thermionic
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Are there many examples of SE FET-input opamps out there? I can think of quite a few which employ complementary stages, but are there many in the SE realm?
Justin
Justin
SE / Non-Complementary FET-Input Opamps?
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Twenty Log
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I am confused perhaps... My bad....
SE output or input (or both), e.g. class A? The words "Non-Complementary" on the input might mean to me that you do not want something like Bi-Fet input that may have the FET be the opposite "gender" (N versus P) of the BJT or somesuch?
OR
Perhaps is it maybe more of a differential input that you may want to avoid in the original proposed question, thusly only one input pin (in that case, some of the buffer chips available might be a close cousin, but gain other than +1 would need some additional circuitry)?
I think the "slash" "/" in the title is throwing me off...
Of course, one can "bias" an output of some class A/B output opamps to be class A output with an external FET style of current source... caveats may apply... Linear Technology has an opamp and app note that highlights this...
So many topologies here...
I suppose that a current mode feedback (CMFB) opamp also has different style of input topologies on each input pin, so that could be an interesting postulate variant on the question... But this may be far removed from the original design goals?
I suppose that a simple "SE" (class A) FET input is easy enough with a single FET coupled with feedback from the Drain to the Gate and stabilized with some modicum of Miller Capacitance in that same feedback circuit leg... Maybe even a cascode configuration... However, diff pair inputs are typically class A already...
The "standard" gain equations of an op-amp with the appropriate feedback resistors and the like for Av = -Rf/Ri or Av = 1+Rf/Ri is a simplification if the op-amp has an internal gain that is HUGE... These equations lose their accuracy as the internal opamp gain goes down.... Yes, the proposed SE input above can have current mirrors/sources and "long tails" that provide huge "resistances" that increase internal gain of said opamp to keep the equations simplified for gain calculations; like a cascode....
My ultimate question is: what is the design goal for this proposed topology, or what issues in other conventional topologies are looking to be solved...
Even the old vacuum tube opamps from the days of yore were differential input (and diff output) and done with 2 dual gain element tubes...
SE output or input (or both), e.g. class A? The words "Non-Complementary" on the input might mean to me that you do not want something like Bi-Fet input that may have the FET be the opposite "gender" (N versus P) of the BJT or somesuch?
OR
Perhaps is it maybe more of a differential input that you may want to avoid in the original proposed question, thusly only one input pin (in that case, some of the buffer chips available might be a close cousin, but gain other than +1 would need some additional circuitry)?
I think the "slash" "/" in the title is throwing me off...
Of course, one can "bias" an output of some class A/B output opamps to be class A output with an external FET style of current source... caveats may apply... Linear Technology has an opamp and app note that highlights this...
So many topologies here...
I suppose that a current mode feedback (CMFB) opamp also has different style of input topologies on each input pin, so that could be an interesting postulate variant on the question... But this may be far removed from the original design goals?
I suppose that a simple "SE" (class A) FET input is easy enough with a single FET coupled with feedback from the Drain to the Gate and stabilized with some modicum of Miller Capacitance in that same feedback circuit leg... Maybe even a cascode configuration... However, diff pair inputs are typically class A already...
The "standard" gain equations of an op-amp with the appropriate feedback resistors and the like for Av = -Rf/Ri or Av = 1+Rf/Ri is a simplification if the op-amp has an internal gain that is HUGE... These equations lose their accuracy as the internal opamp gain goes down.... Yes, the proposed SE input above can have current mirrors/sources and "long tails" that provide huge "resistances" that increase internal gain of said opamp to keep the equations simplified for gain calculations; like a cascode....
My ultimate question is: what is the design goal for this proposed topology, or what issues in other conventional topologies are looking to be solved...
Even the old vacuum tube opamps from the days of yore were differential input (and diff output) and done with 2 dual gain element tubes...
thermionic
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Hi,
Thanks. I'm interested in something that doesn't have Push-Pull stages from I to O. It can be differential, i.e. with an LTP / differential input, but without PP stages anywhere. I'm aware of many BJT-based discrete opamps that are fully SE, but none which have FET inputs. From what I can see at the moment, I would most likely have to contruct such a beast by substituting the front end transistors from a BJT design with FETs and compensating the design. To make a discrete one that outperforms say, a decent FET-input IC, would take a lot of effort...that's not the point, I'm just curious, for research purposes.
