Q
I´ve got a question regarding the application notes of the OP497 Precision
Picoampere Input Current Quad Operational Amplifier. On side 13 at the point
"nonlinear circuits"; Figure 41 Square Root Amplifier, the scaling of I_REF can
be done by the resistor R4. Can I get more information about I_REF/R4 and the
influence of the other resistors? Can I get in general more information about
his topic?
A
Since this circuit works with log/square-root functions, the current and
voltages cannot be zero. As such, the lowest possible voltage at Vout must be
greater than zero. This is accomplished by adjusting the Vbe at Q4. Vbe4 is
determined by the amount of current through it. That current, Iref, is equal to
the current through R3 and R4. The current through R3 is negligible so, for
this design step, just choose R4 such that Iref is at a level to make Vout
greater than zero.