JPH0951237A - Microphone amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the effect due to mismatch between DC-block capacitors by providing two complementary signal inputs, one amplifier stage or over, a means generating a common mode mix signal, and a means coupling the common made mix signal to each of the signal inputs. SOLUTION: A microphone input signal passing through DC block capacitors 30 is amplified respectively by each transistor(TR) 210 and each operational amplifier 220. A resistor 150 is connected between outputs of each of the two amplifiers 220 and a signal at a connecting point of the resistors 150 represents a common mode output signal. The input impedance of each amplifier with respect to the common mode input is increased more than that with respect to the differential mode input. Since common mode bootstrap is conducted after through a capacitor 180, no effect is imposed on a DC bias of the TR 210. Even when mismatch of the capacitors 30 is in existence, the effect is reduced by increasing the common mode input impedance in this way.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a microphone amplifier having a phantom power.

[0002]

BACKGROUND OF THE INVENTION Phantom power supplies are needed by using signal connections from microphones to carry DC voltage in order to power the microphones from a distance. Usually, the DC voltage is supplied by a device (eg, a mixing console) that receives an audio signal from a microphone. This eliminates the need to use local battery power for the microphone.

However, the DC phantom power must be separated from the microphone amplifier at the input stage of the microphone amplifier to prevent saturation of the operational amplifier within the microphone amplifier. To achieve this isolation, previous designs have placed a pair of DC blocking capacitors, one on each input arm of the microphone amplifier. Although its operation is satisfactory in some respects, this type of circuit has several drawbacks.

The main drawback is that the two capacitors must be chosen such that their respective values are almost perfectly matched. The reason is that if the values of the two capacitors do not match, the effect of common mode rejection of the amplifier by producing a differential noise signal from the common mode input is reduced.

[0005]

The values of the two capacitors were made very large in an attempt to satisfy the requirement that the values of these capacitors be exactly the same (which is extremely difficult in practice). This method does not reduce the amount of differential noise, but it can shift its effects to lower frequencies where it is not a noticeable problem in audio systems. However, this workaround also introduces the problem that when the phantom power supply is switched from on to off, the circuitry of the amplifier takes an unacceptably long time to reset. During this reset period, a slight imbalance in the two capacitors can transiently generate a large differential noise signal. Therefore, an object of the present invention is to reduce the above problems.

[0006]

SUMMARY OF THE INVENTION A microphone amplifier according to the present invention amplifies two complementary signal inputs, each microphone connectable via a respective DC phantom power blocking capacitor, and a differential signal at the signal inputs. Two complementary output signals to provide a common mode mixed signal by combining the output signals with one or more amplification stages that provide a common mode gain approximately equal to 1 for the common mode signal at the signal input. Means for generating the common mode
Means for coupling the mix signal to each of said signal inputs.

In the present invention, the common mode signal is returned to the input as a bootstrap signal by almost equality. Compared to conventional bootstrap techniques, this common-mode bootstrap arrangement can increase the input impedance of the amplifier's common-mode input relative to the input impedance of the differential-mode input. By increasing the common-mode input impedance in this way, even if there is a mismatch in the DC blocking capacitor,
The effect is reduced. The reason is that the common mode input impedance determines the differential component generated from the common mode input due to the mismatch of the DC blocking capacitors.
Therefore, the higher the common-mode input impedance,
This is because the differential noise component becomes low. This also reduces the need for very large capacitors (correspondingly large settling times) to avoid differential noise.

Said coupling means preferably comprises two substantially identical resistors connected between the two signal inputs, said common mode mixed signal being connected to the connection point of these two resistors.

In order not to affect the setting of the DC bias of the amplifier, the common mode mixed signal is preferably connected to the connection point of the two resistors via one capacitor.

[0010]

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 an embodiment of the present invention. The illustrated microphone amplifier has a positive and negative balanced input signal 10, 20 via a DC blocking capacitor 30.
Receive.

The purpose of the DC blocking capacitor 30 is to isolate the input of the microphone amplifier from the phantom power source Vp (which may come from part of the amplifier itself) or other DC signal present on the microphone conductor. is there. These capacitors are matched to each other within a nominal value of about 2 microfarads, for example within 1%. They also mount on the circuit board in close proximity to each other so that the thermal effects or drifts of the two capacitors are reasonably well balanced.

The microphone input signal that has passed through the DC blocking capacitor 30 has a transistor input stage (transistor 210) and an operational amplification (device) stage (operational amplifier 22).
Amplification is carried out by two symmetrically arranged amplifiers arranged around 0). The microphone amplifier produces a balanced output that includes positive and negative output signals. The common mode gain of the amplifier is much lower than its differential mode gain. The actual common mode gain of the circuit of FIG. 1 is approximately unity for the components shown.

Two resistors 150 are connected between the outputs of the two operational amplifiers 220, and the signal at the junction of these two resistors represents the common mode output of the amplifier. This signal is returned as a common mode bootstrap signal via capacitor 180 to the junction of the two input resistors 102, 104.

Compared to the conventional bootstrap technique, FIG.
The common-mode bootstrap arrangement of increases the input impedance for the common-mode input of the amplifier compared to the input impedance for the differential-mode input.
This common-mode bootstrap is a capacitor 18
Since it is performed through 0, it is achieved without affecting the DC bias of the input transistor 210.

By increasing the common-mode input impedance in this manner, even if there is a mismatch between the DC blocking capacitors, its effect is reduced. The reason is that the differential component generated from the common mode input due to the mismatch of the DC blocking capacitors is determined by the common mode input impedance, and the higher the common mode input impedance, the lower the differential noise component.

Although the embodiments of the present invention have been described above, it goes without saying that modifications may be made within the scope of the claims.

The values of the main components of the circuit of FIG. 1 are as shown in Table 1. Table 1 Resistors 100 , 102 , 104 100000 Ohms 110 3000 Ohms 120 100 Ohms 130 22000 Ohms 140 680 Ohms 150 10000 Ohms 160 4700 Ohms 170 2400 Ohms Capacitors 30 2 Microfarads 180 47 Microfarads 190 22 Nanofarads 200 150 picofarad inductors 90 40 Micro Henry Semiconductor 210 LM394 Transistor 220 NE5532 Operational Amplifier

[0018]

As described above, according to the present invention,
The DC phantom increases the common mode input impedance of the circuit by bootstrapping the common mode signal, even if there is a mismatch in the value of the DC blocking capacitor placed at the input stage to separate the DC phantom power from the microphone amplifier. The influence of mismatch of the power blocking capacitors can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

[Explanation of symbols]

30 DC phantom power blocking capacitors, 210, 2
20 amplification stages 102, 104, 150, 180 coupling means

Claims (5)

[Claims] 1. Two complementary signal inputs, each connectable to a microphone via a respective DC phantom power blocking capacitor, and amplifying a differential signal at the signal inputs to provide two complementary output signals. , One or more amplification stages that provide a common mode gain to the common mode signal at the signal input that is approximately equal to 1, and means for combining the output signals to generate a common mode mix signal, the common mode mix Means for coupling a signal to each of said signal inputs. 2. The coupling means comprises two substantially identical resistors connected between the two signal inputs, the common mode mix signal being connected to the connection point of these two resistors. Amplifier. 3. The amplifier according to claim 2, wherein the common mode mixed signal is connected to the connection point of the two resistors via one capacitor. 4. The amplifier according to claim 1, wherein the DC blocking capacitors have matched nominal values and are mounted close together on a circuit board. 5. An amplifier according to any one of claims 1 to 4 including means for providing a DC phantom power signal to a microphone.