
Measurements (rf) 147
op-amp, the other half being used to apply to the meter circuit an offset
voltage, derived from R$, equal to the forward voltage established across
the diode by the forward bias current through R$. As the detected voltage
can go below ground and pin 7 of the op-amp has to sink up to 1 mA, a
negative supply rail is needed. Since the diode bias current and the
R%
offset voltage are derived from a +10 V rail, this should be stabilised and
the familiar LM723 was pressed into service (Figure5.10(c)). A three leg
regulator would have worked, but these do not come in a convenient
voltage rating. +10 V is chosen for the positiv ...