I know this is a weird question, but I'm wondering if there is such a thing as a FET based Opamp? It is my understanding that an Opamp is an integrated circuit made up of of transistors among other components, so I'm wondering if any of these Opamps use some form of FETs internally?
Check the schematic on page 5 of http://tinyurl.com/4yq7xtk. The TL07x uses JFETs as input, for instance.
Now you've made me curious.
Is there any sort of design standard for what gets to call itself a "Bi-Fet opamp"? For instance, is the requirement only that a JFET be part of the input structure, or is it more than that?
Quote from: Mark Hammer on October 12, 2011, 12:16:24 PM
Is there any sort of design standard for what gets to call itself a "Bi-Fet opamp"? For instance, is the requirement only that a JFET be part of the input structure, or is it more than that?
I don't think there is any requirement for being to use the term. I believe National Semi used the term first for its opamps which integrated JFETs at the input up the DC input impedance and speed, for technical reasons. They called it "Bi' for the fact that the innards of the opamp are bipolar. "FET" is for the JFETs at the front end.
Later ones may have used a JFET inside.
The technical reasons for using JFETs, particularly P-channel JFETs is that in IC opamps, the frequency response of the PNPs is much poorer than the response of the NPNs because of how the doping has to be done. These "lateral PNPs" are some of the bigger limiting factors on all-bipolar opamps from the first couple of generations. The Bi-FET process sidestepped a lot of that.
There do exist all MOSFET opamps. And probably mixed FET opmaps, I just don't know the details. The TLCxxx line is all MOSFET, as is the LMC line, I believe.
http://www.intersil.com/products/deviceinfo.asp?pn=CA3140 (http://www.intersil.com/products/deviceinfo.asp?pn=CA3140)
check out this input impedance ( 1.5 teraohms ! )
The gates are insulated from the channels by a layer of glass 20 volts thick.