Development of 500Vpp Linear Amplifier - Page 1
Forum section: electronics / beginners

author: GN89
author_rank: Contributor
timestamp: May 31, 2024, 11:50:34 am
content: For some Plasma measurement I'm interesed in the development of an higher voltage linear amplifier. The basic requirement are the following:500Vpp output voltage100mA output current1kHz maximum output frequency (maybe lower)Control voltage -10 to 10VThe design problem I face is how to stack mosfets and control the gates because there are few to no mosfets which have such a SOA so I need to stack them to evenly distribute the voltage.There are some designs available on the internet but there are no real measurements and sometimes there is something obviously wrong with them. Is there any Literature for DC Amplifier design for such high voltages?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 31, 2024, 01:04:08 pm
content: Apex probably make something that will do thishttps://www.apexanalog.com/support/linear_search.html

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 31, 2024, 01:08:15 pm
content: Here is one example, with measurements. And I've seen more in Internet. You may use similar keywords from an article.High-voltage amplifier uses simplified circuitFor 500 Vpp you don't need so much Mosfets. I guess you need a stack of two in a row (so four for all amplifier).It's hard to achieve 10 kHz frequency range or more with that exact topology, but it works quite fine up to several kilohertz (up to 2-4 kHz).

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 31, 2024, 01:09:43 pm
content: I am using GeneSiC G3R450MT17D in linear amplifier at 14 MHzSee datasheet Fig 14 SOA. Plot your Vd vs Id swing range as 2 points on there.TI App Note sluaao2.pd chapter 5 gives an approximate method of derating the SOA to your steady state heatsink temperature

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 31, 2024, 01:15:47 pm
content: Here is exactly 500 Vpp amplifier (designed by Leonid Ridico,Leoniv). I used this his design and it works pretty well. I used another opamp - OP37.

author: M0HZH
author_rank: Regular Contributor
timestamp: May 31, 2024, 02:06:01 pm
content: TSP just published a video on high voltage amplifiers a few days ago, watched it last night, it's gold.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 31, 2024, 03:28:39 pm
content: Bootstrap. In thse circuits the opamp rides the HV rails. Scary but it does work.www.edn.com/bootstrapping-your-op-amp-yields-wide-voltage-swings/http://www.edn.com/contents/images/45890.pdfYou do need transistors thay can hande the full output swing though. P-ch 600V MOSFETs are avaialble.Yes, that signal path video from the other day was great.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 31, 2024, 03:44:58 pm
content: Prices from £120 and up!

author: GN89
author_rank: Contributor
timestamp: May 31, 2024, 08:33:30 pm
content: Thanks for so much hints so far. I looked into the APEX Amplifiers. They don't have the SOA for my application which is why I need to stack some mosfets. Even using the suggested G3R450MT17D needs more than one in series.@mag_therm which design do you use for your 14Mhz amplifier and which voltage swing do you achieve?I found the link to EDN before but that design isn't working at all because the schematic is wrong at some points though this design looks interesting because it seems simple enough.The design by Leonid Ridico looks simple enough to give it a try in a simulator beause there are no floating operational amplifiers which do not work with most models. From what I understand in this design that the gate control is isolated from the operational amplfifier. The operational amplifier build a feedback controller and the LEDs are controlled by the supply rails. Positive output voltage means the upper coupler is on and negative the lower one is on. I think R13 controls the current through the LEDs.The LED intensity directly controls the gate. Higher intensity means the gate voltage becomes higher because voltage over R20 is controlled. This means a positive output voltage of the operational amplifier results in positive output voltage of the amplifier stage (Same applies for the lower rail).I do not understand what purpose R14, R17, R16 and C2 serves. I think that is some negative feedback to compensate the control loop. The feedback loop looks a bit strange to me.VT1 and VT2 are current limiters.What purpose do R7 and R10 serve?Another think is what about lonk term drifts in the opto couplers or drift in temperature or the mosfets? Is there any chance this can drift into a short circuit when there is no current limit in the path?What happens when the 15V supply fails? In my understanding both mosfets are near turned off.

author: Benta
author_rank: Super Contributor
timestamp: May 31, 2024, 09:34:03 pm
content: @GN89, do you have a low frequency limit? I mean, does it have to operate down to DC. or is eg, 20 Hz OK?

