Can somebody take a peek at this circuit for me?

You should learn to say what a circuit has to do in terms of performance, preferably quantifiable. I could say your circuit will "work", because you have not really specified what that means.

I suggest you learn to use a simulator rather than presenting stuff that just cannot work. By using it to examine currents and voltages for specific branches and nodes, you should be able to discover elementary errors for yourself.

The supply value belongs on your schematic.

There is no value on the shunt leg of your divider. So, my prediction is it will divide by "unknown".

When you simulate your circuit, you will find that the MOSFET is always on. I leave it to you to figure out why. A "simulation" done with your mind would be a good way to start. That is a skill you will need no matter how good computer based simulation become.

Unless there is a load, and it is connected to a more positive level than the VSS supply, I would expect no output like what you probably hope for.

Another issue with your circuit, (and boost stages added to op-amps generally), is that it may not be stable. Depending on your load, your circuit could easily oscillate (once you get the MOSFET to not be always on).

Sure. Be sure to ignore the crap that gets thrown around here. It reflects nothing upon you. You may want to take basic questions like this to alt.electronics.basics where there is a little more tolerance.

That said, more care inspecting your schematics before they leave your desk would be good.

That would be one way to put it.

A great many SPICE models for MOSFETs do not have any provision for modeling the rupture of the gate oxide. In fact, I know of no exceptions. My prediction stands.

For purposes of helping the OP learn what he needs to learn, "always on" does the job just fine. That was enough for me, and still is. He can learn about smoke when he starts building real circuits.

Derf transform applied.

"Fred Bloggs" wrote in message news: snipped-for-privacy@nospam.com...

You seem to be repeating yourself, Fred. It may be time for a system checkup and/or reformat.

By the way, I apply the Derf transform for clarity when you actually have something to say that is worth responding to. What survives the filter is related to that purpose, not your desire to post some spew every time I post anything.

When nothing except article references and sigs gets thru the Dreck-Extraneousness-Redundancy-Frivolity filter, I will not be responding at all. So you can rest assured that most of your contributions will go unanswered.

Why should he quantify anything?- You sure as hell never do.

You "could" say it "will" work because he has "not" specified what that means? If that is not confused gibberish I don't know what is. You have almost certainly have severe brain damage.

That is a totally wrong and ass-backwards approach. SPICE is not to be used to "discover" anything- it used to confirm the details of an overall circuit operation that is understood beforehand. But I guess you know better than the original founders and expert practitioners of SPICE simulation- don't you, PIT (pseudo-intellectual trash)?

Oh real smart observation- it's just incredible how you can zoom in on the irrelevant- what a p.o.s. and PIT-bag.

Is that what call "always on"- a MOSFET blown to hell by 400V of applied VGS?

What a bucket of piss...

No kidding ? Really?

You mean killfile pseudo-intellectual trash like yourself....

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If that is not confused gibberish I don't know what is. You have almost certainly have severe brain damage.

"discover" anything- it used to confirm the details of an overall circuit operation that is understood beforehand. But I guess you know better than the original founders and expert practitioners of SPICE simulation- don't you, PIT (pseudo-intellectual trash)?

irrelevant- what a p.o.s. and PIT-bag.

Snipped material re-inserted:

If that is not confused gibberish I don't know what is. You have almost certainly have severe brain damage.

"discover" anything- it used to confirm the details of an overall circuit operation that is understood beforehand. But I guess you know better than the original founders and expert practitioners of SPICE simulation- don't you, PIT (pseudo-intellectual trash)?

irrelevant- what a p.o.s. and PIT-bag.

Snipped material re-inserted:

If that is not confused gibberish I don't know what is. You have almost certainly have severe brain damage.

"discover" anything- it used to confirm the details of an overall circuit operation that is understood beforehand. But I guess you know better than the original founders and expert practitioners of SPICE simulation- don't you, PIT (pseudo-intellectual trash)?

irrelevant- what a p.o.s. and PIT-bag.

Shut the hell up, PIT- until you learn how to format a proper response.

"Larry Brasfield" wrote in news:8HS%d.1$e% snipped-for-privacy@news.uswest.net:

Well - right now I just want to see if it even sort of works - but eventually I want a circuit that can amplify a 0-10V signal to 0-400 with response time of about 1ms and with accuracy to the nearest volt on the output.

I couldn't remember how to add suppy values in Eagle :)

I marked the two voltage dividing resistors as having R and 39R ohms. I wasn't sure what exact value to use for those - but I was thinking I'd probabaly use resistors with values in the megaohms, so that they don't have to dissipate too much power.

I don't understand - say I apply a 0V signal to the input. Assuming the mosfet is off already, vout will be ground, as the load is grounded (which now I realize I failed to mention), and thus the voltage divider will divide 0 by forty - and so both V+ and V- on the op-amp will be 0, and the op-amp will be happy.

The load would be connected to ground.

I was most worried about stability. Now that I'm thinking about it - I would think that this circuit will oscilate - say I give it a 5V input (to v-). If the circuit worked like hoped eventually vout would be 200V, then the voltage divider would divide 200 by 40, and V+ would be 5V to. Since the op-amp amplifies the difference between V+ and V-, Vout from the op-amp would be 0, and it would continue to oscilate back and forth like that.

