JPS5947508B2 - side sound protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in side sound protection circuits using semiconductor elements.

Side sound prevention circuits in telephones and the like are generally constructed of circuit networks using coils, but side sound prevention circuits using semiconductor elements have also been proposed, as shown in FIG. 1, for example.

In the side sound protection circuit shown in the same figure, the receiving signal generated on the lines 1 and 2 is supplied to the receiving amplifier 4 via the capacitor 3, and the output signal drives the receiver 5, and the transmitting signal from the transmitter 6 is supplied to the receiver amplifier 4. is amplified by the transmitting amplifier T and the buffer amplifier 8 and transmitted to the lines 1 and 2, but a part of the signal is supplied to the receiving amplifier 4 via the capacitor 3 in the same way as the above-mentioned receiving signal. The electric potentials at point A and point B are adjusted so that their potentials are equal and their phases are opposite, thereby achieving a side sound prevention effect. Although the side sound prevention circuit shown in FIG. 1 can be put to practical use, the following problems should be improved in order to further improve its usefulness.

1. Since one end of the receiver 5 is not completely connected to the common potential, a part of the output of the receiver amplifier 4 is consumed in the parallel circuit of the resistor 9 and capacitor 10 of the transmitter amplifier 7, and this component becomes a loss. .

In other words, not all of the output of the receiver amplifier 4 becomes a signal for driving the receiver 5, and must be amplified to account for losses. In order to prevent this loss from becoming too large, it is necessary to appropriately select the output impedance of the transmitter amplifier 7, and to reduce the degree of freedom in phase correction, the subscriber line resistance must be controlled within a certain range. The calling distance becomes shorter. 2
In the case of a common grounding type device such as a cordless telephone that has a wireless transmitter/receiver in the telephone body and handset, a transformer is required, and there is no point in using a semiconductor element and omitting the hybrid coil.

In order to solve these problems, the present invention provides a side sound prevention circuit that performs a side sound prevention function using an adder circuit.

An embodiment of the present invention will be described below with reference to FIG.

In the figure, a direct current voltage is supplied to lines 11 and 12 from an exchanger (not shown), and a buffer amplifier 13 functions to stabilize the direct current voltage and to act as an amplifier for the transmission signal, as in FIG. 1. The reception signals now generated on the lines 11 and 12 are led to the first in-phase input terminal 17 of the reception amplifier circuit 16 via the capacitor 14. The reception signal amplified by the reception amplifier circuit 16 is generated at a terminal 21 and a common terminal 22. Since the reception amplifier circuit 16 constitutes an in-phase amplification circuit having first and second in-phase input terminals IT, 18, the in-phase components to both input terminals are canceled out and do not appear at the output terminal 21.

In a normal telephone, the terminal 21 and the common terminal 22 are
The handset is connected, and in cordless telephones the signals developed at these terminals 21, 22 are applied to the transmitter as a modulating signal. The transmitting signal is generated at the terminal 23 and the common terminal 22, which are also connected to a transmitter in a normal telephone, and are supplied with a demodulated signal from a receiver in a cordless telephone.

The transmit signal is then supplied to the base of transistor 24. This transistor 24 is biased by resistors 25, 26 and 27 to form a transmitting amplifier, and the amplified transmitting signal is transmitted to the lines 11 and 12 via the buffer amplifier 13. Further, a part of the transmission signal generated at the emitter of the transistor 24 is supplied to the amplifier 30 via the capacitor 28 and the resistor 29. The capacitor 28 and resistor 29 together with the capacitor 31 and resistor 32 of the input circuit of the amplifier 30 form a phase correction circuit 33 . The output signal of the amplifier 30 is then led to the second common-mode input terminal 18 of the common-mode input amplification circuit 16, and subjected to addition action with the other input. The input terminal 18 is set to a value much larger than the output impedance of the amplifier 30, and the input terminal 17 is set to a value much larger than the impedance of the line. Therefore, the transmitting signal component of the amplifier 30 and the transmitting signal component supplied from the buffer amplifier 13 via the capacitor 14 have equal amplitudes and opposite phases, and are added by the in-phase input amplifier circuit 16. Because it is canceled out, it acts to prevent sidetones. The effects of the present invention described above are as follows. 1
Since the first common-mode input terminal 17 of the common-mode input amplifier circuit 16 forming the adder circuit is selected to be larger than the impedance of the lines 11 and 12 (for example, 600Ω), the line 11
. Therefore, it can be realized with low power consumption.

Further, since the receiving signal is generated at the terminal 21 and the common terminal 22, it can be effectively transmitted to the load. The phase adjustment circuit for two sidetone balance can be adjusted independently of other circuits, and the values of the elements can be selected fairly freely, so that desired transmission characteristics can be easily achieved.

3. Can be widely applied to common grounding devices such as cordless telephones.

4. Furthermore, by using the high input impedance in-phase input amplifier circuit 16 as the adder circuit, there are various advantages.

That is, when the above-mentioned equalizer circuit is constructed using a high impedance element Za as in Japanese Patent Publication No. 49-32606, the equivalent circuit becomes as shown in Figure 3a, and in order to reduce reception signal loss, reception amplification is required. It is also necessary to increase the input impedance of the circuit A, which facilitates electrostatic coupling with the noise source N.
As a result, noise is mixed into the received signal, causing a reduction in S/N. Furthermore, when the high impedance element Za is mounted on a printed circuit board, it is susceptible to the influence of a reader and the like, and furthermore, a situation may occur in which high resistance cannot be maintained due to humidity or the like.
On the other hand, in the case of the circuit of the present invention, the adder circuit equivalently becomes as shown in FIG. , and is less susceptible to implementation limitations and the surrounding environment. Moreover, the high input impedance ZH of the amplifier circuit 16 can be easily realized by applying negative feedback using general circuit branching techniques. Therefore, since the input impedance can be changed by changing the amount of negative feedback, it is also possible to instantaneously set the line end impedance to an arbitrary value using an electric signal.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram showing an example of a conventional side sound prevention circuit using semiconductor elements, FIG. 2 is a circuit configuration diagram showing an embodiment of the present invention, and FIG. 3a is an example of a conventional addition circuit. FIG. 3b is an equivalent circuit diagram of the adder circuit in the above embodiment. 11, 12... Line, 16... Common mode input amplifier circuit,
17... First common mode input terminal, 18... Second common mode input terminal, 30... Amplifier, 33... Phase adjustment circuit.

Claims (1)

[Claims] 1. In a telephone device that transmits a transmit signal to a line and receives a receive signal from the line, a circuit extracts a part of the transmit signal and adjusts the phase of the signal, and an amplifier connected to the phase adjustment circuit. and the receive signal is supplied to a first common-mode input terminal having an impedance greater than the impedance of the line, and the output of the amplifier is supplied to a second common-mode input terminal having an impedance greater than the output impedance of the amplifier. A side sound protection circuit characterized by comprising an in-phase amplifier circuit.