The fine wine of preamp design

Linear and non linear

The fine wine of preamp design

Postby Doug Coulter » Tue Jan 10, 2012 12:49 pm

First, you need the grapes and the barefoot girls...nah, you just need good active devices that have nice characteristics, and a design that doesn't waste them. For truly fine design, these parts should be operating as they were designed to, and be readily available ones that have already stood the test of time - and are still used enough to keep them in the distribution channels. The only concessions to "new" made here are the use of a super good MLCC power bypass capacitor, and surface mount design to keep all the strays low. The rest is classic, and actually does work fine built up with through hole parts on perfboard, just not quite as perfectly as this. Here is the schematic we'll be discussing: (as usual, click the pic to get a big version)
WinePre.gif
Nuclear detector preamp design


Errata: The last transistor is shown as a 2n3906, which is a pnp, but drawn wrong here - look down-thread for a more accurate schematic drawn by Joe Jarski.
What the heck - here it is inline too. Oops, drew that emitter the wrong way, and yup, it matters.
Preamp.gif
*Accurate and clear schematic. Thanks Joe, and Larry for pointing out the error!


/////////////////////////////////
I've come up with a preamp design for nuclear detectors that I'm pretty happy with, even proud, so I thought I'd share it here. For those less up on the arcane details of analog design and interactions, the explanation of how it all works separately and together should be helpful. I have used variations on this topology for a lot of things over the decades, this is specialized for the kind of signals we encounter in our fusion work, but is pretty good general purpose too – with different optimizations. All of those are done by simply selecting different parts and values, so a single PCB layout handles pretty much all uses of this.

The goals here are fairly simple to list:

1. Wide supply voltage range – this shouldn't be a picky eater, and should work over a battery lifetime if desired.

2.Millivolts in make volts out, a good loud signal that won't be bothered by noise around a high voltage device. If possible, enough output swing to directly drive TTL or CMOS with no comparator and variable reference required - a schmitt trigger input should handle our output fine and simplify your system overall.

3.Gain easy to adjust to make the above work well.

4.We want our response to be matched to the input signal, which in this case is a low level negative going pulse with fast risetime and slow decay. A matched filter will get us the best signal to noise possible with our known signal characteristics.

5. Low noise.

6. Tiny, self shielding, convenient.

I want to do a stage by stage analysis first, then show what the feedback does to alter that as a separate issue.

First is the input protection and DC bias. I show here two 1n4148 diodes set up to clamp any input signal to the power rails, plus or minus 1 Vf for a diode. This is to protect the input transistor with a cheaper and easier to replace part. While I personally use phototubes with a negative supply and floating anode that can go “straight in” to the input base, many like the positive 2 wire supply and coupling cap or transformer to a preamp, so I made provisions for either. The issue when you have a coupling capacitor is that it might hold substantial charge versus what a base-emitter junction can handle, and hot plugging such things does happen by accident. Even a tiny capacitor can deliver amperes to a sudden short, and the fairly fat 1n4148's can take that better than the low noise, low power preamp input transistor. They do add a little capacity, even reverse biased, but not much. In a hard wired and carefully built detector design, you could just leave them out if desired.

Next, I wanted this preamp to be mostly DC coupled, and only add various time constants where I wanted them. You don't want too many capacitors making for undershoot and long recovery times from overloads if you can avoid that. This requires a bias source that is relatively independent of the power supply voltage – one goal is to have this work over a fairly wide range. We also require that this voltage have the same tempco as a transistor Vbe, so the output level won't drift around too much with temperature. It turns out that a LED has a one Vbe drop, plus a drop related to the quantum energy of the light frequency it would emit, along with some parastic loss due to series resistance – nothing's perfect. But that nice fixed voltage on top of a Vbe is just what the doctor ordered here to just barely turn on the input transistor. We can fine tune the bias voltage by led selection for coarse adjustments, and series resistor to control the LED current. In this case, we just want to barely bias all the transistors on, so the full supply voltage is available for the output pulse swing. The LED provides just the right temperature-variable voltage to accomplish this – we may or may not put enough current into it to make it visibly light up – that's not the point here.

