Not Quite As Simple CSA

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Richard Hester
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Not Quite As Simple CSA

Post by Richard Hester »

Attached is a schematic for a charge sensitive preamp that runs off a 9V battery, and should be adequate for the output of He3 and B10 proportional tubes. The output pulses will be around 100-200 mV peak, ~2 usec wide with the 0.1-0.2 pC pulses from that sort of detector tube. Circuit hasn't been built yet, but it's pretty straightforward and simulates very cleanly in ORCAD. Those with an appropriate detector tube (I don't have one) may benefit.
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Starfire
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Re: Not Quite As Simple CSA

Post by Starfire »

Excellent Richard - do you have a printed circuit layout for this?
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Re: Not Quite As Simple CSA

Post by DaveC »

Richard -

Clean and simple. Very nice.


Thanks for posting.

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Re: Not Quite As Simple CSA

Post by Richard Hester »

For John - sorry, no layout. If I build one for myself, I'll use perf board or "dead bug" technique and have it built faster than I could lay it out. Dead bug style has the advantage of leaving the summing node up in the air, helping with leakage. A good wash-down with alcohol after all is said and done (hang it up to dry and don't touch it with your grubby fingers -all our fingers are grubby) helps, too.
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Re: Not Quite As Simple CSA

Post by Richard Hull »

Great preamp, Richard! I gotta diddle this one into a reality.

For those not highly electronically inclined or not used to electronic assembly, the first two caps C1 and C2 both need to be rated for ~3kv or more for a decent safety margin. (not seen on diagram)

For my purposes, I like to wash or clean the bodies of all components in contact with the gate lead of the input FET with absolute ethyl alcohol and not board mount them, but instead, keep them in air over the board. Use a good ground plane based board if you go that far and surround the whole set up in a faraday box. for the ultimate set up, try and work a male HN connector directly to the box and cable nothing to the input. This would allow the preamp to directly mount to a stock 3He tube, reducing noise tremendously. All this is worth the effort, especially in a noisey or uncontrolled home environment

The far side of the components would be allowed to be board inserted. This construction is seen in the best charge sensitive preamps and electrometers.

I mount my Princeton Gamma Tech preamp in this manner.
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Doug Coulter
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Re: Not Quite As Simple CSA

Post by Doug Coulter »

Air wiring is good, but gilding the lilly here. At 2.2 meg input impedance (not counting the fact that it's really much less, due to negative feedback), the leakage of a decent board is far into the noise, relatively speaking. Even the rather high leakage of a 1n4148 would be larger (25 nA on the spec sheet). Typical PCB leakage is far above the 100 meg+ range, particularly if the board is post-build coated with something (solder mask or something you add at home) to keep the conductors insulated from any surface contamination (humidity, dust, electro-migration).

This doesn't apply to perf board, which can be pretty bad (forget phenolic), due to the fact you can't really clean them as well after building (all the extra holes collect gunk that seems to re-spread after cleaning attempts). I've only had to resort to air wiring (which also can collect floating dust etc) in cases where I was working with impedances in the e-14 to e-16 ohm range and worried about DC, vs AC errors.

Quote from the National semi apps book:

"Board leakage becomes more of a factor as impedances are raised. It may not be advisable to take advantage of the full potential of the LM11's 5 pA bias current in all cases, when hostile environments are involved. Anyone designing high reliability equipment that is going to be in trouble if combined leakages are greater than 10pa at 125 C had best know what he is about." Which kind of sets where the numbers were back when this was written -- 1980 -- things have gotten better since. Lessee, 10 pa into 10 megs is.... e-11 * e7 -- e-4 volts offset-- that's not all noise, not mostly noise, just DC, varying with humidity and temperature. Tenth of a mv DC error due to that -- and this circuit is AC coupled internally -- C2 and L1 take care of that (and some normal variations in the fet IDss).
So, don't paste peanut butter between the FET leads, and do clean the board. But these impedance levels, no sweat on a PCB if the rosin is cleaned off after soldering. I've never had to clean any but the oldest most corroded badly stored parts for things in this impedance range -- the trashcan is a better choice for those anyway. You're going to have more noise from the bypass electrolytics specified here.

