JP2922018B2 - Balanced output circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission apparatus for converting and transmitting an analog signal such as voice, and a balanced output circuit of a terminal device.

[0002]

2. Description of the Related Art For example, when an analog signal is transmitted over a long distance, the signal is controlled so as not to be affected by noise due to electromagnetic waves or the like and to be affected by a ground potential difference between a driving side (signal transmitting side) and a load side (signal receiving side). Is separated from the ground in a DC manner and is balanced and transmitted using a pair cable.

Conventionally, a circuit shown in FIG. 2 has been used as a balanced output circuit of such a device for transmitting an analog signal in a balanced manner.

In FIG. 2, an internal circuit 1 includes a positive / negative power supply +
Ec and -Ec, and ground G,
Upon receiving these power supplies, an analog signal to be output to the outside of the device is generated and supplied to the output amplifier 2. This output amplifier 2 is also connected to the positive and negative power supplies + Ec and -Ec and the ground G, receives these power supplies, amplifies the input signal, and amplifies the power of the input signal via the impedance matching resistor 3 to the transformer 4. It is applied to one end of the primary winding 4a.
The other end of the primary winding 4a of the transformer 4 is grounded. The secondary side of the transformer 4 is DC-separated from the primary side. Both ends of the secondary winding 4b are connected to output terminals 5a and 5b, and signals appearing in the secondary winding 4b are output from these output terminals 5a and 5b as balanced signals. The impedance matching resistor 3 is provided so that the output impedance of this circuit becomes the characteristic impedance of the pair cable connected to the output in order to suppress the reflected wave.

FIG. 3 shows a second conventional example of a balanced output circuit of a device for transmitting an analog signal in a balanced manner.

In FIG. 3, an output analog signal of internal circuit 1 is applied to output amplifier 9. This output amplifier 9
Are also connected to the positive and negative power supplies + Ec and -Ec and the ground G, and receive the power supply to amplify the input signal. The output amplifier 9 of this conventional example has a differential output configuration, and provides a non-inverted amplified output signal to an output terminal 5a via a DC separating capacitor 10 and an impedance matching resistor 12 sequentially. To the output terminal 5b via the DC separating capacitor 11 and the impedance matching resistor 13 sequentially. The DC separation capacitors 10 and 11 separate the output signal of the output amplifier 9 in a DC manner, thus realizing a balanced output. The impedance matching resistors 12 and 13
Also, in order to suppress the reflected wave, the output impedance is provided so as to be the characteristic impedance of the cable.

[0007]

However, there has been a problem that the conventional circuits become large when both circuits are for low-frequency transmission.

First, it will be described in detail that the conventional circuit shown in FIG. 2 has such a problem. Assuming that the impedance of the transformer 4 is Lt and the load resistance between the output terminals 5a and 5b is RO, the following conditions must be satisfied in order to be able to ignore the transmission loss (power consumption) in the transformer 4.

Ω · Lt >> RO (1) where ω: angular frequency (rad / sec) Therefore, in order to make the transmission loss of the transformer 4 negligible, from the equation (1), The inductance Lt or the angular frequency ω must be increased. Here, the angular frequency ω is determined by the frequency of the signal to be transmitted and cannot be set arbitrarily. Therefore, in order to satisfy the condition of the expression (1), the inductance Lt of the transformer 4 must be increased. Need to be done. As is apparent from the equation (1), the smaller the angular frequency ω, the larger the inductance Lt must be. Therefore, when transmitting low frequency signals,
It is required that the inductance Lt is very large.
The transformer 4 having a very large inductance Lt is actually large, and a balanced output circuit using the transformer 4 cannot be downsized.

Next, it will be described in detail that the conventional circuit shown in FIG. The capacitances of the capacitors 10 and 11 are both C0, the values of the resistors 12 and 13 are both R1, and the load resistance between the output terminals 5a and 5b is R0.
Then, in order to be able to ignore the transmission loss (power consumption) in the capacitors 10 and 11, it is necessary to satisfy the following conditions.