Now for the first gain stage. This is a low noise at high impedance bipolar transistor. I chose a PNP for this slot because transistors turn on faster than they turn off for a variety of reasons, and that's the match for our signal. For fast but not super fast speeds, the audio standby 2n5087 was chosen as at low collector current it has almost theoretic noise figure at the medium impedance level we have here for phototubes and gas proportional tubes. For higher impedances, a FET would be slightly better, but much harder to bias stably – it would have to be tuned for each sample of the FET used. Looking at the gain from base to collector, due to the resistors shown we have a DC gain of a little less than 1 (inverting), but an AC gain of roughly 47, due to the ratio of 4.7k to 100 ohms. The bypassed emitter resistor is one way to futher fine tune biasing in this design, and we have a little high pass from the reactance of the bypass capacitor here in parallel with the 4.7k. I chose the capacitor to be large in comparison with the expected signal pulse width, but other choices might be favored in other applications. Note that the Hfe of the PNP is generally in the range of 300 or thereabouts at the collector current we want to run. We want to produce just about 0.7 volts across the output 4.7k resistor, which works out to 148 uA. This is about where we want it for least noise with some tens of K input impedance. The 100 ohm AC emitter impedance will be boosted up by the Hfe to about 30k – which is less than the 100k bias resistor and input load we show – but we've not covered yet what feedback does for us.

Moving to the second stage, I flipped transistor sex to an NPN, again, because as the first stage turns on, so will this – on that fast risetime or onset of our signal. This stage is set to have a gain of about 21, due to the resistor ratio. Without feedback, this gives us a total gain of 21 x 47 or 987 to work with.
This stage is all DC coupled as you can see. It will bias with a gain of 21 off whatever change occurs to the voltage out of the first stage, which is close to the practical limit without feedback for stability. More gain than that becomes really hard to get stable in DC with various non-theoretical parasitics in the parts (series resistances and such) and even small power supply variations. We want to burn more collector current in this stage for speed, but not so much we get into power issues, so we do – here we burn perhaps a milliamp or a little more. We also want the base of this transistor to not have to wiggle much – here we move 1/21th of the output signal. The reason for this is that the first stage transistor will have some base-collector capacity. The more we let it's output move in voltage, the more the input signal has to be diverted to charging this capacitor, reducing gain and speed. If you think of this as a charge sensitive preamp, that's the main capacity being worked on by input charge, and we don't want it to be multiplied by the miller effect.

The final stage is just a simple emitter follower to reduce the output impedance further and directly drive coax if desired. We therefore use a relatively smaller load resistance, and provide a series match to coax. The other purpose of that series resistor is to prevent creating a one port oscillator with the follower – an esoteric but real troublemaker in some situations, as a piece of coax can look like a tuned circuit under some circumstances – in this case, the series R limits the Q. AC coupling is provided as well as places for pull up or down resistors on the output. When it's all tuned up, the emitter of this resistor wants to be sitting just below the postive supply rail. This is to make the full supply voltage available for output swing, and to reduce quiescent current draw.

OK, now lets look at what the feedback (that 15k resistor) does for us. We had a little too low input impedance. With the feedback (assuming it was coming from infinite gain, but our 987 is “close”) we now move the input emitter to track its own base input. This eliminates the effect of any emitter-base capacity. It also bootstraps the input impedance by roughly the ratio of open loop to closed loop gain, in this case that would be our 987/150 or 6.58 to one. That kind of handles taking our original 30k in parallel with 100k to a point of letting the 100k be the main determining factor of input impedance, sweet! It's now in parallel with a 197k effective transistor input impedance – so we wind up with about 66k input impedance net. Not so bad.