OTOH, the idea of making this into a box that screws directly onto the sensor is a very good one.
Looking at millivolts right near a high voltage supply that can easily make 10's of volts corona noise on a scope probe placed where the detector would be, even when things are "quiet" is hard to do -- at that point, you've got to have 10k to 1 noise rejection just to get a 1::1 SNR! Much cheapo coax isn't 100% shield coverage (if you were going to have the preamp remote), and of course doesn't address ground loop troubles, so having the preamp right there and shielded *tight* is wise indeed. Even then, the key word is "tight" here, as just a couple screws holding a Bud minibox together won't make it tight enough to reject loud pulses on the HV (including the AC magnetic fields those make with most HV wiring schemes that look like a big transformer primary turn).

I'm in fact using something simpler here (just bipolars) and doing fine with it, but have had troubles with EMI getting in through a sloppy aluminum box at the "joints" that swamp all these leakage effects, including the noisy leakage from the protection diodes that "see" X rays to an extent when reverse biased, as well -- they are pretty good "photodiodes" for those energy photons.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

I beg to differ with you Doug, but I saw differences in the pulse response even at these impedance levels between air mounted and board mounted components, mostly in the fall time of the pulse. I did a lot of work with these things a few years ago when researching preamp designs. With the small value of cap used in this design, leakage will start to have an effect. Good construction practice never hurts.

If you insist on using perf board, putting the input components on teflon standoffs is not a bad idea, follwed by an alcohol wash. It also makes mods easier, in case you want a different charge storage cap or decay constant.
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Re: Not Quite As Simple CSA

Post by Doug Coulter »

You can differ with *me* all you want (why should I care, I'm right!), but not with physics and EE math and get away with it. 3dB point of 10 nf and 2 megs is order of 7 hz. It goes up a lot faster if the other end of the 2.2 megs isn't grounded, as here (because it no longer looks like 2.2 megs to ground from negative feedback), but hey -- this still implies something else, not normal clean-board leakage, was going on for you to see any difference whatever on fairly fast pulses. Else virtually all the analog stuff I designed for 30 years would not have worked, but instead it did and made me able to retire early.
I'm way -way past guessing on issues like this one.

Try it in your simulator if you don't believe me, just add another 2 megs straight to ground on the fet input, or to a bypass to ground so as not to mess up DC levels, and see what difference it makes. And board leakage R is hundreds of times higher unless there's visible corrosion there.
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Re: Not Quite As Simple CSA

Post by DaveC »

I think, both of you are correct about different aspects.

Getting the pulse into the circuit is a characteristic impedance issue, which input geometry and the board material will affect. It may take teflon PCB to get the permitivity low enough.

Inbound, the circuit impedance needs to match the tube characteristic impedance... which from geometry has to look something like 75 ohms or therabouts. Air wiring, while asking for trouble in the RF pickup mode (if un-enclosed, that is) reduces parasitic capacitances..

But Doug is correct also, that for the short retention time needed for digitizing, not much leakage will occur with almost any type of decent construction.

FWIW.

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Re: Not Quite As Simple CSA

Post by Doug Coulter »

Still not right, sorry. And this from some of the smarter folks here, re electronics. Sigh. This is not a personal attack of any kind, just an attempt to educate ;~}

From the geometry, the tube "impedance" at RF is in the few hundreds ohms, the center conductor is tiny indeed in these -- to make it 75 ohms with over an inch ID of the outer conductor, the inner one would have to be well over 1/2" diameter (I'd have to go look it up in a handbook, but after awhile the eyes are close enough for things like this). All the tubes I've looked in use a tiny center conductor to get gas gain where the field gets high near the small radius. Tiny wire in big tube, dielectric constant basically one, high impedance compared to most coax. (the old stuff they used for car AM radios is the exception - air dielectric mostly, and tiny wire, same idea) With dielectric == 1, the impedance of a coaxial affair is a function of D/d - a larger dielectric constant makes it lower for a given size ratio.

However, remember that a mismatched transmission line has it's *lowest* resonant frequency (for ringing) at the point where it is 1/4 wavelength -- so let's work that out and see if that's what matters here.

The tube is a little over a foot long, so the place where that kind of thing happens is far higher in frequency than these couple uS pulses (a full wavelength is ~300/F in meters and mhz, which is why the 2 meter ham band is 145 mhz more or less), and the tube looks like a capacitor for all intents and purposes (with a relatively tiny series L), which is why (partly) someone uses the moniker "charge sensitive" preamp for a circuit designed to capture total pulse energy without looking too hard at the shape of the pulse. There's not much useful info in the shape from a gas tube, that depends on things like where the reaction took place inside the tube, which could be anyplace, (and along it vs across it), and the waveform will vary accordingly in all but saturation (geiger) conditions, which aren't relevant here -- we don't run the tube volts up that high to get into that operating regime where we get constant energy pulse output. We're only interested in knowing how many ions happened in the event.

