituting an emitter follower circuit for amplifying the collector output; a first capacitor, a first resistor and a second transistor are installed between the emitter of the second transistor and a reference potential point; A voltage dividing circuit that divides the output voltage is installed on the emitter side of the second transistor using a series circuit of resistors, and the voltage dividing point of the first and second resistors of this voltage dividing circuit and the base of the first transistor are connected. By connecting the electret condenser microphone to the base side thereof via a second capacitor,
between the emitter of the second transistor and the base input of the first transistor;
forming a feedback circuit that returns the output on the emitter side of the transistor to the base input side of the first transistor via the electret condenser microphone, and a connection point between the electret condenser microphone and the second capacitor; A microphone output circuit characterized in that a third resistor is connected between the and a power source.