We also get some DC feedback, and in this case the model is a little different, as the input stage 4.7k emitter resistor is also in play. So we're only trying to close the loop on the DC gain of the second stage here. This cuts that DC gain down from about 21 to about 3 – the ratio of the 15k and the 4.7k, so the thing becomes more stable to small DC variations caused by temperature and parts variations.

The resulting circuit satisfies all the goals, hands down. One can tune all this by using different parts or values. For example, to get more speed, at a cost of lower input impedance, one could reduce the first stage resistor values and burn more current there (which helps discharge capacities quicker and turn off the transistor faster). You could, at the limit, start looking for a faster PNP to put there too, within the noise and current gain limits you decide to tolerate – low noise high gain PNP RF transistors don't exactly hang out on street corners! Ditto the other stages – you could reduce the resistor values, keeping ratios about the same, throughout. This will cost you some noise, but mainly a lot more supply current – instant death to batteries! But you might not care about that if you have the need for speed.

To reduce the drain, the bias is probably the place to fiddle. You could in theory go all the way to the point the output transistor is just plain off – or even the second gain stage. This would create an implied (but real) threshold on the input signal, but if you don't go too far that way, you'd get free “baseline clipping” from that as well as a reduction in power use. In general you'd not want to go much below about half the collector current on the first stage shown here – the characteristics of this transistor don't scale forever. You would get a little higher input impedance for least noise figure doing that, but only to a point. In truth, noise from other sources starts to be the big problem here, with this high impedance and high gain, which is why the PCB has one side all ground – point that side at any source of noise, and the top right at the signal producing device!

To change the gain, just change that 15k resistor. If you change it a lot, you might want to redo the bias as well via either changing the LED current, or the 4.7k emitter resistor in the input PNP.

As shown, the values are more or less optimized for a photo tube with floating anode, direct into the input base, to produce TTL level outputs and with speed to match fairly slow scintillators – which would include the ZnS:Ag in a hornyak detector, NaI:Tl, BGO's longer time constant (it has two), or a gas proportional tube, for which this is faster than needed as is.

I'll post up some scope traces of putting the thing through its paces when the boards get here and I get to build one up all surface mount. This board design takes care of not having excess stray capacity where it will hurt, and in just plain having very little parasitic C or L anywhere, so it should squeeze the best possible performance out of the parts used. The bottom all copper, and a ring around the top should help with noise pickup issues as well. I included a minor RC network to take the real bad fuzz off the power input, but not enough that you should run 10 foot long unshielded power wires by a high voltage arc on the way in – do be careful there.

/////////////////////////////////
Here's what the board looks like as laid out by Joe Jarski. And yes, we'll be making these available either built or as kits at DJ's real soon now. Although there will of course be a labor charge for building and testing these - unless you can do SMD pretty well, you should probably spend the money and get the built version. These won't be real easy to make, but worth it to get the size and performance.
WinePreBd.gif
Board layout, top view. Box is 1 inch.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: The fine wine of preamp design

Postby Doug Coulter » Tue Jan 10, 2012 1:00 pm

Some other picky details. This is the same design we used in our Hornyak detector that went to HEAS to be tested against Richard Hull's gear. In that detector, we had a fairly low gain phototube - about 10e5. This required the high gain and input impedance we are showing here. No problems, though, as it's also about right for many proportional tubes, and if you need less gain it's always easy to go down. Even in that, we had a few inches of lead required between the tube and the preamp, so we used coax there. This was in the same box with a very noisy high voltage supply, and we wanted to completely minimize pickup of its 50khz square wave. So, in this case, we used the filtered power rail as the coax shield, since that's really the input signal "ground" for this design. The other end of the coax (up inside the phototube case) was left floating. This board layout makes that filtered DC rail available for that use on a pad. Best of all is not to need coax in the first place - less input capacity involved to attenuate input charge to voltage conversion.

