> Is the bias difference going to be any worse than trying to match your own JFETs for PIMETA v1, or taking the Tangent lottery on matched sets?I do not know. Long ago, when I cared, it was not uncommon for "identical" BJT parts to be 20mV off at the same current, implying 2:1 off in current at the same Vbe. But I think wafer-fab has improved a ton, and on-tape parts will generally be same-wafer and probably adjacent dice, therefore very much the same. Others report +/-2mV deviation on BJTs from a reel.
I put some coal under my SPICE to see what it says. Yes, it says a simple JFET is much better than a couple BJTs. The problem is variability; general-purpose JFETs may be 2mA or 8mA Idss, and similar range of Vp, meaning 3:1 or 4:1 range of actual current. Which maybe won't bother the op-amp, but is sure to upset some fussy builders.
There are for-purpose Idss-selected JFETs. DigiKey can't find any, Newark does not stock, Mouser does. But Mouser's in-stock parts start at $1.02, and the good-range parts are $1.66 for +/-20% selection. Not a budget-breaker but not trivial. But this is a one-part no-fiddle solution to off-biasing the opamp chip, so may be "cheaper" in some way than a collection of $0.29 parts and PCB space or trims.
Part Number Min _ Nom _ Max _Zd
SST504 ___ 0.600 0.75 0.900 0.5Meg
SST505 ___ 0.800 1.00 1.200 0.4
SST506 ___ 1.120 1.40 1.680 0.3
http://www.vishay.com/docs/70711/70711.pd...
FWIW: the "Typical" dynamic impedance (not shown) is mighty near what SPICE gives for a JFET plus a 2K resistor to set current. The Min (shown) is far higher than we need here.
+/-10% parts are $34!!! At this point you can buy a $13 DVM and a box of resistors to trim with.
posted 02-10-2009 01:07 AM CST (US) 
