DESIGNING PHOTODIODE AMPLIFIER CIRCUITS WITH OPA128
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The OPA128 ultra-low bias current operational amplifier achieves its 75fA maximum bias current without compromise. Using standard design techniques, serious performance trade-offs were required which sacrificed overall amplifier performance in order to reach fem to amp (fA= 10–15A) bias currents.
UNIQUE DESIGN MINIMIZES PERFORMANCE TRADE-OFFS
Small-geometry FETs have low bias current, of course, but FET size reduction reduces transconductance and increases noise dramatically, placing a serious restriction on performance when low bias current is achieved simply by making input FETs extremely small. Unfortunately, larger geometries suffer from high gate-to-substrate isolation diode leakage (which is the major contribution to BIFET® amplifierinput bias current).
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