Analog challenge:High power/high speed amplifier for physics experiment - Page 1
Forum section: electronics / projects

author: Mazo
author_rank: Regular Contributor
timestamp: February 21, 2020, 04:57:36 pm
content: Hello everybody,I am tasked with designing an amplifier for physics experiment and I would be very glad to hear all opinons and receive helpSummarised the device should fit in the following.Powered by30-100V(+- so 60-200V)Output current up to4-5AOutput voltage swing->Not of high importance,anything reasonble will sufficeGain-everythingabove 20dB(voltage gain)is acceptable.Now the hard part-it should do all that @0Hz-1MHzoutput frequency(pure sine).Some gain droop is acceptable.Harmonic distortion should be low(say below a few %)Operating into an coil placed on a wildely varying magnetic materials so the load impedance is inductive...ish.(Load isolation resistor?)The expected load indutance range is~20-200uHAll requirements aren't set in stone so feel free to deviate a little.What have I thought about for the moment:GaN based class D amplifier switching at ~2.5-3MHz and a very good output filter-Hard to implement(I have zero experience with GaN) and blows the budget as the device should be relatively low cost-ish(think a few hundred EUR(USD))Classical voltage feedback discrete amplifier as in a audio amplifier-out of the question as required GBWP is in the order of 10-100MHzOpen loop amplifier(Harold Black doesn't like this) -Hard to get linearity out of?Current feedback topology of some kind using either discrete only devices or a "boosted" op-amp:What I think is the best version for now:The output is the big "+" at the middle the op-amp is the LT1818 and the trannies are non standard LTspice models,otherwise I would have provided the sim.(It is slow as hell anyway)Any values you see are gut sense based for nowAny ideas and suggestions are welcome.P.S All parts even obsolete are ok(if one can still find them) since it is a one-off.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 21, 2020, 05:13:37 pm
content: What output voltage swing are you expecting? <nevermind...I see you want rail-rail>>BTW, 4-way connections are a bad drafting practice.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2020, 05:18:49 pm
content: The requirements don't look that unusual. So rather similar to a slightly faster Hifi amplifier.The only unusual part would be the variable power supply and the idea to get close to the rails. This would normally be things not important for a one off. Who cares about a few 10 W of extra heat in such a case.4-5 A is not such a high current.My suggestion would be a more or less normal class AB amplifier, just with slightly fast transistors. No need to go exotic, just the regular better audio ones.An inductive load is not a problem for voltage driving. The tricky part would be of the coil/ load shows self resonance.An inductive load may only need a little bit more care with the SOA - here it helps the current is not that high.

author: pwlps
author_rank: Frequent Contributor
timestamp: February 21, 2020, 06:22:57 pm
content: Your specs look similar to those of MRI gradient amplifiers. Googling for "MRI gradient amplifier" I see many papers, you might find some inspiration there.

author: duak
author_rank: Super Contributor
timestamp: February 21, 2020, 06:37:33 pm
content: I expect the output stage will have thermal runaway issues. The idle current in the output stage is a function of the quiescent current of U11 and the temperatures & VBEs of Q13, Q16, Q6 to Q10 and Q1 to Q5. Forward biased diodes in series with R15 & R16 as well as R13 & R14 would help but may require rethinking their values. I might consider temperature compensated current sinks/sources to replace these resistors.R1 to R10 may also be a bit low to help current sharing among the devices in the output stage.Q13 & Q16 have to stand off almost the full supply rail to common voltage and thus dissipate extra power for no reason. I would reduce their VCEs to a much lower value. Perhaps use the cascode CB base bias voltage?In the end, the slew rate of the output stage is established by how much current can be quickly driven into and taken out of the bases of the output devices. The driver stage may need voltages outside of the rails or to accept a lower output voltage swing to speed up the turning off of the output devices. It may be possible to use peaking capacitors, inductors or even T-coils to help.

author: dietert1
author_rank: Super Contributor
timestamp: February 21, 2020, 06:54:51 pm
content: That 2p2 capacitor looks strange considering the sheer size of the device. Can't you make the capacitive divider into 2n2 / 22pF or 2n2 / 100 pF (then with 23x gain)?We have an 8 channel audio amplifier "Sirus Pro MXC8" with a very similar schematic and those amplifiers perform very well.Regards, Dieter

author: fcb
author_rank: Super Contributor
timestamp: February 21, 2020, 06:56:00 pm
content: Pockels cell amplifiers have some of the same characteristics you are after.Also. Is 100V enough? At 1MHz, 20uH has an XL of 125ohms - so you'd need 500V to drive 4A through your minimum inductance. Probably look at a cascode output stage to give you some headroom for the future - also less slew across the output devices.

