Low-Power,High-Speed(10MHz),High-Voltage(+-100V),amplifier for ion trap physics - Page 1
Forum section: electronics / projects

author: universal_101
author_rank: Newbie
timestamp: June 26, 2021, 09:44:12 am
content: Hi all, I am tasked with developing an amplifier circuit, to drive the ion trap electrodes(5-10 pF capacitive load) of our physics experiment, I don't have an electronics background but I understand a fair bit. The input voltage is single sided 1Vpp, the output should be capable of bipolar 200Vpp at 10 MHz. One direct solution would be to buy apex linear opamps, but they are prohibitively expensive(we need tens of them).I did research about the ways of making an amplifier, it seems that a folded cascode amplifier(bjt) followed by emitter follower to boost the voltage should be able to achieve all the parameters of operation, my question is, am I correct about it? And if someone can help me with some sort of reference design.Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2021, 09:58:08 am
content: The requirements look a little similar to the video amplifiers for old CRT based TVs / computer monitors. Some may be available for cheap as left overs. The analog scope defefection amplfiiers are also similar, though often slightly faster. For the older (e.g. 20 MHz range) scopes plans are sometimes available.Many old TVs used a simple class A cascode. So in the simple case just 2 NPNs, but quite some heat. The folded cascode is more like a thing for low voltage and with multiple stages. AFAIK the ion traps would run a single frequency only and thus would not need an expecially flat frequency response.

author: universal_101
author_rank: Newbie
timestamp: June 26, 2021, 10:10:58 am
content: Hi Kleinstein,You're correct the flat frequency response is not a requirement, you mentioned simple two NPN cascode will produce appreciable amount of heat, but my power requirement are very low less than a watt, so what would be the source of the heat, the collector and emitter resistors? And if there would be heat then I suspect there would be voltage drift also because of change in gain. So, should I go for the simple two NPN cascode or should I go for CRT deflection amplifier look alike?Thanks a lot for your input.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2021, 10:59:54 am
content: The power loss in class A amplifier is from the collector load (resistor or active current source). So while there is low output power the internal loss is quite high. I would expect some 2-5 W for the simple 2 transistor stage, mainly at the high voltage NPN and the collector resistor.A have faint memory of something like a 33 K high power collector resistor, a high voltage transistor like BF459 and a rather conventional NPN (e.g. BC547 like) for the input part. The old TV sets are a bit slower (only need some 5 MHz BW, but should be relatively flat response) and the voltage can also be a little lower (like 150 V supply and 100 V_pp for one side).The scope defelection amplifiers look similar, though they usually work differential, with differential in and differential out to drive 2 electrodes with an opposite sign signal. Except for the differential part and thus 2 power stages the circuit may not be that much different.With a limited frequency response (especially no need for DC) one could improve the efficiency a little by using a choke at the collector instead of just the resistor.

author: TurboTom
author_rank: Super Contributor
timestamp: June 26, 2021, 12:43:13 pm
content: Quote from: universal_101 on June 26, 2021, 10:10:58 amYou may want to take a look at myscopeclock schematic. Though the deflection output amplifiers aren't as fast as you specified, they are extremely low power. I'm sure with some modifications, you will get them to meet your requirements (IIRC, there should be some margin power-wise). You may try to replace the low-side driver transistor in common-emitter configuration of my circuitry with a cascode... For experimentation, I attached the LTSpice file of the deflection circuit.Good luck with the project!P.S. To use the attachment with LTSpice, remove the .txt extension required to be able to attach it here in the forum.