In this case, the ringing frequency of the mismatched line impedance of the tube itself would be of the order ~100mhz (5 ns per half cycle) so....doesn't matter here, the rest of the circuit will just lowpass all that out and give the correct answer anyway -- Ft on the pnp is lower than that, and the output darlington even lower I think (didn't check, but darlingtons are slow in general). With another piece of differently mismatched coax between the tube and preamp, the number changes, of course, and a long enough cable *could* get you in trouble there.

Basic theory. Known correct. Well tested.

An event creates a certain amount of charge, which we want to collect and amplify. This amounts to a current multiplied by a time, and an integrator is what is most often used for that, though you could usefully put a preamp in front of all of it and integrate later on -- and get better S/N by allowing some voltage drop to occur so that a *power* is developed with that current. The reason most put the integrator first is it's more elegant that way, and cuts the HF noise (from other things) early on in the chain. And the integrator is rarely a perfect one (else the pulse would never return to baseline among other things!) -- so it works out, even though not technically ideal -- an impedance matched preamp would be better for S/N, but luckily we don't need the best we can possibly get here. You'd couple more signal power out of the tube if you didn't load it with so low an impedance, allowing a bit of voltage drop to occur so the old currnent times volts times time = joules thing is happening and the rest has some actual power to work with to swamp its own noises.

Technically speaking, this isn't even a charge sensitive preamp. Even the miller capacity of the FET is suppressed via the low input impedance cascode pnp here! That's where the integration usually is in a CSA, or around the entire amp loop, and there is no explicit C is across the feedback 2.2 meg R here (always some actual C, of course and at 2.2 megs, the parasitic C is about right, actually). Does that actually matter? Only to a pedant, as the circuit will work fine anyway -- it's a current to voltage converter with current gain and both high and low passing, not all explicitly drawn, but there none the less.

The real time constant of the input circuit can't be known without also knowing the R in series with the HV supply for the tube, btw -- the tube is a current source when there's an event, and open circuit when not -- essentially infinite impedance at all times, just one that draws some current when a neutron hits it, for a little while.

The real impedance the input coupling cap sees is that resistor (not shown or specified) in series with the amp effective input impedance (zero by comparison). Talk about a mismatch -- we have about a meg or more at the tube end (with a tiny C across it, intrinsic to the tube and connectors), and basically zero at the amp. But this is an easy signal processing job, and that pedant stuff doesn't matter as much as you may think in this case -- remember the active tech in use when these tubes were really in use was thermionic triodes.... This is, though, why the HV power has to be quiet, as any noise on it will get in here via the series resistor to the supply, and show up as an inpu current (the supply R is a voltage to current converter for noise on the HV).

Depending on the open loop voltage gain of the amp, which I've not looked at hard, but it's high, essentially the transconductance of the fet driving 82 k ohms via the cascode pnp with elimination of the DC bias current of the fet via the clever inductor. Fets are all over the place on IDss, which is why the source R is also a good idea. We have a high pass filter or differentiator due to the inductor.

The effective value of the 2.2 meg resistor for frequencies the amp can pass (see that inductor - it's not a DC amp) is....zero, yup, or pretty close! (check this in the simulator) -- it would be 2.2 meg divided by the amp open loop voltage gain which is a fairly large number (100?) so call it 20k. And even though I spit out that figure above as 7 hz -- no one caught it (heh;-), so I now know that no one knows basic opamp closed loop theory (at least who has yet posted, other than myself). Or didn't realize that it applies here. So these words need to be read and understood.

Just because you didn't draw the opamp symbol doesn't mean you didn't make one -- and this is pretty close to some current opamp (and transimpedance amp) designs in most of the ways that matter. It just lacks a non inverting input (as did many of the older designs from Philbrick). And it has several things in it doing high pass too -- all those bypass caps but mainly the inductor which probably sets the main corner high pass frequency and the rest don't really matter for skinny pulses in the few uS range.

