Have you seen an analysis of the classic op-amp-controlled MOSFET current-source? I needed this for a design, plus I'd like to squeeze it into the x-Chapters, before the mid-August printing deadline. https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 In the formulas, I assume the opamp's output signal is controlled by Vin-Vs and R2 C2. I'm still checking the math, so won't post the results just now, but one early formula helps to determine the opamp's output load, giving us Zin = Vg/i3 at the MOSFET's gate. Zin = R1 (1 + fT / f) + 1 / s C1. Where fT is the MOSFET's fT = gm / s Ciss. It's high at low frequencies, as expected, but drops toward R1 as f approaches fT, and eventually becomes capacitive = Ciss + R1. If R1 is low, e.g., 5 ohms for 2A pulsing, the opamp will need help from a driver, like an BUF634. But if it has a BUF634, it probable won't need R2 C2 either. Hah, I'll post an example of that scene shortly, x-Chapters 3x.20 Precision 1.5 kV 1us Ramp. -- Thanks, - Win

# op-amp-controlled MOSFET cucrrent-source

Started by ●July 31, 2019

Reply by ●July 31, 20192019-07-31

On 31 Jul 2019 08:43:16 -0700, Winfield Hill <winfieldhill@yahoo.com> wrote:> Have you seen an analysis of the classic > op-amp-controlled MOSFET current-source? > I needed this for a design, plus I'd like > to squeeze it into the x-Chapters, before > the mid-August printing deadline. > >https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 > > In the formulas, I assume the opamp's output > signal is controlled by Vin-Vs and R2 C2. > > I'm still checking the math, so won't post > the results just now, but one early formula > helps to determine the opamp's output load, > giving us Zin = Vg/i3 at the MOSFET's gate. > > Zin = R1 (1 + fT / f) + 1 / s C1. > > Where fT is the MOSFET's fT = gm / s Ciss. > > It's high at low frequencies, as expected, > but drops toward R1 as f approaches fT, and > eventually becomes capacitive = Ciss + R1. > > If R1 is low, e.g., 5 ohms for 2A pulsing, > the opamp will need help from a driver, > like an BUF634. But if it has a BUF634, > it probable won't need R2 C2 either. Hah, > I'll post an example of that scene shortly, > x-Chapters 3x.20 Precision 1.5 kV 1us Ramp.I think that R2 can often be omittted, when R1 is high. Save half a cent or something. That would be fun to Spice. R3 also keeps the fet from RF oscillating on its own. I theorized that the open-loop output impedance of the opamp would damp the fet oscillation, and was proven wrong. Spice models of fets never seem to demonstrate the follower oscillation hazard. They don't often include the wire bonds. -- John Larkin Highland Technology, Inc lunatic fringe electronics

Reply by ●July 31, 20192019-07-31

On 7/31/19 11:43 AM, Winfield Hill wrote:> Have you seen an analysis of the classic > op-amp-controlled MOSFET current-source? > I needed this for a design, plus I'd like > to squeeze it into the x-Chapters, before > the mid-August printing deadline. > > https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 > > In the formulas, I assume the opamp's output > signal is controlled by Vin-Vs and R2 C2. > > I'm still checking the math, so won't post > the results just now, but one early formula > helps to determine the opamp's output load, > giving us Zin = Vg/i3 at the MOSFET's gate. > > Zin = R1 (1 + fT / f) + 1 / s C1. > > Where fT is the MOSFET's fT = gm / s Ciss. > > It's high at low frequencies, as expected, > but drops toward R1 as f approaches fT, and > eventually becomes capacitive = Ciss + R1. > > If R1 is low, e.g., 5 ohms for 2A pulsing, > the opamp will need help from a driver, > like an BUF634. But if it has a BUF634, > it probable won't need R2 C2 either. Hah, > I'll post an example of that scene shortly, > x-Chapters 3x.20 Precision 1.5 kV 1us Ramp. > >It might be enlightening also to use a slightly more sophisticated op-amp model that doesn't assume no phase shift and infinite bandwidth, like e.g. just a GBW/s integrator, as sometimes people think "Oh hey it's a DC current source and the op amp doesn't have to provide squat for output current if R1 isn't that small" and then use the junkiest LM324 they can find and don't know why their current source is oscillating like nuts - I don't think the equations as written show potential for oscillation but the circuit definitely can at low frequency, even with a gate stopper to damp RF self-oscillation on the MOSFET