author: mawyatt
author_rank: Super Contributor
timestamp: June 26, 2021, 04:48:14 pm
content: If I read correctly, you are using a 1VPP input and desire a 200VPP (+-100VP) output @ 10MHz, that's a GBP of 2GHz. This will require a little power to get this level of performance, and since the supplies will need to be >+-100V for +-100VP output likely not a simple straightforward design.Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2021, 05:01:55 pm
content: Quote from: Kleinstein on June 26, 2021, 10:59:54 amMore particularly, the less bandwidth you need, the less power you must pay for. By giving up DC, you can replace resistors with inductors, but now you must decide what lower cutoff frequency you shall have -- if not zero (DC), then what?If the signal is very close to 10MHz (not changing much or at all), and also its amplitude or phase change at a similar rate (bandwidth), then the inductance can be reduced more and more, and you have a tuned amplifier. Then the only power required, is whatever's enough to achieve the bandwidth, or to achieve the desired voltage drop across the inductor/output.In the extreme, superconducting inductances are used, to achieve very high Q factors, typically for purposes of coupling pulsed AC power (from a klystron or other powerful, high frequency oscillator) to particle beams (LINACs). The Q factor is so high, the beam energy presents a significant load on the system, i.e. efficiency is high. (Inductance is a bit of a misnomer, as they're really resonant waveguides, but the superconductor is there for the same purpose.)BTW, note that you can always couple DC bias back into the load, and this might be done with large value resistors and small capacitors (resistance coupled) -- again, you're paying for bandwidth, so if the "DC" doesn't need to be adjusted quickly, you don't need much, whereas if it needs to change quickly, you need an RC time constant comparable to that rate. Basically you're chopping up the bandwidth, so there's a narrow band at 10MHz, and a narrow band near DC, rather than the whole DC-10MHz+ flat band.Power savings doesn't usually mean very much for lab equipment, and I would be inclined to do the cascode because it's easier to get right and doesn't require tuning or RF amplifiers. If you really do need to save power, then this is a good way to do it, consider the bandwidth requirements and go from there.Tim

author: penfold
author_rank: Frequent Contributor
timestamp: June 26, 2021, 08:35:33 pm
content: Quote from: universal_101 on June 26, 2021, 09:44:12 amWords I rarely hear in the same paragraph.How precisely does the voltage need to be maintained and do you need to preserve the phase of the original signal? The suggestion that you would have needed 10's of the apex amps made me a little curious as to what type of trap it could be and wondering if it's one trap with multiple excitations.Adding inductance to resonate with the trap's electrode capacitance is a great option, though because of the fact that the electrode voltage is more so defined by the capacitance of the electrodes (whichcanvary a bit, though may not in your setup, but definitely, if you're using coax to connect) than with a conventional RF source (maybe with matching network). It is worth thinking at an early stage whether or not you need to sense the electrode voltage for feedback in regulating the RF supply because it can be made hugely easier (and so much more effective) if you get a chance to think about how you're going to make sense connections and get the wire back out of the vacuum as early as possible.

author: jbb
author_rank: Super Contributor
timestamp: June 26, 2021, 08:50:11 pm
content: Don’t forget your cable capacitance!

author: David Hess
author_rank: Super Contributor
timestamp: June 26, 2021, 11:30:32 pm
content: Oscilloscope CRT vertical amplifiers are lower voltage because of higher bandwidth requirements which is why there was so much effort placed into increasing deflection sensitivity.A much better match is the z-axis amplifier which almost 100 volts at much higher than 10 MHz. They use a shunt feedback configuration with cascode output stage and constant current load, and in higher performance examples have a cascode on the constant current load and AC drive to it as well. The limit here will be finding fast high voltage PNP transistors for the positive side.There is also the problem of doing a level shift to get a bipolar output which suggests to me that a complementary common emitter output stage with output cascodes is the way to go. There is a high voltage operational amplifier configuration which can do this but I don't know if it would be fast enough. My own early designs topped out at 1 MHz at lower voltage but much higher current and I never tried to get that kind of voltage and speed so I am not entirely clear about what kind of performance is realistic.You say that you need 200 volts peak-to-peak at 10 MHz but is that the full power bandwidth or the small signal bandwidth? The former implies 6000 volts per microsecond which may not be impossible, but is a major design consideration.

author: Marco
author_rank: Super Contributor
timestamp: June 27, 2021, 02:35:31 am
content: The old video BJTs of yore were probably better than anything you can get new, BFQ252 has a 1 GHz ft ... something modern would probably have 100 MHz. Yay for progress.

author: magic
author_rank: Super Contributor
timestamp: June 27, 2021, 10:28:36 am
content: That puppy is 100V only, though.On DIYAudio KSC3503/KSA1381 seem popular for high voltage / high speed these days.But SOA isn't great and even with an efficient push-pull stage you could need one pair to drive the load (6V/ns into 10pF is 60mA peak) and another pair to drive parasitics of the transistors themselves (5pF per complementary pair). Forget class A unless you parallel even more.

author: Marco
author_rank: Super Contributor
timestamp: June 27, 2021, 04:52:49 pm
content: A low Coss MOSFET class A works in simulation at least with a bit of inductive peaking, the necessary source voltage swing in the simulation is only half a volt at 10 MHz. It's a room heater though, 10W per MOSFET and 16W in R1 ... probably needs to be tweaked a bit, but I doubt the simulation is worth a damn (increasing R1 to reduce power consumption craters bandwidth).Variation on a tube class A driver fromthis paper. The tube driver will probably work, but isn't cheap either.