author: Marco
author_rank: Super Contributor
timestamp: February 21, 2020, 08:31:03 pm
content: How about a class A amplifier with a high side buck current source? Same approximate efficiency as a push-pull amplifier, but simpler in some ways.PS. actually that's not right, nevermind.

author: Mazo
author_rank: Regular Contributor
timestamp: February 21, 2020, 09:35:30 pm
content: Thank you for your answers I will try to reply and give some additional info:Quote from: fcb on February 21, 2020, 06:56:00 pmNumbers given are maximums,so yes the 4-5A is just the maximum current and it won't be able to drive it @1MHz into 20uH.(the sought after output variable is voltage).No probelms with that.The output is a cascode,miller kills any hopes otherwise.About the Pockels cells,it is the first time I hear about that effect and I will research the topic,thank you.Quote from: dietert1 on February 21, 2020, 06:54:51 pmYes you are right,the capacitive divider will get bumped so the lower capacitor dominates the op-amp input capacitance(probably 2n2/22p will suffice will have a look in the datasheet)Quote from: duak on February 21, 2020, 06:37:33 pmThe idea was the output stage to be biased in a hard class B operation,so no quiescent current as I thought that by having a fast slew rate will make the crossover distortion not that of an issue.Am I right?Thank you about the cascode CB base bias voltage idea,I was wondering where to return those collectors anywayI plan on using slightly oversized devices so that deviations in current sharing in the low tens of % will be in spec.(pictured "inner"trannies are at the edge of their spec,so no go).Will think about including some peaking circuitry to speed up things.Also I am at a bit of a loss how to evaluate the stability of the circuit considering it's 2 effective feedback loops?Do I just have a look at the global one and if it's stable call it a day?Quote from: pwlps on February 21, 2020, 06:22:57 pmThank you,will have a look in those.Quote from: Kleinstein on February 21, 2020, 05:18:49 pmSorry I have been misunderstood the output voltage compliance is of the least concern,so a few volts more or less doesn't matter.I have started with a standard approach with emitter follower output but the 5*2SC5200 and 5*2SA1943 seem like a pain the ass to drive at that speed and swing(probably should just brute force the problem by throwing current and power at it)Are there better(read faster) trannies that can withstand 100V while passing 0.8-1A current(pure inductive load case)My research found nothing faster than the 2SC5200 and the 2SA1943?My concern is that the output trannies parasitic capacitance and the load inductance resonant frequency can make for a nice high powered oscillator,so probably will just include a isolation(or Q-killing if you look at it that way) resistor.Quote from: Wimberleytech on February 21, 2020, 05:13:37 pmMy bad on the 4-way connection should have tidied up the schematic before uploading.Seems the wording about the output voltage swing on my original post was bad so I will edit it.I will research and revise the circuit and post again,still any ideas and comments are welcome.P.S If somebody is curious about the physical experiment itself,PM me I will gladly explain.

author: duak
author_rank: Super Contributor
timestamp: February 21, 2020, 11:59:15 pm
content: Mazo, have you run a simulation of this circuit? What did it tell you about the turn on and turn off times of the output devices? I'd expect the turn off times of the output devices to be a very large part of a 1 MHz cycle. If the CE device (Q1 to Q10) of the output stage saturates, the turn off time will become even longer. If this happems, you'll have to add a Baker Clamp to prevent it.Stability will be a big issue with this circuit. I would start with trying to stabilize the output loop through R12 first with the overall loop perhaps set to a higher gain.I almost forgot, since the load is inductive, you will need some clamp diodes from the output to the supply rails to handle recirculation currents.