Since we've made a virtual ground (for AC) with the 2.2 meg feedback from the output, the effective AC *input* impedance there is basically zero, or low anyway -- and hooking even a big leaker across that makes no difference (unless the leaker is noisy, like a diode being hit by X rays! Or one with shot noise or 1/F noise in the leakage), any more than it does in an inverting opamp circuit when you hook a resistor from the - input to ground at the virtual ground point at the minus input. when you do this, you lose the advantages of low noise at high impedance a FET has, but it really doesn't matter that much here. The signal is, gratefully, loud anyway.

Guys, I'm not making this up! Sorry if I sound negative here -- not intended -- but I just can't let factual error pass completely unmolested, as we're not the only people listening here -- some people are just learning electronics, and big misconceptions will lead them astray in later efforts. This is dead center of my expertise. Other things, sure, there are people who know more or have more experience. This stuff, nope.

If you like, I can get a "real" analog chip designer on my team (Joe Sousa who works for LTC in that capacity) to comment further...this is a fine design, but perhaps not as well understood here as it could be. You probably own some of his (working!) designs now and use them every day. He maintains an antique opamp forum too, at http://www.philbrickarchive.org/
BTW. Way not dumb on this stuff. Designs A/D's for a living for LTC, mainly.

I was simply pointing out that going for certain things (like insulation) in extremis was indicative of a misunderstanding and superstition -- neither are good for being able to repeat results like we demand in science.

Sure, no harm keeping things clean, but this is like thinking you need an atomically sharp screwdriver tip to turn a 10-32 screw. You don't! And as I pointed out, shielding may make far more difference, since it has to have 10k to one goodness to even get to 0 dB signal to noise(!), and it's not trivial to get a couple hundred times that to really ignore noises around a fusor. That's one of two reasons to put the preamp right on the tube. The other is that not only is coax not a perfect shield, but it's also a capacitor in parallel to ground, which will "eat" some of the charge signal from the tube.

Remember, these are not working in a range where the resonance of a mismatched cable of reasonable length makes *any* real difference. Off by orders magnitude unless you've got a whole spool of wire in there.

Changing the leakage currents from a ten thousandth of what matters to a millionth really doesn't make much difference. Anything you think that did, there's something else going on -- where the rosin spattered or peanut butter smeared while you were breadboarding or similar could do things like that for example. Of course, you keep things clean no matter how you do it, that's just basic prudence and hygiene, and some things like rosin can be hygroscopic and become quite conductive with time due to that and other effects, like electro-migration (electroplating by another name, only not desired here).

But I've had air wiring fail too -- amazing how a cat hair can enter into the equation....They seem to seek out things like that to bridge, collect dust and dampness, and so on downhill we go.

For a reality check on what I'm saying, take an ohmmeter and measure track to track leakage R on a clean PCB. I want to see the ohmmeter that will do that -- they exist (and I have one), but not the Radio Shack, bench-grade that only go up to 20 megs. The number is going to be giga-ohms in nearly all cases. The number is higher yet if the board has solder mask or a good coating. And a cat hair laying on a coating may not affect that -- it can actually be better than air in that regard, given the dust issues.
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Carl Willis
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Re: Not Quite As Simple CSA

Post by Carl Willis »

Hi Richard,

Thanks for your efforts with this design, and your previous contributions. I'll have to give it a try.

If I had one suggestion from just looking at your design, it would be to increase the decay time constant. Charge collection in proportional counters takes a long time, probably on average a few microseconds if the response is to remain linear in energy. So I'd advocate 30+ uS for the product of R1*C2 for that reason. An unrelated reason to make the decay time longer is that the pole-zero adjustment on commercial NIM amplifiers typically works between 50 microseconds and infinity. I think the only downside of using a few dozen MOhm would be the lack of availability at Radio Shack or in most junk boxes...what do you think?