1 / (ω · C0 / 2) << (2R1 + R0) ω · C0 >>>> 1 / (R1 + R0 / 2) (2) The resistance values R1 and R0 are determined by the characteristic impedance of the transmission cable. Yes, a fixed value. Therefore, from the equation (2), in order to make the transmission loss in the capacitors 10 and 11 negligible, the capacitor 10
And the capacitance C0 or the angular frequency .omega. As described above, since the angular frequency ω cannot be set arbitrarily, it is necessary to increase the capacitance C0 of the capacitors 10 and 11 to satisfy the condition of the expression (2). As is apparent from the equation (2), the smaller the angular frequency ω, the larger the capacitance C0 must be. When transmitting a low-frequency signal, the capacitance C0 is increased.
Is required to be very large. Therefore, the capacitors 10 and 11 are actually large, and the balanced output circuit using them cannot be downsized.

The present invention has been made in view of the above points, and an object of the present invention is to provide a balanced output circuit which can be downsized and is particularly suitable for low-frequency transmission.

[0013]

SUMMARY OF THE INVENTION In order to solve this problem, the present invention relates to a balanced output circuit for amplifying an analog signal generated by an internal circuit by an output amplifier and outputting the amplified signal to a balanced transmission line. And a supply power separating means for separating the power supply and the ground of the output amplifier from the power supply and the ground of the internal circuit in a DC manner, and providing a differential output of the output amplifier. This is used as it is as a balanced output.

[0014]

In the case of balanced output, it is necessary that the DC component of the signal output to the balanced transmission line be independent of the DC component of the analog signal generated by the internal circuit. Therefore, DC separation is required, but in the present invention, the signal itself is separated by a DC separation capacitor interposed between the internal circuit and the output amplifier. But,
In the case where the power supply and the ground of the output amplifier are the same as the power supply and the ground of the internal circuit, the complete DC separation is not performed even in this case. Therefore, a power supply separating means for separating the power supply and the ground of the output amplifier from the power supply and the ground of the internal circuit in a DC manner is provided, and complete DC separation is realized. With this configuration, the differential output of the output amplifier is a balanced output, and there is no need to provide a DC separating capacitor on the output side of the output amplifier.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings. Here, FIG. 1 shows the entire configuration of this embodiment, and the same reference numerals as those in FIG. 3 denote the same or corresponding parts.

As shown in FIG. 1, this embodiment is different from the second conventional example (FIG. 3) in the installation position of the DC separating capacitor and the power supply configuration to the output amplifier 9.
That is, a capacitor 17 for DC separation is interposed on the signal transmission path from the internal circuit 1 to the output amplifier 9, and no capacitor is provided on the output side of the output amplifier 9. The positive and negative power supplies + Ec and -Ec for the internal circuit 1 are supplied to primary / secondary separated DC / DC converters 15 and 16, respectively, to form power supplies + Vc and -Vc for the output amplifier 9. The output is supplied to an output amplifier 9. Further, the ground G of the internal circuit 1 and
The ground E of the output amplifier 9 is also separated.

The DC / DC converters 15 and 16
Without providing the DC power supply, the power supply + Ec, -Ec and the ground G of the internal circuit 1 and the output amplifier 9 are common if the insertion position of the capacitor 17 for DC separation is changed from the conventional one. Can not do (equilibrate).

The primary / secondary separated type DC / DC converters 15 and 16 can be realized by, for example, a switching regulator configuration as shown in FIG.
FIG. 4 shows the detailed configuration of the DC / DC converter 15, but the DC / DC converter 16 can also be realized by a similar configuration.

In FIG. 4, an input positive power supply + Ec to the ground G, that is, a DC voltage is supplied to a switching circuit 20 driven and controlled by a switching control circuit 21, and is switched by the switching circuit 20 to form a rectangular wave voltage. (AC voltage) and applied to the primary winding 22a of the converter transformer 22. Thus, an AC voltage is induced in the secondary winding 22b of the transformer 22, which is rectified by the rectifier circuit 23, and a predetermined DC voltage + Vc with respect to the ground E on the secondary side.
Thus, the power is supplied to the output amplifier 9 as described above.

The DC / DC converter 1 having such a configuration
The balanced output circuit according to the embodiment in which 5 and 16 are newly provided and the capacitor 17 for direct current separation is provided before the output amplifier 9 performs balanced output processing as follows. The analog signal generated by the internal circuit 1 is supplied to the output amplifier 9 after the DC component is removed via the DC separating capacitor 17. The output amplifier 9 receives the power supplies + Vc and -Vc from the DC / DC converters 15 and 16, power-amplifies the transmission signal from which such a DC component has been removed, and converts the non-inverted amplified output to the impedance matching resistor 1.
2 to the output terminal 5a and the inverted amplified output via the impedance matching resistor 13 to the output terminal 5b.
Give to. Thus, the balanced output of the transmission signal to the pair cable, which is the balanced transmission path, is performed.

