Going by my memory (which I admit may be a bit dim) of the time in the late 1970's when these were introduced, I believe the semiconductor fabrication processes in use at the time permitted fabrication of p-channel JFETs alongside the BJTs used in the rest of the op amp, allowing for much-reduced input bias currents (the main point of having FET inputs). As I recall, MOSFETs were not compatible with those fab processes.Opamps like the TL07x have JFET inputs. Why not MOSFET or CMOS? Very often guitar effects have either such a JFET opamp on the input or a JFET transistor configured as a buffer. Why not some type of MOSFET transistor configuration?
I was thinking along the lines of linearity, noise, speed etc. Fab processes never occurred to me, thanks! And thank you for the document, it looks interesting.Going by my memory (which I admit may be a bit dim) of the time in the late 1970's when these were introduced, I believe the semiconductor fabrication processes in use at the time permitted fabrication of p-channel JFETs alongside the BJTs used in the rest of the op amp, allowing for much-reduced input bias currents (the main point of having FET inputs). As I recall, MOSFETs were not compatible with those fab processes.
Someone may come along with a better explanation, but I believe that's more or less the way it was.
The attached document provides some history.
Wow, that ADA4530 is $25 at digikey! Must be an expensive process.MOSFET input op amps exist. I thought there was one way back. See: https://prom-electric.ru/media/CA3140E.pdf
This modern one has a MOSFET input for extremely low bias current:
https://www.analog.com/media/en/technical-documentation/data-sheets/ADA4530-1.pdf
20 femtoamps is a mind-boggling low current. The PCB design and assembly is a lot of fun to insure that level of performance by eliminating leakage currents. Leakage paths have to be greater than 50,000,000,000,000 (5e13) ohms to 1 volt. They are pretty much only used as preamps. Special connectors are used to attach to sensors.I was thinking along the lines of linearity, noise, speed etc. Fab processes never occurred to me, thanks! And thank you for the document, it looks interesting.
Wow, that ADA4530 is $25 at digikey! Must be an expensive process.
That changed in the 80's. Intel had a process they called BiCMOS; their process designater was P651. I don't think there were every any real technical hurdles that prevented anyone from doing it. In the early 80's designs for memories and, later, microprocessors, were transitioning from bipolar to NMOS and then to CMOS (though some companies had a PMOS only process).As I recall, MOSFETs were not compatible with those fab processes.
Bob Pease wrote quite a bit about working with the very high input impedance devices and many of his insights are quite valuable. The summary is that it takes a lot more effort to be successful in dealing with those systems.20 femtoamps is a mind-boggling low current. The PCB design and assembly is a lot of fun to insure that level of performance by eliminating leakage currents. Leakage paths have to be greater than 50,000,000,000,000 (5e13) ohms to 1 volt. They are pretty much only used as preamps. Special connectors are used to attach to sensors.
This note by TI compares the three input types, BJT, Mosfet, and JFET: http://www.ti.com/lit/ml/slyt701/slyt701.pdf Comparison of input impedance and bias currents begins about page 11. Also of note, input bias current of CMOS inputs is affected by protection circuity that may be included.I was thinking along the lines of linearity, noise, speed etc. Fab processes never occurred to me, thanks! And thank you for the document, it looks interesting.
Wow, that ADA4530 is $25 at digikey! Must be an expensive process.