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Re: Not Quite As Simple CSA

Post by Richard Hester »

I have some extra intelligence on that, Carl - I was looking up data on Boron-lined counters, and a group in Washington state is evaluating them as a replacement for He3 tubes in portal monitors due to the He3 shortage (sound familiar?). They were talking about using a short time constant to aid in gamma discrimination. It's something that could be played with, as we don't expect a lot of gamma action with the fusor, and it doesn't take a lot of lead to screen out X-rays and such.The paper is one of the first things that pops out if you do a search for "B10 counter tube" or "B10 proportional tube" or something similar. Anyway, it's easy to tailor the time constant by playing with the discharge resistor. The larger you make that resistor, though, the more chance you have of leakage starting to affect your results. I also don't trust normal -type resistors too much over 10M or so. I'd keep the charge storage cap small because of the low amount of picocoulombs involved. When you have a big, fat pulse like from a PMT, then you can splurge on the picofarads.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

To Doug - I really dont care, either. I messed with CSAs for upwards of a year developing various circuits and know what I've seen. The circuit posted is a simplified version of a CSA available from Bicron - I even posted the schematic in files.
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Re: Not Quite As Simple CSA

Post by nemesistech »

How bout I ask simple questions that I am sure most of you can answer with ease. I am still in the learning stages with building circuits, so forgive me if these questions bore you.
First, should I assume you would want to use metail film resistors instead of carbon?
Second, on Q3 transistor, is that one piggybacked on the other, if so, what is the purpose of this?
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Re: Not Quite As Simple CSA

Post by Doug Coulter »

Richard, sorry, bad day at the office, you know. And for once it's Carl being polite and me abrasive ;~) Not really called for....I do what I do and see what I see too, but when that conflicts with predictions and book learnin, I try and find out why -- I suspected it may have been something not obvious in that case as I do also have a lot of experience with things like this. I agree with Carl on the time constants here -- this stuff is on the way slow side compared to things like PMT's.

To Mike , that last transistor is two in one package, in the Darlington configuration, used as an impedance reducing device (eg it will drive more current with this than without it). In common collector mode, it's just a gain of approximately one for voltage, but has a ton of current gain. This way the design can use that 82k resistor in the pnp for good voltage gain, without having the load you put on the preamp affect that (the output of the pnp is a current which the resistor changes to a voltage), and it will drive a hefty load (which helps with noise pickup downstream, keeps the gain stable, and other good things like that).

So think of it as a gain of one (for voltage) buffer that doesn't need much current to drive it, but will put out far more current than it takes to drive it. You can do this with two transistors in separate packages too, and the net current gain is that of the two transistors multiplied -- so the current gain gets big quickly. Darlingtons are usually slow, particularly in turn off, as there is no resistor to discharge the base-emitter capacity of the second transistor when the first turns off (or maybe there's one inside the package and just not shown in this case, I'm not familiar with that particular number).
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Re: Not Quite As Simple CSA

Post by Starfire »

Less talk more build - suck it and see - general comment not direct at any one in particular
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Re: Not Quite As Simple CSA

Post by Carl Willis »

Hi Richard,

I have a preamp at home, derived from probably the same Bicron preamp you mention, on which I use a regular 10M resistor and a 5.6-pF, NPO-grade ATC cap. The cap and resistor, as well as the protection diodes, form a self-supporting "dead-bug" structure. It works well with proportional tubes in my experience, but I cannot comment on its temperature stability or noise figure or anything like that. I do know that the observed time constant--when fed from a charge-terminated pulser--is not any different from what it should be ideally.

Thanks for clarifying your thinking about the short time constant. I had no idea B-lined tubes were making a comeback.

To your point about construction technique, I'll just add that I think it matters a lot. My feeling is that stray capacitance is probably what causes observable differences from the ideal when building the input stage solely on PCB pads, but it could be surface leakage as well, or some of both. I am in the habit of building the rest of the boards by "Dremel etching" like a total amateur, but the input stage is indeed unpredictable if built this way. Ground loops are a huge issue with preamps. Microphonics are a problem also. Power supply filtration is an issue. Cleanliness is a huge issue especially if the load resistor and HV components are in the preamp also. Leaky BNCs...leaky Victoreen resistors...rosin threads...humidity...all very relevant issues, not easy to avoid. I wash with methanol ("Heet") dried over zeolite pellets to get rid of rosin and humidity. Does not always effect a cure, though.

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Re: Not Quite As Simple CSA

Post by Richard Hull »

As I first noted above, I just got used to cleaning and floating gate stuff in electrostatic/electrometer systems I have built and worked with and have seen it done on virtually every single charge preamp from GeLi preamps to 3He and BF3 NIM preamps. I'll do it reflexively until the day I die. Once the faraday case or shell closes, it should be a sleeping dog for a long time and require no revisiting.

I do hope to construct this thing soon, just 'fer grins and googlers.

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Re: Not Quite As Simple CSA

Post by DaveC »

In response to Doug's earlier comments on characteristic impedances.