author: magic
author_rank: Super Contributor
timestamp: June 27, 2021, 05:34:49 pm
content: Its Coss is only low at very high drain voltages. 70pF at 25V, 20pF at 50V

author: Marco
author_rank: Super Contributor
timestamp: June 27, 2021, 05:41:12 pm
content: Given the biasing in that particular bullshit simulation I don't think it's relevant

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 27, 2021, 06:35:05 pm
content: I think you misread that schematic a bit... (it is rather cluttered, and also a JPEG, ugh!). They seem to have a mu stage, with tetrode biasing.https://www.tubecad.com/2009/10/blog0173.htmFor a capacitive load, bandwidth can be slightly more than doubled, using T-coil peaking (2nd or 3rd order). AFAIK, this does not apply to resistive loads (where only series or shunt peaking can be done, at a modest ~30% improvement). The EOM paper might not have used this as often as it could. It does depend on whether the load is nearby and lumped-capacitive equivalent, or distant and terminated or travelling-wave.The minimum required for a 10pF load at 10MHz flat bandwidth, with T-coil peaking, is about 5MHz unpeaked or R = 1 / (2 pi F C) = 1.6k. The amplifier output capacitance loads this as well, so expect somewhat less in practice. Choose a transistor with as low capacitance as possible, while having enough SOA to handle the power. Yes, 2SC3503 is an excellent choice, and can do 200Vpp from a >200V supply, say 250V (it's rated for maximum 300V so this is acceptable). The 1.6k resistor is biased at half supply or 125V drop, or 78mA, and dissipates 10W, as does the transistor -- we will need at least two transistors in parallel then, as the 2SC3503 is only rated for 7W at Tc=25C, and that's if you can water cool it, which you can't. (This is a good application for AlN insulators, or Al2O3 extruded heatsinks -- low capacitance with excellent heat transfer.)Using multiple transistors also hints at an opportunity to do a distributed amplifier, but that's probably unnecessary; tuning one is not trivial. The advantage is, if you're willing to do the work -- you get bandwidth proportional to the number of transistors (up until device losses dominate), in this case potentially saving a little power dissipation. You're still limited by the lumped capacitance load; it just saves the amplifier's capacitance, reducing required power a little bit.For the tuned amplifier case, we need to know required bandwidth and maximum losses first. Take a modest 10% BW case for example: this can have loss as low as 8kohms (same calculation as above, BW = 1MHz), and the 10pF load (plus transistor and other capacitances) resonates with 25uH (or less). That is, a Q factor of 10. If the load resistance is much higher, it may actually be better to introduce loss, to keep the Q factor modest; alternately, neutralization (shunt feedback) can be used to reduce the amplifier's output resistance, to the same end (be careful of stability, what with its reactive load of course!). Since the resistance is now higher, the required bias current is much lower, and a single 2SC3503, MJE340, maybe even MPSA42 (and many modern equivalents, and MOSFETs) will do. Also, with the inductor supplying DC directly, we don't need a 250V supply; 125V is adequate, maybe call it 150 for dynamic range.If the amplifier and trap need to be distant, cable capacitance will very quickly dominate, until at 1/4 wave distance it's practically impossible (in fact, it's upside-down!). In this case, a better approach is to match both ends to 50 ohms, keeping SWR on the cable low. The amplifier's inductor can be tapped for a direct transformation ratio, or any impedance divider or matching network can be used. If the amplifier has a 16kohm output resistance, then a 320:1 impedance ratio or 17.9:1 voltage ratio is required (which is higher than the Q factor, which excludes some networks, like a straight capacitor divider, or L-match). The same network is used, in reverse, at the load, optionally with a load resistor to terminate it (but this isn't required, we can still have the damping provided by the amplifier).In fact, if we're matching the amplifier, we no longer have the supply voltage requirement, and can use something much more conventional like a 12V supply, and a cheap 30-60V transistor (still rated for a watt or two, so not a 2N4401, something just a bit bigger). In that case, its tuned network has a different ratio, or maybe even none at all: 1W into 50 ohms is only 10V peak, and we can use a very conventional driver circuit for that, including just a plain old buffer IC.Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2021, 01:57:09 pm
content: Quote from: magic on June 27, 2021, 10:28:36 amThose are on my list as the best modern parts for this sort of application. Besides sufficient voltage rating, they are especially suitable with low capacitance; they are really designed for this application. The KSA1142/KSC3902 are a lower voltage alternative but would require a stacked output stage for for greatly increased complexity.In a cascode design, the transistors driving the KSC3503/KSA1381 can be low voltage and higher speed without the limitations of capacitance.QuoteAll of the circuit configurations I would choose easily support parallel transistors though emitter degeneration to increase current and power capability.

