1. Field of the Invention
The present invention relates to a push-button dial circuit for a push-button telephone set.
2. Description of the Prior Art
Since a dial circuit used in each of conventional push-button telephone sets for generating a multi-tone dial signal is, in principles, composed of a buffer transistor and a resistor connected in parallel thereto, the AC impedance of the dial circuit is substantially equal to the value of the parallel resistor due to a high AC impedance of the buffer transistor, and the DC resistance of the dial circuit is provided in the form of a combined resistance of the respective resistances of the buffer transistor and the parallel resistor.
Incidentally, the push-button dial circuit operates by a power source current supplied from the central office; therefore, in order to ensure its stable operation even in a case of a long subscriber's line, the push-button dial circuit is required to operate at a low voltage and its DC resistance must be small in value. On the other hand, the AC impedance of the push-button dial is required to have a large value for sending out a dial signal at a proper level.
In conventional push-button dial circuits, since the AC impedance and the DC resistance are determined by the resistor connected in parallel with the buffer transistor as described above, there is such a shorcoming that an increase in the AC impedance causes an increase in the DC resistance, whereas a decrease in the DC resistance causes a decrease in the AC impedance. Further, the AC impedance depends on the resistance value of the resistor connected in parallel to the buffer transistor and assumes a constance value irrespective of the length of the subscriber's line, with the result that the output signal from the dial circuit is also sent out at a constant level.
On the other hand, since the subscriber's line has an AC loss substantially determined by the line length, the level of the dial signal reaching the central office comes to have a value obtained by subtracting the AC loss of the subscriber's line from the output signal level of the dial circuit. In practice, since the loss of the subscriber's line is large, use is made of a manual or automatic signal level control circuit in the prior art push-button dial circuit for controlling the signal level in a case of a short-distance line, but in a case of a long-distance line, the signal level is not controlled, whereby the range of the level of the signal reaching the central office is made narrower than the range of loss of the subscriber's line.