12
R104 (Both 2.2K ohms) allowing around 1.4mA to flow into pin-12. This current flows out of
pin-3 and approximately half of it flows through D38 / R83 and the other half to the
emitter of common base amplifier Q34. The current flowing in the collector of Q34, pulls
the high side of R120 to ground potential, allowing again around 14V to be dropped
across R120. The current through R120 also appears at R127, as in the operation of the
positive half.
What determines where things switch? On the positive half, the +45V supply is first filtered
(R5/C9) and then divided down in voltage (R5, R4, R152 and R151 through D69). This
divided-down supply reference is present on the-non inverting input of U6 (pin-9). With no
output signal, the inverting input is always kept one diode drop below the non-inverting
input, as D69 is biased via R151. With the non-inverting input higher than inverting, pin-7 is
high, and the FET is still switched off. When the output swings more and more positive,
the divided-down signal is presented to U6-10 via R112, D56 and R151. When Pin-10 goes
above Pin-9, the switch turns-on. D69 and D68 are present to limit the voltage range to
the comparator inputs to +/-0.7V: This increases the switching speed.
R153 and D70 are the positive switch’s turn-off hysteresis network. When the switch is
finally activated (about 10V below the 45V rail), D70 is forward biased and R153 drives
U6-9 slightly lower. The result is that when the output signal eventually starts to decrease
in level, the switch will actually switch off at an even lower level (around 12V below the
+45V supply). This hysteresis is important so that the switch only turns on and off once as it
follows the output signal. Picture a switch without this hysteresis: When the output was
hovering around 10V below +45V the switch could continually toggle b90V and
+45V. This can cause all sorts of thermal, reliability and distortion problems.
The action of the negative detector is identical to the positive half described above.