author: Mazo
author_rank: Regular Contributor
timestamp: February 22, 2020, 09:58:25 am
content: Quote from: duak on February 21, 2020, 11:59:15 pmAm I missing something or if the CE devices get near saturation can't I just move the CB base bias supply "further" away and thus add some voltage to the C-E drop and prevent saturation?The simulation is almost hopeless,time domain simulations are in the ps/s range,and if given enough time they provide some weird ass results like MV voltages and MA currents,the bottleneck seems to be the 2SC5200 and 2SA1943 models.I have thought about the recirculation diodes,just tried to strip the sim out of everything that isn't required for the operation in an effort to fix the sim(didn't work).Should have a look into the capacitances of such diodes that the DUT will have to swing around,or just put diodes that are say 1A continuosly rated and rely on the fact that the output will have not that much energy stored in it and the diodes will survive the transient.Working on a new schematic,will post soon.

author: fcb
author_rank: Super Contributor
timestamp: February 22, 2020, 10:28:04 am
content: Quote from: duak on February 21, 2020, 11:59:15 pmOoooooh. Baker clamps. Dave should do a video on transistor saturation.

author: magic
author_rank: Super Contributor
timestamp: February 22, 2020, 07:30:57 pm
content: The common emitter combined VAS/output stage topology you chose has high open loop output impedance and relies on feedback to bring it down to reasonable territory. If you want low impedance "voltage" output, you will need plenty of GBW. Consider removing those paralleled transistors and adding a unity gain buffer stage instead.If a few % THD is enough, you may get away with leaving the buffer outside any global feedback loop. Try to simulate such an output stage alone, experiment with Darlington and Sziklai pairs. To measure distortion, disable "compression" in LTspice control panel and take FFT of the sim output waveform, the option is there.

author: duak
author_rank: Super Contributor
timestamp: February 22, 2020, 07:46:27 pm
content: Mazo, I'm not surprised the simulation gave you trouble. I would start with just half of the driver and output stages (Q1 to Q5, U1 to U5 & Q16) with a simple resistive load to understand how they work and how fast they are. I'm not a SPICE user so I can't offer much help there.Regarding saturation, I expect the simulation to act differently from the real world because the bias voltage of the CB stage is not current limited. This will just make the CB transistor saturate instead of the CE transistor. One way to look at saturation is that when the circuit limits collector current to a value less than what the base current and hFE demand, the transistor's VCE becomes lower than its VBE. This increases the turn off time because the transistor is flooded with current carriers and it takes time for them to be swept out. Why I wouldn't be surprised if saturation occurs is because the op-amp will try to drive as much current as it can into the output stage until the output voltage comes to the correct value. Moving the CB bias voltage away from the supply voltage won't make a big difference although it may shorten the turn off time of the CB transistors by sweeping the current carriers out quicker.Magic's suggestions also make sense and are certainly worth considering.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2020, 01:50:43 am
content: Yeah, nothing crazy, a friend has built amplifiers of conventional architecture (diff amp, VAS, emitter follower) with several MHz bandwidth. A beefy VAS, preferably complementary cascode (compare to conventional, where the VAS load is a passive CCS), and outputs with lots of drive (Darlington with fairly stiff bias?), is the way to go. The VAS in turn might be driven by current mirrors from a single diff pair, or a complementary diff pair with no mirrors needed.If you're just driving coils, mind you can use less voltage with smaller coils and fewer turns. The 200V range seems... excessive?Also, note that you need at least as much power dissipation as VAR you want. Probably more like double. A linear amp does not return reactive power to the rails, it's 100% dissipated. And it's dissipated in the worst part of operation (e.g. when the output voltage swings low, the current is pulling down, so the high side transistor has to drop Vcc + Vout(pk) while delivering Iout), so shop for transistors with the widest SOA.So, if you're talking say 200 VAR, expect to need maybe >400W capacity.The cascode output stage is alright in some respects, but it's a nightmare to stably bias, and as mentioned, optimizes voltage slew rate which may not be the most desirable here.Quote from: duak on February 22, 2020, 07:46:27 pmTimestep too small (or some platforms will just keep chugging along on garbage (ps/s) rather than terminating) is often a sign of unrealistic circuits, or poor models. Note that .MODEL statements are just that, transistor models, no package parasitics or anything -- you need to add those manually if you want a realistic circuit. And those will be critical in a circuit pushing the performance of the output transistors at least.Tim

author: dmills
author_rank: Super Contributor
timestamp: February 23, 2020, 01:10:23 pm
content: That spec look a LOT like the design Win Hill shows in the X chapters (and that is discussed extensively over on DIY audio) :https://www.diyaudio.com/forums/solid-state/287023-winfields-100w-dc-10mhz-1000v-amplifier-12.html100V (Not 1kV), 5A, 5MHz... Impressive.