The thinking here is to put the preamp inside the metal tubing with a phototube in cases where that's what your sensor is. The parts side goes up against the phototube base, and the backside shields the mess from noise that might be coming from behind, as if perhaps we also had a power supply in the same tube with the phototube. Wires can go around the board, or through a hole drilled in some of the copious extra ground around the circuitry.
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Re: The fine wine of preamp design - scope shot

Postby Doug Coulter » Tue Jan 17, 2012 8:01 pm

OK, Joe did a fine job on the boards and I got them today. I optimized the bias a little bit via changing the bypassed part of the input stage emitter resistor up to 6.8k (7.02k is optimum, but not much different).
The preamp performs quite well, even overdriven (which was easier than building a good attenuator for my sig gen, but I'll get there at some point). Here's what that looks like on the scope:
preamp.gif
Scope shot, preamp running on 8v at 600khz pulse input rate. Quiescent current about 2.4 ma, about 10 with this signal.
preamp.gif (4.44 KiB) Viewed 5424 times

The blue trace is the crummy input signal.

Note the 250 ns or so delay, but the fairly quick rise-fall times. This could be made much faster by burning more current in the transistors, but for most of what it would be used for (1uS pulses more or less) this is fine and even ideal just as it is. This shows a fairly overdriven input, which is AC coupled through the on board capacitor. This of course draws more with a signal, due to driving the output through that 220 ohm resistor in the final stage - and with the bias set so that it sits about .4 volt off the supply rail, it's also most of the quiescent current.

Touching the input pad through a tweezer point is enough to fully overdrive this with the noise from my CCFL bench lamps. When you remove your input, it's pretty silent. Here's what the output looks like with the input floating - and the CCFLs nearby turned off - it still picks them up a little even with nothing wired to the input pad.

preampNoise.gif
Floating input, output noise about 4 mv pk-pk. Not too shabby. (gain = 150)
preampNoise.gif (7.02 KiB) Viewed 5424 times


Look for a listing for these on DJ's coming soon - I have to buy some 6.8k resistors in 0805 to build up the boards Joe Jarski laid out before we can ship these with optimum bias for the 6-9v range. The board will actually run well below or above that range with the negative feedback in place, but will start base clipping below about 5.5v when run "wide open" eg, no 15k feedback resistor.

And of course, here's what it looks like on my bench.
Preamp.jpg
Here's preamp!
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Re: The fine wine of preamp design

Postby Laurence Upjohn » Wed Jan 18, 2012 7:50 pm

Doug;
I am a newby here and need to catch up on a set of initials you have noted in the above description. Who is DJ and are these PC boards available yet? Thanks and Happy New Year! Larry Upjohn.

P.S. I have prototyped this pre-amp as well as the MCP 6024 based pre-amp and find them both excellent designs even on the proto-board. I have previously built up a design based on the Analog AD 823 which is okay..but not consistant. I am using these pre-amps with my standard GEO Bicron 1.5 X 1.5 NaI scintillator and ultimately with my homebrew XP2102-plastic scint. When I get time I will attempt somesort of final comparison. Thanks again, LRU.
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Re: The fine wine of preamp design

Postby Laurence Upjohn » Wed Jan 18, 2012 8:50 pm

Doug;
I used the search and found DJ's. I am learning! Thanks anyway and I will follow up when you have the preamp boards in supply. More later, Larry Upjohn.
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Re: The fine wine of preamp design

Postby Doug Coulter » Wed Jan 18, 2012 9:41 pm

I kinda had a few issues with the MCP preamp - it didn't handle overdrive recovery too well, and just didn't work as well in bad situations near its count rate limits, which I thought were too low in some cases.