author: Marco
author_rank: Super Contributor
timestamp: June 28, 2021, 02:47:00 pm
content: Quote from: David Hess on June 28, 2021, 01:57:09 pmThey are really right on the edge if spice is to be believed. ~10x current amplification left at 10 MHz and 200V collector swing, good enough but just about to run out of steam.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2021, 05:32:39 pm
content: So you're saying they're still good at 50MHz?Cascode doesn't need much current gain, and you're still getting power gain as long as the voltage gain more than covers for it!Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2021, 06:07:59 pm
content: Quote from: Marco on June 28, 2021, 02:47:00 pmIn what configuration? I would not even bother with common emitter or common collector in this application. That is why I specifically said:In a cascode design, the transistors driving the KSC3503/KSA1381 can be low voltage and higher speed without the limitations of capacitance.I described z-axis amplifiers which operate in exactly this configuration with similar transistors and achieve at least 33 MHz full power bandwidth.Actually what I described is slightly different from real designs. Below is the Tektronix 7834 z-axis amplifier which if you squint a little uses differential pairs in place of cascode pairs, but the idea is the same and other contemporary designs used a cascode on the bottom side and a simple current source on the top side.This may be a good place for the right power MOSFETs as the cascode transistors, but it is not something I have tried; I just have not had any need for such a fast high voltage amplifier. Some very late CRT amplifiers did use MOSFETs this way, however they were lateral parts with better high frequency characteristics than commonly available vertical parts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2021, 06:18:47 pm
content: Hah, complementary diff pair, of sorts. It's weird because, whereas a long-tailed pair is current steering, the complementary one is on-off like a common emitter stage. And, like a CE, it has a Vbe offset, more specifically two Vbe between inputs, plus bias. Looks like they're using a pair of them in push-pull, and mainly using the high side for bias, with a little AC action to sharpen it up a bit?Tim

author: Marco
author_rank: Super Contributor
timestamp: June 28, 2021, 07:30:15 pm
content: Quote from: David Hess on June 28, 2021, 06:07:59 pmDoesn't really matter for what I stated, but I was assuming it would be used in a class AB output stage, to meet the Low-Power requirement from the OP. A bootstrapped cascode for the NPN/PNP pair could still lighten the load on a previous class A stage, but when the current amplification runs out it's all over.I don't actually ever see bootstrapped cascodes on AB output stages though ... why is that?

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2021, 12:33:47 am
content: Quote from: T3sl4co1l on June 28, 2021, 06:18:47 pmBut the double Vbe offset is removed by the shunt feedback to the base of error amplifier Q2236, and the Vbe offset of Q2236 is irrelevant in this particular application because it forms a transimpedance amplifier with current input. Some earlier Tektronix designs (7704 and 7704A which despite naming are unrelated designs) replace Q2236 with a differential pair removing the offset, but I do not know why. (1) Maybe they dropped the differential pair for increased speed.It is not required in a z-axis amplifier application because the output is unipolar, but in a bipolar design I would make the feedback to the emitters of a complementary pair, like an OPA860/OPA861, but I would not use this part, and this is exactly what complementary current feedback amplifiers do. It is also what the voltage feedback amplifier implementation of this circuit (shown below) does, however I do not know if it would be fast enough.In the circuit shown below, the output stage of the operational amplifier can be replaced with a diamond buffer which then drives the output transistors with local feedback directed to its output as shown. There is a Linear Technology application note which shows the same idea with an LT1010 in place of the diamond buffer.The discrete version of this topology that I experimented with long ago achieved 600 kHz full power bandwidth out of 2 MHz output transistors, and that was without the output cascodes, so I still think it is a good candidate for a high voltage, high speed, high impedance applications.(1) How could I know that? For reasons I have forgotten, many years ago I conducted a survey, albeit incomplete, of Tektronix z-axis amplifier designs and it was in my notes.QuoteThat is right; DC control is through the lower pair, and AC coupling to the upper pair doubles the transconductance of the output stage doubling slew rate. This is also common in Tektronix horizontal CRT amplifier designs but they are much slower.

author: Marco
author_rank: Super Contributor
timestamp: June 29, 2021, 02:38:56 am
content: How about something like this, with an opamp driving a class A in closed loop to get the 110V+200Vpp sine to drive the AB stage?

