Photodiode Amplifier Circuits with OPA128

Photodiode Amplifier Circuits with OPA128

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 femtoamp (fA = 10E–15 A) 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® amplifier input bias current).

Replacing the reverse-biased gate-to-substrate isolation diode structure of BlFETs with dielectric isolation removes this large leakage current component which, together with a noise-free cascode circuit, special FET geometry, and advanced wafer processing, allows far higher Difet ® performance compared to BIFETs.

How To Improve Photodiode Aplifier Performance
An important electro-optical application of FET op amps is for photodiode amplifiers. The unequaled performance of the OPA128 is well-suited for very high sensitivity detector designs. A few design tips for photodiode amplifiers may be helpful