JPH0951236A - Microphone amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To exclude a stray magnetic field by providing a transistor(TR) input stage having an inductor for increasing/decreasing a frequency band consisting of a troidal core wound by a wire with plural turns at an equal pitch. SOLUTION: The input stage has a troidal ferrite core 310 wound by a wire with plural turns 300 at an equal pitch. The pitch 320 of the turns 300 is made equal to each other to the utmost so that a stray magnetic field in any direction is coupled with each winding parts with an equal distance and in an opposite direction thereby cancelling any of induced currents. The two inductors are mounted on a printed circuit board close to each other so that a magnetic field picked up by one inductor is made nearly equal to a magnetic field picked up by the other inductor. Since the induced noise signals are common mode noise signals with a different polarity with respect to bipolar signals carried by the inductors, the bipolar signals are cacelled as balanced signals in the case of further processing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microphone amplifier.

[0002]

BACKGROUND OF THE INVENTION The performance requirements of microphone amplifiers for high quality speech processing systems are very demanding. They must have high gain and sensitivity and must compensate for the low level of the microphone's electrical output signal, yet they are susceptible to induced noise from other sources, despite their high sensitivity to low input signals. Don't

Microphone amplifiers are usually configured as a two-stage circuit with an open loop high gain transistor input stage and another operational amplification (instrument) stage. Basically, the operational amplifier stage is part of a gain limited feedback loop around the transistor input stage.

However, it has been found that the total bandwidth of the transistor input stage is often greater than the total bandwidth of the operational amplifier stage. Since the gain of the transistor input stage is limited by the feedback provided via the operational amplifier stage, the gain of the transistor input stage will not be controlled at frequencies outside the response range of the operational amplifier stage, thus It is easy to pick up the noise induced at the frequency and greatly amplify it. Therefore, the imbalance in the frequency response characteristics of the transistor input stage and the operational amplification stage can be a noise source in the microphone amplification circuit.

The standard technique for reducing the bandwidth of an amplification stage is to add either (i) resistance and capacitance or (ii) inductance to the amplification stage. However, adding resistance to the transistor input stage of the microphone amplifier is undesirable because any resistance is a noise source. The noise generated at the input of the amplifier due to such resistance adversely affects the noise characteristic of the entire microphone amplifier.

Similarly, the first of high gain amplifiers of this type
The inductor in the stage is susceptible to picking up magnetic interference from other parts of the circuit or external sources. This induced noise is amplified by the rest of the microphone amplifier (and the amplification stage further connected downstream thereof). Due to this problem, attempts have been made to avoid placing an inductor in the input stage of the microphone amplifier, or to magnetically shield the inductor using an expensive and complicated case made of so-called mu metal.

[0007]

SUMMARY OF THE INVENTION The present invention solves the seemingly contradictory problem of reducing the bandwidth of a transistor input stage in a microphone amplifier and avoiding magnetic interference through an inductance that reduces the bandwidth. It is something to solve.

[0008]

The present invention provides a toroidal
The above-mentioned problems have been solved by using an inductance in which wires are wound around the core at substantially equal intervals. With such a symmetrical arrangement, whatever the angle of incidence of the stray magnetic field on the inductance, the magnetic field couples with windings of equal length and opposite direction. Therefore, any induced current tends to cancel out.

Although the present invention is advantageously used with a single transistor input stage, the amplifier has another transistor input stage with the two transistor input stages operating in parallel with complementary polarity (positive). And negative) balanced input signals, the bandwidth reducing inductances of the two input stages should be mounted close together. In this case, since the second inductance is arranged next to the first inductance, the same noise component is picked up. However, since the second inductance carries a signal of opposite polarity to the first inductance, any residual inductive noise components can be canceled out by common mode signals when the two complementary polarity signals undergo further balanced signal processing. Becomes

In order to enhance the similarity of the induced noise between the two inductances, it is preferable that the two inductances are mounted so that their sides are close to each other.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings. In the drawings, corresponding parts are designated by the same reference numerals. FIG. 1 is a circuit diagram showing a microphone amplifier to which the present invention can be applied. The microphone amplifier of FIG. 1 accepts positive and negative balanced input signals 10 and 20 via a DC blocking capacitor 30 and removes phantom power (DC power supplied to the microphone) and other DC signals present in the microphone leads. . Each input signal is a transistor input stage (transistor 210)
And amplified by two symmetrical respective amplifiers arranged around the operational amplification stage (operational amplifier 220).