This seems the best of them all so far (it better be, it was also much more work). Using an opamp section as a comparator didn't work well in that particular opamp chip, there was some cross coupling back to the preamp that took fiddling to overcome. Other chips that did work better drew more current than this does, and didn't work as well, or as fast. So this one seems to be "it" for all mid speed things - as an audio preamp, it has much less than .01% distortion due to all the negative feedback involved, and this is much more stable at DC than the original "ultimate preamp" design was. I was astonished that in testing, without the 15k nfb resistor (so the gain was 900 something) that between 6v and 10v the output voltage off the positive rail only changed 20 mv! That keeps the full swing available for negative going pulses no matter the battery state if you're running off a 9v battery. Sweet! I'm using an earlier version of this with a Hornyak detector we plan to sell along with one of these just driving a CMOS counter input (the aux input on the standard counter we sell) - nothing else needed, and it's rock solid there - will be a good adjunct to the standard counter for fusioneers as it works out to almost exactly 1k cpm per 1 million neuts/second output - so it will span any normal fusor range fine. Also, needs no bulky moderator and is almost EMI proof with an 8v signal out to coax.

I see this as a pretty ideal preamp for a gamma spectrometer due to the nice matched filter effect and the right speed for NaI:Tl heads too, since the linearity it has is more or less unheard of at these speeds.

DJ's is a dumb name for a collaboration between myself (Doug) and Joe Jarski. We just pulled out out of a hat. If anyone has a better name, I guess we'd like to hear it - we aren't in love with it, it's just our initials.

We have a lot of pretty cool things planned....whatever we wind up calling it all. The two items nearest the top of the list, after the neutron detector stuff (this was part of that), are a pump station controller and turbo pump driver for those cheap turbos off ebay, and a multichannel analyzer for gamma spectrometry (which will also use this preamp - thinking ahead we are), both of which we should be able to do a lot more affordable than the current stuff out there - only with better quality on top. People are pretty wowed on the counter.

I expect to have boards available in about a week, we've not set prices on them yet. I just need to get some 6.8k 0805 resistors in stock first. Already built up another 4 except for that one part - the bane of hardware, if you're missing just one part, you have nothing.
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Re: The fine wine of preamp design

Postby Joe Jarski » Sat Jan 21, 2012 7:43 pm

I got the first of my preamps built to use with the Russian B10 tubes and hooked it up to the Standard Counter. This is still a temporary setup to test everything. I'm using several coax runs that wouldn't do well in a noisy environment. The final deal will have the preamp mounted in a metal housing directly to the B10 tube. Earlier tonight, a little before sunset I was getting background counts of 1 every few minutes with a few doubles or triples occasionally and then 8-10 minutes of nothing before it starts over again. Does that sound about par?

Well, since the sun went down, it's gotten a bit quieter - about half the activity that I had earlier.
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Re: The fine wine of preamp design

Postby Doug Coulter » Sun Jan 22, 2012 10:56 pm

We just got grazed by a huge coronal mass ejection - expect a few strange readings for a day or a few. I saw some funny stuff here.

Here's what I've been up to with this:
HornyakProtoParts.jpg
Hornyak proto parts

I'm working up a standard neutron detector to go with standard counter...(it's working now, by the way, but needs some tuning).

And here's how I'm mounting the preamp. I folded that copper flashing over at the ends and soldered the backside of the board to them, it goes right up in the phototube housing - no emi in there. Works great, though it has too much gain for this particular setup - easy to change that of course. In this case, I'll probably just cut down the input impedance as this is running right off the phototube anode, DC coupled.
PreampMount.jpg
Mounting right up inside the sensor


But your readings don't sound super far off - these neutron tubes don't do much unless they get a real hit from a cosmic ray - or neutrons, which are pretty rare till you start making a lot of them.
You might wind up changing the gain too - the idea would be to have the preamp just clip on a real neutron hit, then you can drive ttl kinds of things directly with the analog output from this.
Standard counter likes this signal fine - that's a shmitt trigger input so it's got a little hysteresis built in.