author: magic
author_rank: Super Contributor
timestamp: June 29, 2021, 05:44:14 am
content: Where do they sell opamps with 2GHz GBW, ±100V supply and 6000V/µs slew rate?The discrete topology of choice would be cascoded common emitter stage, because there is nothing that can drive your cascoded emitter follower without already almost being able to drive the OP's load

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2021, 07:53:40 am
content: Heh, speaking of emitter followers:There is a slight advantage to putting an emitter follower after the cascode, as you can use transistors of a couple pF total, gaining some bandwidth into a say 10pF load. This was typical strategy in the CRT days; I still have some BFQ221/241 (100V 100mA 1GHz, complementary pair, emitter follower) and BFQ225 (basically identical to BFQ221, in TO-202 so good for a couple of watts, cascode output) salvaged from I think a handful of iMacs I found dumpster diving, years ago. Somewhat later designs integrated everything on a hybrid module, or monolithic (LM2471 and FA4111 come to mind). Bandwidth over 100MHz was common -- that being required to drive a 1600x1200 display at 85Hz refresh.These don't have quite the voltage ratings required though, and I don't think they ever made transistors with higher voltage ratings with anywhere near as much speed (let alone that are still in production).Heh, though if you don't mind a matching network (again, reducing bandwidth because it's also a filter), one of those chips would certainly do a fine job here, wired in whatever way (a paralleling network could be used to increase power output, if a single channel doesn't happen to be enough).Tim

author: Marco
author_rank: Super Contributor
timestamp: June 29, 2021, 09:35:07 am
content: Quote from: magic on June 29, 2021, 05:44:14 amThat's not really relevant to a composite class A amplifier, a cascode MOSFET will take care of most of the voltage gain.QuoteAlmost from a physicist point of view perhaps, it can still provide a little over an order of magnitude reduction in the necessary current. So a class A stage goes from a room heater to a heatsink warmer.Since a wideband amplifier in this range is really running into the limits of what you can accomplish with heatsinking and non parallel transistors, a 10+ times reduction is more than worth it. The transistors in the AB stage will dissipate a couple Watt each and in the class A stage not even that. The only alternative to an AB output stage to keep power moderate with a wideband amplifier is a current driven output stage (ie. PNP on the upper rail, NPN on the lower) which would be a pain to get stable.

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2021, 04:37:42 pm
content: Quote from: Marco on June 29, 2021, 02:38:56 amHow are you getting a bipolar output to drive the class AB output stage without level shifting?I do not see anything in the original requirements which allow an AC coupled output.

author: magic
author_rank: Super Contributor
timestamp: June 29, 2021, 09:49:41 pm
content: Perhaps the classic CFA architecture could be adapted.The trick is to have a well controlled current gain and close to 180° phase in the current mirrors over the whole bandwidth of interest. Then compensation is a matter of loading the output with enough capacitance. There is no unity gain buffer stage.The topology occurred to me some time ago as a solution to the problem of driving capacitive loads, but it's not very exciting for any sane application because it sucks in a million other ways (low gain, crap input stage, gm doubling).

author: Marco
author_rank: Super Contributor
timestamp: June 30, 2021, 12:37:14 am
content: Can it run with less bias current?

author: magic
author_rank: Super Contributor
timestamp: June 30, 2021, 06:10:03 am
content: Maybe. But R3~R6 simultaneously set current gain and quiescent condition, so it takes some compromise and/or tweaking to get what you want. I ended up with 20mA on Q3,Q4 and 10mA or less would suffice (for phase and bandwidth), Q9,Q10 run on 24mA which is about the average current consumption at full output so who cares, but chances are the circuit would be OK with less if you can tweak the resistors to get there. Maybe insert a small voltage source (schottky?) in series with R5,R6 but mind thermal drift. Or say "screw DC" and parallel a smaller AC-coupled resistor with R5,R6, then you gain a lot of freedom.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 30, 2021, 06:33:25 am
content: Quote from: magic on June 29, 2021, 05:44:14 amwell, while you were asking: they are not cutting it for the particular use case discussed here, but quite impressive anyway.https://www.apexanalog.com/products/linear_selector.html

author: magic
author_rank: Super Contributor
timestamp: June 30, 2021, 07:05:35 am
content: Yes, they have been mentioned before. I frankly expected worse, but the fastest one is 3000V/µs with ±90V swing and 9MHz bandwidth at gain of 20x, so not really far awayStill suffers from the usual problem with off the shelf opamps - way overcompensated, if you want some serious gain.MOSFET fanboys will be happy, it has source follower outputs.Quote