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 31, 2024, 10:58:59 pm
content: Hi GN89, 14MHz is homebrew design ( qucs) and build. Vd is from a variable DC converter 13.8V to 50 ~ 200 Vdc depending on Tx mode Topology is like linear class B push pull.Drain current ave DC is 0.7 A ave , peak Id per side is about 1.0 Amp. Case temp at continuous full power is about 68C, but in ham radio usage overall duty cycle is less than 50%I think you will be able to find TO247 ( or similar) mosfets to do your audio freq job.Regards

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2024, 12:00:27 am
content: 500 volts peak-to-peak or +/-250 volts at 0.1 amps does not seem that demanding for a traditional class-AB audio amplifier design. Before higher voltage transistors became available, an output stage like that shown below was used.Now the problem becomes the high voltage VAS stage, but since that can be NPN or n-channel transistors, higher voltage parts can be used.

author: langwadt
author_rank: Super Contributor
timestamp: June 01, 2024, 12:21:12 am
content: https://eu.mouser.com/ProductDetail/Apex-Microtechnology/PA194?qs=TiOZkKH1s2S8Qje%2FmQorJg%3D%3D&_gl=1*e6560q*_ga*MTE5MTA0MTg5NC4xNzE3MjAxMjA2*_ga_15W4STQT4T*MTcxNzIwMTIwNi4xLjAuMTcxNzIwMTIwOC41OC4wLjA. ?

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 01, 2024, 02:21:21 am
content: Kepco BOP 500M. This unit does +- 500 volts, up to 80mA, and up to 6 KHz depending on operating mode.

author: CaptDon
author_rank: Super Contributor
timestamp: June 01, 2024, 02:32:23 am
content: It would seem that nearly any audio amplifier of 100 watt RMS capacity driving a step up transformer would work for you. As for the control voltage that would take place at the signal generator. I have a Telulex SG-100 that I believe can be externally voltage controlled. At the very least the output voltage can be controlled through the serial data port. Since your plasma impedance will vary you may need a voltage sense circuit to control the absolute output voltage of your amplifier. Loading and impedance shift will cause the output voltage to sag even though the amplifier is 'linear'.

author: GN89
author_rank: Contributor
timestamp: June 01, 2024, 07:42:03 am
content: I need DC output for that application so audio amplifier even down to a few Hz with a transformer is not an option. I think the audio output stage looks familiar and might be from the audio power amplifier design by Douglas Self. In this case the VAS stage is also critical because it might require a high voltage current source and an npn transistor but this transistor can't be controlled directly by an opamp (maybe here an opto isolator would be possible). I don't really see how the voltage is split across transistors in the output stage.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2024, 08:48:09 am
content: Quote from: GN89 on June 01, 2024, 07:42:03 amYes.QuoteHmm, yes, so normally a high voltage series transistor PNP stage is still required.QuoteThe resistor divider R1-R2-R3, or R4-R5-R6, divides the output voltage between the output and supply rail roughly in two, and then drives the series transistor.

author: GN89
author_rank: Contributor
timestamp: June 01, 2024, 09:19:39 am
content: QuoteThanks I got that. A single VAS transistor is still required and to be perfectly safe with the SOA I would like to stack them but I'm not familiar with the topology of the VAS stage for higher voltages.

author: magic
author_rank: Super Contributor
timestamp: June 01, 2024, 10:13:53 am
content: There is one more topology which reduces HV electronics to absolute minimum. Main disadvantage is needing a floating HV supply for each channel, not a big deal if there is only one channel. Another drawback is that the supply's earth capacitance appears as a load on the output stage, but that should be tolerable at 1kHz.Note that an alternative way of dealing with limited SOA is paralleling output devices. For instance, Leonid Ridico's amp seems to be OK with about 40mA limit, so three such MOSFET pairs would get you to 100mA.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2024, 12:20:48 pm
content: Quote from: GN89 on June 01, 2024, 09:19:39 amThe VAS stage is a transconductance (current) output with shunt feedback (capacitor). So add the second transistor in series like you would do with a cascode VAS, and then a voltage divider between the output and negative rail to control the base of the second transistor. If the voltage divider needs to be buffered, then an emitter follower will have to be made with 2 series transistors since it will see the entire supply voltage also, and the emitter follower can buffer its own divider.I am not saying this is the way to go, but that a high voltage traditional class-AB amplifier has been a solved problem. As an alternative I might extend the example below to higher voltages, and add short circuit protection. This design avoids bootstrapping the operational amplifier so input common mode range is not a problem requiring positive feedback. The cascode and output transistors have to be doubled up in series of course. Current limiting can be added with a diode across R3 and R4.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 01, 2024, 04:11:23 pm
content: Quote from: Terry Bites on May 31, 2024, 03:28:39 pmBootstrapping won't work here because of the gain needed. The amplifier needs to amplify 10 V to 250V so will have 240V from input to output.

author: GN89
author_rank: Contributor
timestamp: June 04, 2024, 10:21:20 am
content: I made a first attempt to simulate the suggested circuit and extend it to a higher voltage by stacking mosfets. I had no luck simulating it with ngspice in a DC analysis. The result seems odd and the convergence is bad. Can you guys look into the schematic and tell me if the topology is correct so i can build a simple prototype?