Reply by ●July 31, 20192019-07-31

On 7/31/19 12:14 PM, John Larkin wrote:> On 31 Jul 2019 08:43:16 -0700, Winfield Hill <winfieldhill@yahoo.com> > wrote: > >> Have you seen an analysis of the classic >> op-amp-controlled MOSFET current-source? >> I needed this for a design, plus I'd like >> to squeeze it into the x-Chapters, before >> the mid-August printing deadline. >> >> https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 >> >> In the formulas, I assume the opamp's output >> signal is controlled by Vin-Vs and R2 C2. >> >> I'm still checking the math, so won't post >> the results just now, but one early formula >> helps to determine the opamp's output load, >> giving us Zin = Vg/i3 at the MOSFET's gate. >> >> Zin = R1 (1 + fT / f) + 1 / s C1. >> >> Where fT is the MOSFET's fT = gm / s Ciss. >> >> It's high at low frequencies, as expected, >> but drops toward R1 as f approaches fT, and >> eventually becomes capacitive = Ciss + R1. >> >> If R1 is low, e.g., 5 ohms for 2A pulsing, >> the opamp will need help from a driver, >> like an BUF634. But if it has a BUF634, >> it probable won't need R2 C2 either. Hah, >> I'll post an example of that scene shortly, >> x-Chapters 3x.20 Precision 1.5 kV 1us Ramp. > > I think that R2 can often be omittted, when R1 is high. Save half a > cent or something. That would be fun to Spice. > > R3 also keeps the fet from RF oscillating on its own. I theorized that > the open-loop output impedance of the opamp would damp the fet > oscillation, and was proven wrong.Slew rate? the op amp in question might have the small-signal GBW to still have a low enough (small-signal) output impedance on paper up there to damp it but when the FET wants to oscillate it wants to swing rail-to-rail, the op amp output stage has to have the slew rate to tame it I think> Spice models of fets never seem to demonstrate the follower > oscillation hazard. They don't often include the wire bonds. > >

Reply by ●July 31, 20192019-07-31

On Wednesday, July 31, 2019 at 12:14:46 PM UTC-4, John Larkin wrote:> On 31 Jul 2019 08:43:16 -0700, Winfield Hill <winfieldhill@yahoo.com> > wrote: > > > Have you seen an analysis of the classic > > op-amp-controlled MOSFET current-source? > > I needed this for a design, plus I'd like > > to squeeze it into the x-Chapters, before > > the mid-August printing deadline. > > > >https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 > > > > In the formulas, I assume the opamp's output > > signal is controlled by Vin-Vs and R2 C2. > > > > I'm still checking the math, so won't post > > the results just now, but one early formula > > helps to determine the opamp's output load, > > giving us Zin = Vg/i3 at the MOSFET's gate. > > > > Zin = R1 (1 + fT / f) + 1 / s C1. > > > > Where fT is the MOSFET's fT = gm / s Ciss. > > > > It's high at low frequencies, as expected, > > but drops toward R1 as f approaches fT, and > > eventually becomes capacitive = Ciss + R1. > > > > If R1 is low, e.g., 5 ohms for 2A pulsing, > > the opamp will need help from a driver, > > like an BUF634. But if it has a BUF634, > > it probable won't need R2 C2 either. Hah, > > I'll post an example of that scene shortly, > > x-Chapters 3x.20 Precision 1.5 kV 1us Ramp. > > I think that R2 can often be omittted, when R1 is high. Save half a > cent or something. That would be fun to Spice.When I leave R2 out, I often have problems. (I think that's true for the NPN version of the circuit too.. but not sure.) I do this hand-wavy R2/ C2 time constant thing... but I don't really understand it. Maybe it's C1 that R2 is working with? An analysis would be nice. George H.> > R3 also keeps the fet from RF oscillating on its own. I theorized that > the open-loop output impedance of the opamp would damp the fet > oscillation, and was proven wrong. > > Spice models of fets never seem to demonstrate the follower > oscillation hazard. They don't often include the wire bonds. > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics

Reply by ●July 31, 20192019-07-31

bitrex wrote...> >On 7/31/19 11:43 AM, Winfield Hill wrote: >> Have you seen an analysis of the classic >> op-amp-controlled MOSFET current-source? >> I needed this for a design, plus I'd like >> to squeeze it into the x-Chapters, before >> the mid-August printing deadline. >> >>https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 >> >> In the formulas, I assume the opamp's output >> signal is controlled by Vin-Vs and R2 C2. >> >> I'm still checking the math, so won't post >> the results just now, but one early formula ...> I don't think the equations as written show > potential for oscillation but the circuit > definitely can at low frequency, even with > a gate stopper to damp RF self-oscillation > on the MOSFET.I'll post the feedback-loop set of equations later. They should give an indication. All the elements are there, with phase shifts. Except to simplify, I assumed the opamp GBW is well above the R2 C2 bandwidth. That's easy, and faster opamps usually have a higher Iout, etc., to help it drive the MOSFET's Zin. -- Thanks, - Win

Reply by ●July 31, 20192019-07-31

George Herold wrote...> > On Wednesday, July 31, 2019, John Larkin wrote: >> On 31 Jul 2019, Winfield Hill wrote: >> >>> Have you seen an analysis of the classic >>> op-amp-controlled MOSFET current-source? >> >> I think that R2 can often be omittted, > when R1 is high. ... > > When I leave R2 out, I often have problems.Well, maybe not for high enough R1.> An analysis would be nice.Coming up shortly. -- Thanks, - Win

Reply by ●July 31, 20192019-07-31

On a sunny day (31 Jul 2019 10:40:58 -0700) it happened Winfield Hill <winfieldhill@yahoo.com> wrote in <qhsjra0p7a@drn.newsguy.com>:>George Herold wrote... >> >> On Wednesday, July 31, 2019, John Larkin wrote: >>> On 31 Jul 2019, Winfield Hill wrote: >>> >>>> Have you seen an analysis of the classic >>>> op-amp-controlled MOSFET current-source? >>> >>> I think that R2 can often be omittted, >> when R1 is high. ... >> >> When I leave R2 out, I often have problems. > > Well, maybe not for high enough R1. > >> An analysis would be nice. > > Coming up shortly.Here is my version: http://panteltje.com/panteltje/tri_pic/tritium_decay_experiment_black_box_circuit_diagram_IMG_3883.GIF Been working OK for what's it 6 years ? now. All calijugatiations for everything are on the same A4. My neural net designed it. Zorry http://panteltje.com/panteltje/tri_pic/

Reply by ●July 31, 20192019-07-31

Jan Panteltje wrote...> >Here is my version: >http://panteltje.com/panteltje/tri_pic/tritium_decay_experiment_black_box_circuit_diagram_IMG_3883.GIF > >Been working OK for what's it 6 years ? now. >All calijugatiations for everything are on the same A4. >My neural net designed it.Sorry, I can't read the part values. -- Thanks, - Win

Reply by ●July 31, 20192019-07-31

On a sunny day (31 Jul 2019 11:18:25 -0700) it happened Winfield Hill <winfieldhill@yahoo.com> wrote in <qhsm1h0ruo@drn.newsguy.com>:>Jan Panteltje wrote... >> >>Here is my version: >>http://panteltje.com/panteltje/tri_pic/tritium_decay_experiment_black_box_circuit_diagram_IMG_3883.GIF >> >>Been working OK for what's it 6 years ? now. >>All calijugatiations for everything are on the same A4. >>My neural net designed it. > > Sorry, I can't read the part values.Fair enough, neither could I, so I looked at the PICTURE http://panteltje.com/panteltje/tri_pic/tritium_decay_experiment_black_box_electronics_top_view_IMG_3873.GIF that showed me the caps are indeed all 1 uF and the opamp drive is via 470k 120K (that is a lowpass for PWM from a PIC BTW) but the IRLZ34N is on the back with the gate resistor and feedback resistor so still did not know. From Jan Panteltje's handwriting I think the gate - as well as the feedback resistor from the current sense resistor to the opamp is also 100 kOhm. One would think the gate resistor to be 10O Ohm, but looks like it is 100 k, forms a nice lowpass with the gate capacitance, also note drain is decoupled with 1 uF to ground, preventing oscililililations. Neural net was right Once your job will be taken over by AI you will see more and more of these solutions. No questions asked, no explanations given, NN uses what is in the junkbox, or the resistors at hand. <click he if this answer was helpful> <click here if in despair> Press power or reset button on PC to remove this text.