Justin
Thanks. I'm interested in something that doesn't have Push-Pull stages from I to O. It can be differential, i.e. with an LTP / differential input, but without PP stages anywhere. I'm aware of many BJT-based discrete opamps that are fully SE, but none which have FET inputs. From what I can see at the moment, I would most likely have to contruct such a beast by substituting the front end transistors from a BJT design with FETs and compensating the design. To make a discrete one that outperforms say, a decent FET-input IC, would take a lot of effort...that's not the point, I'm just curious, for research purposes.
Justin
Twenty Log
Well-known member
Ahhh... Sorry for running off into the weeds... my mind can be overactive... Told you I was confused 
As I mentioned Linear Technology had an opamp circa the mid to late 1990s, that had an app note to bias the output with a FET with about 2 mA to keep the the bottom transistor in the A/B output from turning on and biasing the top transistor into essentially class A... Still trying to find it... Most simple (1980s/1990s vintage) opamps may not be A/B until the last stage... Maybe someone else can chime in with an opinion on that concept as I reserve the right to be wrong...
The only thing I can find is the datasheet for the LT1010 which uses a LM334 for about 2mA bias on the output of the opamp... but the distortion is not as nifty as say the new National modern op amps...
But I wonder if you could run a comparator into linear mode (it can be done with some glue circuitry stuff, but it may be helpful to know how they designed the chip exactly).. My last microchip design experience was with GaAs HBTs, so not FET in that regard, but I was experimenting with SiGe zero threshold FETs ("zero" Vt if you can imagine that, +/- 10mV of course, dancing on that fine line between depletion mode and enhancement mode)....
Notwithstanding, the newer high speed comparators from the likes of Analog Devices are SiGe, but I do not know if they are FET (they might be, but I could be wrong, as it seems most modern chip design at least for the front end is some sort of FET, perhaps with BJT output)... They are designed to be open Drain/Collector or open emitter whatever or for ECL, RTL, CML et cetera logic... low voltage though, but I wonder of the max specs for output stages... There is latching on some of these comparators so maybe this idea is half baked and a no-go?
You can see some of the simplified schematics if they disclose it to get an idea on how to proceed for research purposes.. it's always fun...
Cheers...
As I mentioned Linear Technology had an opamp circa the mid to late 1990s, that had an app note to bias the output with a FET with about 2 mA to keep the the bottom transistor in the A/B output from turning on and biasing the top transistor into essentially class A... Still trying to find it... Most simple (1980s/1990s vintage) opamps may not be A/B until the last stage... Maybe someone else can chime in with an opinion on that concept as I reserve the right to be wrong...
The only thing I can find is the datasheet for the LT1010 which uses a LM334 for about 2mA bias on the output of the opamp... but the distortion is not as nifty as say the new National modern op amps...
But I wonder if you could run a comparator into linear mode (it can be done with some glue circuitry stuff, but it may be helpful to know how they designed the chip exactly).. My last microchip design experience was with GaAs HBTs, so not FET in that regard, but I was experimenting with SiGe zero threshold FETs ("zero" Vt if you can imagine that, +/- 10mV of course, dancing on that fine line between depletion mode and enhancement mode)....
Notwithstanding, the newer high speed comparators from the likes of Analog Devices are SiGe, but I do not know if they are FET (they might be, but I could be wrong, as it seems most modern chip design at least for the front end is some sort of FET, perhaps with BJT output)... They are designed to be open Drain/Collector or open emitter whatever or for ECL, RTL, CML et cetera logic... low voltage though, but I wonder of the max specs for output stages... There is latching on some of these comparators so maybe this idea is half baked and a no-go?
You can see some of the simplified schematics if they disclose it to get an idea on how to proceed for research purposes.. it's always fun...
Cheers...
thermionic said:
My APP2055 (fets input opamp) has a voltage gain stage that's SE and the out stage is PP, but this second can be easily modified to became SE. If you are interested on, I could modify some APP2055's for full SE mode for you. APP2055 is considered a very good opamp here in the forum and outside it (it's not a banal/simple SE opamp). Just PM me if you are interested on.
Pier Paolo