Hey - I said "75 ohms or thereabouts"


Z0 depends on the Log of the diameter ratios, not ratio directly... thus pretty insensitive to size, right?

Further, within a factor 2 or 3 is "Exact" for physics discussions, and not even bad for arm waving electronics.

So what's your beef, eh?


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Re: Not Quite As Simple CSA

Post by Doug Coulter »

No beef at all Dave -- you are absolutely correct on that one, and here it's just not an issue, as we are far shorter than any length that constitutes transmission line behavior for the risetimes involved anyway.

My apologies, I guess I should more often calm down before posting sometimes.
On email, they can't see you grinning and armwaving, so I get in trouble.

Ever notice that stuff that comes off as mere agitated excitement in person sounds pretty bad (or can be taken as such) on email/print? This one bites me now and again. People who have met me in person know it's just me spouting off, but others can't know. Sorry 'bout that.

I have a two transistor circuit I'm using on the 3He tube (and in a lot of other places) I like better myself, and it doesn't need all the capacitors, which is nice both for complexity and for recovery times on overload. This circuit in that use also has builtin adjustable threshold for gamma discrimination.
As I have two B10 tubes, I may try a version optimized for them (that's a somewhat tougher signal) and let everyone know.

The circuit under discussion seems to be derived from the second one here, also in the same book.
(1980 linear apps handbook)
There are some interesting variations of it that provide for almost zero input capacity, due to bootstrapping the fet G-S cap, and eliminating the G-D miller cap by use of the cascode -- the drain basically is driving an emitter, and there's almost no signal volts there -- it's all current.

Which made it a great place for the inductor -- since that's a low impedance node, you don't need one as large, and using an inductor protects the rest of the circuit from the wild variations in Idss most fets tend to have from unit to unit (if not from moment to moment, and temperature to temperature).

Joe Sousa and I try to eliminate all L's and C's where we can and it makes sense. Old habits -- he's a chip designer where you can't have much of either (but where taking a few transistors to make a current source is nothing), and I'm an old cheapskate with a weird idea of what constitutes "elegant", and who used to have to fix up behind designers of mass market stuff, and guess what made all the troubles -- caps, which at least we could get, and inductors, which in some random TV set or stereo, just try to find out what it even *was* so you know what to replace it with, and more often than not the Q and self resonance was as important as the inductance to the thing working right. Nightmare for a tech, which is what I was, coming up in the '60s. I later graduated to EE, then Scientist, but I didn't forget some fantastic lessons I learned about repeatability and reliability as a service tech. We saw what really worked and of course, what didn't -- many times a day.

I first saw this circuit in a very good early solid state Marantz reciever, used in the tone controls, then some years later another version was in the National Semi apps book used at the other extreme of impedances for a moving coil phone cartridge preamp that was near noise theoretic at room temp.

By changing some speeds and feeds, it's in use on my 3He tubes, my BF3 tubes, and some phototubes, all just by changing some values. Very simple, low noise if tuned right for the impedances involved, cleanly closed loop system that can also be an integrator of the type wanted here if you add a cap across the NFB R. Works like a champ, and is not real sensitive to power supply issues. You can use a fet for the input device if you make some small changes for bias issues.
I don't usually have to, with the current gain of the transistor pair with NFB, the input impedance can be plenty high with plain old bipolars (and therefore more rugged, and in some eyes, more elegant).

Here's a scan from the National book. In most other uses, you don't use their fancy low impedance transistor pair, but the good old 2n3904/3906 kind of thing, much lower currents, and in the case of the tubes that put out a negative pulse, use a pnp in the input, then npn (power supply reversed) since the tube will turn on the pnp better with the negative pulse.... and then you can fool with the bias to put a threshold on there -- no parts. In my practice here, I do that via a simple pot across the signal input (after the input cap) to adjust the tube net gain to match tyhe Vbe threshold with no other input bas at all. (I run my tubes higher volt than spec to get into a good gas-gain region and they drive this fine -- out come 5v pulses at decently low impedance).

So basically, all the R's in the national circuit are much higher value, and for some things I do the NFB differently -- sometimes none if I just want a count/no-count situation and am using input bias (or the lack of it) to set the threshold.

