A hook switch circuit is shown wherein two high-voltage bipolar
transistors, a PNP transistor Q1 and a NPN transistor Q2. that are
connected in a regenerative feedback manner to form a bi-stable latch.
The regenerative structure permits the use of low beta transistors that
may be turned on with a low control current, but still conduct a
sufficient off-hook current. Also shown is a polarity steering
regenerative switch (MP1, MP2) that provides a power supply voltage from
a telephone line pair and may be adapted for a polarity signal and can be
combined with a current mirror (MP7) to produce a current signal
proportional to the line voltage (LV1).