I'll have more to say on this on the hornyak thread when I get there - but this is a good way to do things, and why I wanted this board so small. In your case, you might have to look out for corona noise at the voltages that tube likes to run at and put the backside of the board (the ground plane) towards the HV stuff so it won't pick that junk up. FWIW, it looks like my homebrew hornyak head is only about half as sensitive as the official one from Eljen (which is $150 last I checked). I'll have to look into that one further - or perhaps offer a choice of head (at a price...).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: The fine wine of preamp design

Postby Laurence Upjohn » Tue Jan 31, 2012 3:08 pm

Doug;
I have built up a through hole version of the WINE Pre-amp using a protoboard with its inherent problems. I even made the upgrade to the 6.8k resistor in the first emitter leg as you observed above. I have been running the proto off a 3.6v 1/2 AA lithium battery with the following observations. I am using a 1.5 X 1.5 Bicron NaI/PMT (Square) at about 650VDC + from a Bertan 2Kv PMT positive supply. The PMT is capacitatively coupled with about 300pf/6000V cap. I get positive "square" pulses out around 100nano seconds width with no apparent Tail and that nearly reach the positive supply rail. (Sorry I can't provide a photo as my TekScope (7804) doesn't photograph well digitally yet). It appears that the amp is peaking which will count fine but I am interested in recovering peak height information for input to PRA for gamma spectral analysis. I have not reduced the 15k NFB resistor for now. I just wanted to check in with you to assure I was on the right track here. Thanks for your thoughtful input. More later, Larry Upjohn.

P.S. The final stage transistor in the schematic is labeled a 2N3906 PNP but the symbol is for an NPN. When building the circuit I just put in a second 2N3904 NPN and discovered my oversight when debugging later. Also when purchasing the 2N5087 I ended up with a substitute generic part which turned out to be for either a 2N5087 or a 2N3906 so I have replaced the generic with a 2N3906 which I have many of in the parts store. This needs to clarified for the above question to be full disclosure. LRU>
Last edited by Laurence Upjohn on Tue Jan 31, 2012 4:11 pm, edited 1 time in total.
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Re: The fine wine of preamp design

Postby Doug Coulter » Tue Jan 31, 2012 4:07 pm

You're on the right track. In my early tests using this for our new Hornyak, it has too much gain for a good phototube, so reducing the 15k is what's needed - you can also reduce the load R for the tube to reduce impedance (speeds things up with whatever capacity is stray in that line), and signal level - I cut that 100k resistor to the input base down to 15k here for that one (my tube is direct coupled, negative supply type). But I'm surprised you are getting decent operation at 3.6v power - mine all have "turned off" by then, there's no quiescent drop across the 220 ohm output resistor at anything less than about 5.8v or so supply input. That means it's going to clip the bottoms off pulses too, and adds some DC recovery issues in fast counting. If you have to run it off 3.6v, you'll probably need more bias to pull that 220 ohm off the positive rail a little bit (nominal is about 200 mv - but any is good, too much just wastes current and swing).

There are two ways (at least) to get this happening. One is to reduce that 6.8k again to a lower value, that is, if there's any drop across it in the first place for that transistor to amplify into the collector R. The other is to use a smaller resistor to turn on the led and get a little more bias in the first place. Either will work, neither is super sensitive. If you want to make it sensitive enough to let you be a perfectionist, get the bias right without any NFB resistor at all, then add it back. Then it won't have to be done again for future gain changes. You want just a little drop across that output resistor (measured directly across it) or you'll have other issues down the road for pulse fidelity. Letting it go to saturation on that side opens the NFB loop and can cause longer recovery delays after a pulse.

If you want 5v output for a homebrew MCA - or some factory ones that don't have a preamp - you really want a higher supply voltage for this. Once "in range" it's not real picky about the exact voltage but obviously with a 3.6v supply and 200mv or so loss, plus one Vbe on the output emitter follower, you can only have about 2.8v output pulses.

I should add here that there's a pretty good reason I used the lowest noise for this application input transistor money can buy (even though it's cheap at Digikey). You might get by with a generic, but it won't be as quiet - and noise will affect line widths on MCA kinds of applications.
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