In the third one we see the bootstrapping, which can only work if you have a gain == 1 node someplace to play with (however you get it, I've seen people use dividers off a gainy circuit to make that happen). Pretty cool, actually, and this is close to what we wound up with to take EEGs with pure capacitive input coupling -- no shaved heads and no goo needed.
Same class of problem but at a very different passband.

I have a bias toward real pc board building, since it's really not hard these days, and when I did this for a living, of course that was all there was. I use a freeware software tool (Protel TraxEdit, very old dos program, and IMO better than their current multithousand buck offering by far -- simple), a laser printer, and pre-sensitized PCB (available all over). Even if I'm not real sure the thing will work, it's easier to cut the odd track or add the odd wire, change parts etc, and get going.
And once it's right -- it's right forever, every one you make is the same re all the leakage R's, parasitic C's and interwire coupling....worth the effort for me, because once you get set up, it's not much effort at all. Then I can send the same files to AP circuits in Canada and get them made really nice, pretty cheap, and very fast if I need a bunch. I have zip troubles with leakage unless I'm getting way out there on something. Real high volts and real high R's I air-wire as required -- there it doesn't take much conductance to make trouble, but at 5 volts, nah. In this case, the tube supply resistor wouldn't live on the PCB at all, of course, and neither would the input coupling cap as one end is very hot. At the normal solid state volts, up to about 10 megs, no issues whatever with PCB leakages, even if you need a pretty precise 10 megs. Above that, well then -- first find good enough parts at all, then yes, you have issues especially at high volts where tiny amounts of corona play hob with SNR.
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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Attached is the output waveform for the circuit in question built dead bug style on a small piece of copper clad board. The excitation source is an LED/Photodiode pulser similar to the circuit described in "files". The charge storage cap used was a 1 pF +/- 0.25pF NPO disk. The coupling capacitor (4.7 nF) was moved to the input rather than inside the feedback loop to allow greater freedom of choice for the charge storage cap. The voltage rating for the charge storage I chose is unknown (though I suspect it is either 500 V or 1kV rating). The input, output, and summing nodes were built off the board on small standoffs for isolation and/or ease of connection. The picture shows a 1/e time of about 2.9 usec, which is somewhat longer than expected, but not too far off considering expected stray capacitance and the small value of charge storage cap.

Will post more info and pictures tomorrow as time permits. This represents ~15min of effort (the data taking , that is - the circuit took a couple of hours of effort, including digging up the fet for the input). The circuit came right up and required no extra effort. I spent a lot more time trying to find my pulser....
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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Here is the actual build of the CSA circuit using dead bug construction. The circuit was constructed on single-sided copper clad board. An additional piece of board was tacked to the main board (superglue works ok) to serve as a Vcc bus. The circuit was laid out on the board to flow from left to right pretty much as presented in the schematic. It's not pretty, but it works. Small standoffs were used for input, output, and summing node connections. The summing node could very well have been an "air connection" (and it would have worked better that way), but I wanted a little more support for the components. The nailhead terminals on the input and output connectors make connection to the board a little easier. The large disc cap at the left hand side of the device is the input coupling cap . It's about as big around as a nickel, which gives you an idea of the overalll size.
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Carl Willis
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Re: Not Quite As Simple CSA

Post by Carl Willis »

That's a nice little project, Richard.

I'm thinking that on account of its diminutive size and simplicity it might work well as a front-end for CDV-700s, Ludlum 3s, etc. to enable their function with proportional detectors.

Thanks for the update.

-Carl
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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Test results are in a post a little farther up the chain - maybe I'll ask Mr. Hull or the Professor to move it down the line so it is in chronological order. I based the charge storage cap size on the numbers given in the Reuter-Stokes data sheet for their He3 detector. If i remember correctly, their charge/event was specified at 600V. As you stated earlier on in the boron tube thread (I think), this number will be higher for higher bias voltages. Getting the storage cap value up to near 10pF would help repeatability, and also make the cap a little easier to obtain. Having said that, a teflon wire "gimmick" with some heat shrink around the wires to preserve the twist would make for a high quality charge storage cap - make up a couple if inches of twist, and cut it to the desired capacitance...
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Re: Not Quite As Simple CSA

Post by Tyler Christensen »

I built this amplifier today and it is working great, another successful build. It is a huge improvement over my previous "pathetically simple pre-amp". I really like the battery power source since it simplifies on getting power to it noise-free.

Worked great on my 3He tube, I'll test it on a 10B tube at some point but I'm sure it will work just as well.
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