author: langwadt
author_rank: Super Contributor
timestamp: February 23, 2020, 01:51:35 pm
content: Quote from: dmills on February 23, 2020, 01:10:23 pmhe posted a schematic in sc.electronics.design,https://www.dropbox.com/s/elhs1whl2dfn0i7/AMP-70A-2_sch.pdf

author: David Hess
author_rank: Super Contributor
timestamp: February 26, 2020, 11:23:18 pm
content: Quote from: Mazo on February 21, 2020, 04:57:36 pmSo it needs roughly audio amplifier impedance and power.QuoteMany audio power amplifier designs have almost the bandwidth required but not the gain. The current feedback design I experimented with before I knew what I was doing achieved 500 kHz of bandwidth with 2 MHz output transistors. It used current feedback but with a common-base pair at the input instead of a diamond buffer so had no common mode rejection in the driver and output stage.QuoteNothing precludes using a current feedback design. Also, a voltage feedback audio amplifier would be decompensated to operate at a minimum gain anyway yielding a higher gain-bandwidth and slew rate. Or a unity gain stable feedback stage might control a fixed gain amplifier to multiply the gain-bandwidth and slew rate.QuoteOscilloscope CRT amplifiers operate like that. The vertical CRT amplifier is usually just a transconductance amplifier with series feedback for high bandwidth and tame response but the horizontal CRT amplifier also includes shunt feedback to improve linearity and typically only has a bandwidth or 1 or 2 MHz.QuoteI would probably try to scale up the unconventional voltage feedback operational amplifier boosted by current feedback into its output which is what your example shows. But I am not sure it would be up to handling the crossover distortion; I think something would need to be done to control the output stage biasing.

author: Marco
author_rank: Super Contributor
timestamp: February 26, 2020, 11:31:04 pm
content: I wonder if a fast optically isolated floating driver to drive the output push-pull transistors wouldn't be easier to get high bandwidth with than having to ping-pong from the rails for voltage amplification.

author: Mazo
author_rank: Regular Contributor
timestamp: February 27, 2020, 09:13:05 pm
content: Been busy lately but thought about the design alot,Reading all of them I thought I should go for a different approach,so I came up with this:Basic folded cascode which gives me a push pull gain node rather than a CCS loaded one and one gain stage should make for a easy compensation of the amplifier.The thing which buggers me with the design:the KSC/KSA trannies are operated very near their limits both powerwise and barely fitting in the SOA->cooling should be aboslutely superb.The high quiescent currents are all in order to obtain a 500-1000V/us Slew rate (I have estimated 30-50pF input capacitance of the output stage).The input bias current is also rather large which makes for a hundreds of mV output DC offset(I guess I will have to swallow that one?)Emitter follower output makes for easier biasing(Vbe multiplier should sit in the place of Vbias->is ommited for clarity)The R38-R39-C3 trio should be adjusted IRL and should bypass the output stage(presumed slower than the folded cascode part) at high frequencies to prevent excessive phase shift.I have calculated and adjusted alot and this design seems to stand a chance,any obvious shortcomings that you see?Thank you for your contributions.P.S Alot of needed stuff is omitted for clarity so the schematic doesn't get cluttered,should I include in the real circuit base resistors for all the emitter followers(especially I am afraid the pretty fast 2SC3519A/2SA1386A+the large input capacitance of a 2SC5200/2SA1943 is a recipe for VHF disaster)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2020, 09:30:00 pm
content: Folded cascode means there's no current gain between the diff and VAS, which gives shite for loop gain (perhaps a bit of a plus, to be fair), and means the diff needs to run stupid hot, so, needing so much base current as you note. You could tack on emitter followers to buffer the inputs (so it's, not quite a darlington diff), or put emitter followers in the diff collector circuits to boost them up (which would make a bit of a, I guess, cascaded normal-diff-pair into complementary-diff-pair for instance, depending on how exactly you choose to implement it), or use regular cascode VAS (thus driving the bases from the collectors, giving full current gain as usual).Could maybe do something with JFETs or MOSFETs, but that's kind of ehh with respect to what kinds of parts are actually available -- JFETs are all 40V or less, and MOSFETs are almost all switching types with way more Ciss than Pd; that is to say, not good for wideband and small-signal amplification purposes. (There are power RF MOSFETs, in high voltages, but only N-ch of course.)Could also maybe do something with low voltages, getting high gain and bandwidth with minimal phase shift, then level shifting (via cascodes most likely) to a (again cascoded?) VAS and output stage.Incidentally, Apex makes amps very much like this, which as incredibly expensive as they are, are still looking like a very good investment compared to the effort to design and build this thing! If nothing else, it might be worth a look just to see how they do things -- they usually give equivalent circuits.Tim