The microphone amplifier produces a balanced output containing positive and negative output signals. The common mode gain of this amplifier is much lower than its differential mode gain.

The operational amplifier 220 and the input stage transistor 210 are connected through a common feedback loop which acts to limit the gain of the transistor input stage. But,
Since the transistor input stage has an open loop bandwidth greater than the bandwidth of the feedback loop of the operational amplifier, the inductor 4
The zero is connected to the emitter lead of transistor 210 and limits the bandwidth of the transistor input stage. The actual value of the inductance of each inductor 40 is 40 microHenry, and the structure and orientation of the inductor 40 are as shown in FIGS.
See below for more details. The values of other main components are as shown in Table 1. Table 1 Resistors 100 100 000 ohms 110 3000 ohms 120 100 ohms 130 22000 ohms 140 680 ohms 150 10000 ohms 160 4700 ohms 170 2400 ohms capacitors 180 47 microfarads (electrolysis) 190 22 nanofarads 200 arithmetics 210 220N LM4 transistors 210 LM39 transistors. amplifier

FIG. 2 shows a toroidal ferrite core 3
FIG. 1 shows a toroidal inductor for use with the present invention having a plurality of wire turns 300 wound at equal intervals around 10. The spacing 320 of the turns 300 should be as equal as possible so that stray magnetic fields in any direction will combine with windings of the same length but in opposite directions to cancel any induced current. In the case of the 40 microHenry inductor used in FIG. 1, a wire with 10 turns has an outer diameter of 8
Wound at equal intervals around a millimeter toroidal ferrite core.

FIG. 3 shows a pair of toroidal inductors 40 ', mounted closely on a printed circuit board 330.
40 "is a diagram showing 40". In this case, the inductor 40 'may be the inductor shown in the upper part of FIG. 1 and the inductor 40 "may be the inductor shown in the lower part of FIG. The two inductors are juxtaposed on the printed circuit board 330 so that the magnetic field picked up by one inductor is approximately equal to the magnetic field picked up by the other inductor. However, because one inductor carries the positive going signal to be amplified and the other inductor carries the complementary negative going signal, the induced noise is in the opposite direction with respect to the positive and negative signals carried by the inductance. Since they become in-phase node noise, they are canceled when these positive and negative signals are further processed as a balanced signal.

[0016]

As described above, according to the present invention,
The contradictory issues of reducing the bandwidth of the transistor input stage in a microphone amplifier and avoiding magnetic interference via an inductance that reduces the bandwidth can be solved.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a microphone amplifier to which the present invention can be applied.

FIG. 2 is a diagram showing a toroidal inductor used in the present invention.

FIG. 3 is a diagram showing a pair of closely arranged toroidal inductors.

[Explanation of symbols]

40,40 ', 40 "inductance, 210 input stage transistor, 300 wire turns, 310 toroidal core

Claims (3)

[Claims] 1. A microphone amplifier including a transistor input stage having a bandwidth-reducing inductance consisting of turns of a plurality of wires wound approximately equidistantly around a toroidal core. 2. Including another transistor input stage,
The amplifier of claim 1, wherein the two transistor input stages operate in parallel to amplify a balanced input signal of complementary polarities, and the bandwidth reducing inductances of the two input stages are provided in close proximity to each other. 3. The amplifier of claim 2 wherein the two inductances are mounted laterally adjacent to each other.