BACKGROUND....
Regarding circuits (or opamps) that prize their low bias current....
Adding input protection in the form of back-biased small signal diodes, often kills the circuit's desired low bias current, from excessive leakage.
As mentioned for example, in the OPA627 datasheet (FIGURE 7)....
http://focus.ti.com/lit/ds/symlink/opa627.pdf
Robert Pease (among others) has also written about using the C and B (E open) pins on a typical transistor say, a 2n3906...or the G and D+S pins of the best N-FET for the job...2n4117.
And TI has an app note... http://focus.ti.com/lit/an/sboa058/sboa058.pdf
QUESTION...
I am wondering if there are disadvantages, of these schemes, (besides the bias/leakage issue discussed above), rather than the typical 1n419, 1n4148, 1n547 small signal diodes, if very-low leakage will not be considered as essential.
BTW, in my tests, I was able to find lower-than-average leakage in the "A" versions of the 1n457, 458, 459 series, over the typical 1n914/4148 types.
Here is my test layout.
Of course, these diodes need to be shielded from light during the tests.
Various protection diodes...(I am referring here, to D5, D6 input protection diodes)
=FB=
Regarding circuits (or opamps) that prize their low bias current....
Adding input protection in the form of back-biased small signal diodes, often kills the circuit's desired low bias current, from excessive leakage.
As mentioned for example, in the OPA627 datasheet (FIGURE 7)....
http://focus.ti.com/lit/ds/symlink/opa627.pdf
Robert Pease (among others) has also written about using the C and B (E open) pins on a typical transistor say, a 2n3906...or the G and D+S pins of the best N-FET for the job...2n4117.
And TI has an app note... http://focus.ti.com/lit/an/sboa058/sboa058.pdf
QUESTION...
I am wondering if there are disadvantages, of these schemes, (besides the bias/leakage issue discussed above), rather than the typical 1n419, 1n4148, 1n547 small signal diodes, if very-low leakage will not be considered as essential.
BTW, in my tests, I was able to find lower-than-average leakage in the "A" versions of the 1n457, 458, 459 series, over the typical 1n914/4148 types.
Here is my test layout.
Of course, these diodes need to be shielded from light during the tests.
Various protection diodes...(I am referring here, to D5, D6 input protection diodes)
=FB=