Interfacing and Processing 2658.5.2 Scope
The parts most at risk from excess input voltages are the solid-state active devices,
particularly discrete BJTs, and most monolithic IC op-amp input stages.
Valves are relatively immune to input voltage abuse. They are most likely to be harmed by
gross overdrive conditions that bias the grid positive so a damagingly high current fl ows.
J-FETs and MOSFETs are next most rugged. MOSFETs are most susceptible to gate-
source overvoltage, but gate-source protection is straightforward and effective.
IC input stages are the most fragile. Due to IC structure, even FETs, when monolithic,
may have parasitic weak points. For long-term reliability, currents fl owing into or out of
IC op-amp pins^12 must always be kept below 5 mA.
8.5.3 Harmful Conditions
There are two kinds of potentially damaging input voltages: (1) common mode and
(2) differential mode. Either may occur when a power amplifi er is in (i) the on state or
(ii) the off state, giving four possibilities.
8.5.4 On-State Risks
When an amplifi er employing BJTs at the front of its input stage is on, powered up, and
settled down, it can sustain relatively high differential (signal) voltages without damage.
Generally, in high NFB op-amp and other dual-rail based designs, the max differential
voltage is a volt below the supply rails, hence a maximum differential voltage ranges
from /–14 V for input stages working from /–15-V supplies, up to /–30 V or even
over /–100 V, where the input stage transistors operate from the same or else similarly
high supplies, as the output stage.
Long before differential overload, the input stage will be driven strongly into clip. Provided
the amplifi er has clean recovery, an overvoltaging may pass unnoticed if the high differential
voltage only lasts I mS. Yet this is plenty long enough to damage a semiconductor junction.
In BJTs, the most vulnerable junction is the base emitter, when reverse biased.
Under the same powered-up conditions, common-mode voltages above /–10 V can
damage unprotected BJT input stages. In large systems, the common-mode voltage
can be this high, commonly comprising 50/60-Hz AC and harmonics, and arising from
differences in grounding or AC power potentials.