Here, the DC / DC converters 15 and 1
In FIG. 6, the internal circuit 1 and the output amplifier 9 are connected via a capacitor 17 using the primary-side secondary-side separated type shown in FIG. It can be completely separated in direct current, and its output becomes a perfectly balanced output.

Next, the physical size of the DC separating capacitor 17 with respect to the change of the interposed position will be examined by comparison with the conventional circuit shown in FIG.

Conventionally, two capacitors are provided, but in this embodiment, only one capacitor 17 is provided. In this respect, the circuit can be made smaller. In addition, since the input resistance of the output amplifier 9 can be made very large, the capacitor 1 is placed at a position before power amplification.
By providing 7, the capacitance of the capacitor 17 can be reduced so that it can be easily estimated from the equation (2), and the circuit can be reduced from this point as well. Accordingly, the output amplifier 9 can output a low-frequency signal in a balanced manner without being restricted by the above-described equation (2). That is, unlike the related art, a large capacitor is not required.

Next, the DC / DC converters 15 and 16
Consider the physical size for the provision of. In this study, the DC / DC converter 15 will be described.

In the DC / DC converter 15, the switching frequency of the switching circuit 20 may be within the range of the performance of the switching element.
Select a high frequency of ~ 100 kHz. Therefore, transformer 2
As can be seen from the above-mentioned equation (1), the inductance of the power supply 2 can be made very small (normally, the load resistance of the power supply is quite small.
The load current of the power supply is considerably larger than the signal power supply of the circuit). Therefore, the size of the transformer 22 can be reduced. That is, a large transformer is not required unlike the related art.

As described above, according to the above embodiment, the physical size occupied by the DC separating capacitor 17 can be made much smaller than in the past, and the DC / DC converters 15 and 16 are provided. Even if a new occupied volume is required, the overall size can be reduced. Such an effect is particularly remarkable in a circuit that outputs a low-frequency signal in a balanced manner.

In the above embodiment, the internal circuit 1
And the output amplifier 9 operates by receiving the supply of positive and negative power. However, the present invention is applicable even if the internal circuit and the output amplifier operate by receiving either the positive or negative power. can do.

Although the present invention is particularly effective when outputting a low-frequency signal in a balanced manner, it is naturally applicable to a case where a high-frequency signal is output.

[0029]

As described above, according to the present invention, the DC separating capacitor interposed between the transfer of analog signals between the internal circuit and the output amplifier, the power supply and the ground of the output amplifier,
A power supply separation means that separates DC power from the power supply and ground of the internal circuit is provided, and the differential output of the output amplifier is used as a balanced output as it is, so a small balanced output circuit that does not require a large capacitor or a large transformer. Can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an embodiment.

FIG. 2 is a block diagram showing a conventional configuration (part 1).

FIG. 3 is a block diagram showing a conventional configuration (part 2).

FIG. 4 is a block diagram showing a detailed configuration of the DC / DC converter of the embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal circuit, 9 ... Output amplifier, 12 and 13 ... Impedance matching resistance, 15 and 16 ... DC / DC converter, 17 ... DC separation capacitor, + Ec, -Ec, G
... Power supply and ground for internal circuit 1, + Vc, -Vc
, E: power supply and ground for the output amplifier 9.

Continued on the front page (58) Fields surveyed (Int.Cl. ⁶ , DB name) H03F 1/42-1/56 H03F 3/20-3/36 H03F 3/42-3/44 H03F 3/50-3 / 52 H04B 1/02-1/04 H04B 1/76-3/44 H04B 3/50-3/60 H04B 7/005-7/015

Claims (1)

(57) [Claims] 1. A balanced output circuit for amplifying an analog signal generated by an internal circuit by an output amplifier and outputting the amplified signal to a balanced transmission line, wherein a DC separating capacitor interposed between the transmission and reception of the analog signal between the internal circuit and the output amplifier. And a power supply separating means for separating the power and ground of the output amplifier from the power and ground of the internal circuit in a DC manner, and using the differential output of the output amplifier as a balanced output as it is. Balanced output circuit.