Thanks,

-M. Noone

I did post my little HV amplifier circuit to a.b.s.e. a while back. It uses an opamp driving a pair of high-voltage optocouplers, with the phototransistor sides as the output totem pole across the 400 volt supply. This eliminated all sorts of level shifting problems... lets light do it! It should be about good for 1 mA drive from 400 volt rails.

What are you going to drive?

John

Sorry, Michael, your circuit is not even close. First, you have no load on the drain of the FET, unless you consider your feedback resistor to be a load. Second, where do you apply the 400V supply, to the Vss pin? Does Vss = -400V (normally Vss is taken to mean more negative voltages (i.e. "sources of N-channel FETs) and Vdd is taken to mean more positive voltages (drains)? No, the BSS92 is a p-type FET, so Vss must be +400V. Oops, Third, the FET will blow its brains out when you turn on the 400V because 1) you exceeded the +/-20V Vgs rating, and 2) you exceeded the 240V Vds rating. Fourth, after solving the drain-connection problem, and the excess voltage problems, you'll find the feedback network is unstable, because it has too much loop gain, and too much phase shift. Fifth, you'll learn FET capacitances and load capacitances (e.g. a connecting coax cable) are a big issue, so you'll need an active way to both pull up and pull down the output. In general, Sixth, with MOSFETs, you'll also need a way to isolate the opamp from driving the high gate capacitance of typical high-voltage power MOSFETs. Even the BSS92, which is not a true high-voltage power MOSFET, has nearly 200pF of gate capacitance at low voltages (see the Siemens datasheet page 7).

Seventh, Spice is NOT a good way to evaluate this type of circuit, in part because once you find a good 500V FET, and if you investigate sufficiently, you'll find that the power MOSFET spice models are very poor for low-current-density linear operation. They're designed for analyzing switching performance. And most are rather poor at that.

Spice is also not a good way to study such circuits because you'll learn very little, compared to studying MOSFET linear operation, and applying simple analytical circuit-operation calculations. I suggest you get a copy of our book, The Art of Electronics, and read chapter 3. In it you'll also find an example of a high-voltage FET amp that works.

John Larkin wrote in news: snipped-for-privacy@4ax.com:

Hi John - it will be driving some sort of fluid load. I've been told that it should be able to sync 0-20ma though this fluid. I looked through ABSE and didn't find your circuit. My NG server has about 30 day retention - so it should still be there... Do you have any idea what the filename or topic name was? Thanks,

-Michael

Winfield Hill wrote in news: snipped-for-privacy@drn.newsguy.com:

Oops - I always get vss, vdd, vcc, etc. confused. I meant +400V by it though. Also - the fet choice was almost entirely random - I just chose it because it was the proper type.

Thanks for your note about driving FETs - I hadn't realized that was an issue.

Spice tis not a worry - being that I haven't a clue how to use it :)

I looked through chapter three and didn't see it. Or are you speaking of the piezo driver circuit?

Thanks for your help,

-Michael J. Noone

Hey, Michael, I posted it again. This uses the V+ and V- power supply currents of the opamp to drive the led sides of high-voltage optocouplers. I use a variant of this in a box I built for some guys who make electron-microscope sorta things, where not much current is needed. But 20 mA is probably pushing this too hard... the opto ctr's are low, and the couplers could well fry from power dissipation.

But the general idea of using optocouplers as analog level shifters is sorta handy to keep around. We did discuss cascoding lower-voltage optos with depletion-mode mosfets.

Is the fluid one of those electro-rheological thingies?

John

.... [Old and resolved topic (on or blown FET) cut.]

You seem to be supporting my point already.

That would be the ideal circumstance. A more complete mental simulation would include more dynamic effects. Then, stability could become more of an issue.

The load's actual (E versus I versus (t or f)) characteristic needs to be known or bounded to permit real design.

This is not the place to go into stability. It is a whole subject of study. Perhaps the most useful thing I can tell you (if you believe it) is that stability is not anything that you want to assume take for granted in feedback systems. Too many discover this to their chagrin.

If you look at recent sim circuits posted under this thread, (in both fora you seem to have selected for it), you can see some reasons for such an idea.

You're welcome.

After two days- the fraud Brasfield responds to the juvenile punk with his usual drivel...

Not really pseudo-intellectual, the study of stability is part and parcel with the study of electronics. But you wouldn't know that because you studied neither subject with any success...and this explains why your work is so inferior, and also why you have never been employed as an engineer.

Especially to look at yours which doesn't even work at DC- not a real good sign that it's of much use.

You just love to assume the OP thanks you for your specious and non-informational trash posts....

Hmmm, yes. Wire the LEDs in series and the phototransistors in parallel. Use more than +-5 for the opamp supplies to make up for the LED voltage drop. Idle power dissipation could be an issue here.

Take a look at the datasheet for the optocoupler to see how hard it could be safely pushed. I think they want a b-e resistor at high voltages, too.

I bet there's a way to stack the phototransistors for more than 400 volts swing!

John

John Larkin wrote in news: snipped-for-privacy@4ax.com:

Very interesting circuit! In reality current should be in the 1ma-5ma range, do you think it could handle that? Or would it be possible to somehow wire the couplers in parallel? Thanks,

-Michael