op-amp-controlled MOSFET cucrrent-source

Started by July 31, 2019
``` 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

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
```
```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
>
>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

```
```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
>
> 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
```
```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
>>
>> 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.
>
>

```
```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

```
```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
>>
>>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
```
```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
```
```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/

```
```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
```
```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.