Ring trip detection circuit

A ring trip detection circuit for detecting an OFF-hooked state of a telephone receiver during ringing of a telephone set by detecting a change in a voltage across two terminals of a transmission resistor through which a ringing signal is supplied to the telephone set from a ringing signal source, said ring trip detection circuit comprising:

• a voltage dividing circuit, coupled to the two terminals of the transmission resistor, for dividing voltages at the two terminals and for outputting divided voltages;
• a first pair of transistors having bases for respectively receiving the divided voltages from said voltage dividing circuit;
• a constant current source;
• a first pair of resistors having first terminals respectively coupled to emitters of said first pair of transistors and second terminals coupled in common to said constant current source, said first pair of resistors having same resistance values;
• and
• a current mirror circuit having an input and an output thereof respectively coupled to collectors of the first pair of transistors, for outputting an output current which indicates whether or not the telephone receiver is off-hooked, an output signal of the ring trip detection circuit being derived from a node which connects the output of said current mirror circuit and one of said first pair of transistors,
• said voltage dividing circuit including a first resistor circuit for dividing the voltage at one of the terminals of the transmission resistor, and a second resistor circuit for dividing the voltage at the other terminal of the transmission resistor, said first and second resistor circuits dividing the voltage with the same voltage dividing ratio,
• said current mirror circuit including a second pair of transistors having emitters which are coupled to a power source voltage, bases which are connected to each other and collectors which are respectively coupled to the collectors of said first pair of transistors, one of said second pair of transistors having the base and collector thereof connected to each other, the other of said second pair of transistors outputting the output current from the collector thereof.