author: Mazo
author_rank: Regular Contributor
timestamp: February 27, 2020, 09:54:23 pm
content: Quote from: T3sl4co1l on February 27, 2020, 09:30:00 pmThe kinda lowish loop gain that will result from a cascode is to me as you said a bit of a plus->I don't need that stellar of a performance distortion-wise.Besides the high quiescent current makes for a high gm and the solid degeneration of the sinks+the fairly high input impedance of the output stage should make for a reasonable loop gain.I thought about making the inputs from sziklai pairs,improving the input bias current,enhancing the transconductance of the diff pair,and improving the offset voltage that will be present from real world unmatched KSCs.I considered using a matched 2N5551 pair(I forgot the manufacturer,diodes inc probably??) as I don't want to add another cascode and that is the only high voltage matched pair,buut I am afraid such an arrangement will make up for a high frequency(few MHz to few tens of MHz) peaking of the response that I should then tame.Thought about jfets and mosfets also but seems they are out of the question.I know about Apex and searched their portfolio,if I am not mistaken they seem to not have a amp that is fast enough and powerful enough at the same time.

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2020, 01:03:37 am
content: Quote from: Marco on February 26, 2020, 11:31:04 pmI have tried that and direct level shifters are much faster if only because they can take advantage of capacitive bypassing to drive a high voltage node with low AC impedance. Consider the DC restorer circuit which can drive a signal thousands of volts away with transition times of nanoseconds.

author: thm_w
author_rank: Super Contributor
timestamp: February 28, 2020, 01:50:20 am
content: Looks like the fastest apex on digikey is 65V/us, maybe 100kHz?https://www.digikey.ca/en/products/detail/apex-microtechnology/MP118FD/4916546Their site has some better ones,https://www.apexanalog.com/products/mp108.html150V/us 200V supply 10Ahttps://www.apexanalog.com/products/pa107.html3000V/us 180V supply 1.5Abut nothing matching the spec.

author: boblevy4321@sbcglobal.net
author_rank: Newbie
timestamp: July 11, 2020, 10:53:28 pm
content: helloI am trying to do something similar.I want a power amplifier for my function generator.I have a design from LINEAR app note 87 that uses a lt1210 in a floating 100 volt arrangement .I am retired and not that up on spice modeling.I calculate the gain at -10.44My question is what to drive this input with and what is the input current or voltage at this point.I have an idea of using a lm7171 to drive this. it has low output impedance and 100mA of current.I'm not use to dealing with current opamps.All help is welcome and appreciated.Thank you

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 12, 2020, 05:04:44 am
content: I would try lm7171 (with +- 15VDC power or higher). It seems to me there is no need in a large input current or voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 12, 2020, 08:24:31 am
content: The bootstrapped amplifier circuit from LT looks odd. The 300 Ohms resistors in the feedback are rather low.Bootstrapping circuit are known to not like larger loads and may tend to oscillate. Usually it takes some measures to slow down the variations in the supply (the FET internal capacitance may do the job).

author: David Hess
author_rank: Super Contributor
timestamp: July 12, 2020, 09:43:58 am
content: Quote from: Kleinstein on July 12, 2020, 08:24:31 amThe LT1210 is a 1 amp output current feedback amplifier so 300 ohm feedback resistors are a reasonable value.

author: Marco
author_rank: Super Contributor
timestamp: July 12, 2020, 04:53:30 pm
content: Quote from: boblevy4321@sbcglobal.net on July 11, 2020, 10:53:28 pmThat's so janky, I love it. Takes a twisted mind to come up with that.

author: duak
author_rank: Super Contributor
timestamp: July 12, 2020, 06:02:00 pm
content: I thought the 300R feedback resistor was a typo, but the op-amp isn't configured the way I thought it was. I chased down the original article where the op-amp is considered to be "suspended". The author goes a bit deeper in to the circuit operation.