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2024, 12:40:16 pm
content: Quote from: ejeffrey on June 01, 2024, 04:11:23 pmIt can be done by adding a positive feedback loop operating in parallel with the negative feedback loop. Doing this allows high gain while swinging the operational amplifier's common mode voltage to stay in range of the output.Figure 6 on page 5/124 of the attached PDF shows how this works in the non-inverting configuration and gives the math.

author: macaba
author_rank: Regular Contributor
timestamp: June 04, 2024, 01:08:37 pm
content: Quote from: magic on June 01, 2024, 10:13:53 amI have used this topology before with good success. I haven't come across any source material on this (aside from being a favourite of SMU designers) - I would like to read into the background more. Does your image come from a book or article?

author: langwadt
author_rank: Super Contributor
timestamp: June 04, 2024, 01:57:58 pm
content: Quote from: macaba on June 04, 2024, 01:08:37 pmI seem to remember the name "TransNova"I think it was QSC that used it with sziklai pair output, so the heat sink cn be grounded and the output transistors don't need isolation

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 04, 2024, 05:01:27 pm
content: Quote from: GN89 on June 04, 2024, 10:21:20 amHow high do you want to go? What Vpp do you want? That circuit is quite simple and just works (possibly after adjusting source resistors for your needed output current and simple source current adjusting).Here is one more circuit with this exact ideaClass AB inverting amp uses two floating-amplifier cells.It can deliver about 1000 Vpp. The only drawback I remember - the more stacked Mosfets the slower it became (but still has >= 1-2 kHz bandwidth).For a 100 mA output current, you need to use 10 R Source resistors (+-50%).Pros and cons of those two ideas (Bootstrapping vs Floating-amplifier cells):1) Floating-amplifier cells with N-Mosfets:pros: uses only N-Mosfets which are easy to find. If something goes wrong you lose only MOSFETs and optocouplers. It can be considered quite reliable and repairable.cons: it is slow, it works up to 1-10 kHz. Source current depends on the heatsink temperature and may vary.2) Bootstrapping:pros: very fast. It can be almost as fast as used opamp itself.cons: typically needs both N and P Mosfets (or NPN & PNP bipolar transistors), which are hard to find now (high voltage ones). If something goes wrong - you lose MOSFETs together with opamp (more loss).I would use bootstrapping topology up to 200 Vpp, and Floating-amplifier cells topology for higher voltages (if slow speed is not a problem).

author: magic
author_rank: Super Contributor
timestamp: June 04, 2024, 09:50:45 pm
content: Quote from: macaba on June 04, 2024, 01:08:37 pmThe image is from Edward Cherry's Electronics World article "Ironing out distortion, part 2", where it is only used to illustrate similarities between emitter follower and common emitter output stages. There is no discussion of complete amplifiers of this kind and the topology is unattractive for audio due to the multiple PSUs requirement.I imagine the simplest complete amplifier would add Miller compensation around this output stage and an operational transconductance amplifier in front to drive it.

author: langwadt
author_rank: Super Contributor
timestamp: June 04, 2024, 10:32:37 pm
content: Quote from: magic on June 04, 2024, 09:50:45 pm

author: macaba
author_rank: Regular Contributor
timestamp: June 05, 2024, 08:16:46 am
content: Thanks for the responses regarding the grounded output topology. From a browse of audio forums, this topology for audio usage seems to originate in 1982. As we all know, electronics doesn’t tend to have an outright best answer, it’s more about which compromises are acceptable to you. In my case, for my lab instrumentation build, I was happy to have the downside of per-channel transformer windings (with low pri-sec capacitance) in return for the reduced part count, low voltage frontend & speed.It looks like this thread has hit all the major topologies so it will be interesting to see what OP goes with.

author: magic
author_rank: Super Contributor
timestamp: June 07, 2024, 10:05:43 am
content: For the record, this is what I meant by a simple, one chip solution for driving this grounded emitter power stage. Simulation seems to agree that it should work.

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2024, 12:50:30 pm
content: Quote from: magic on June 07, 2024, 10:05:43 amBesides massively exceeding the breakdown voltage of the output transistors, the open loop frequency response changes with operating point, so frequency compensation will be very difficult.

author: GN89
author_rank: Contributor
timestamp: June 07, 2024, 12:58:01 pm
content: Quote from: Vovk_Z on June 04, 2024, 05:01:27 pmFor the first attempt I'm happy with the 400Vpp but later versions might be higher. More bandwidth is always good for the use case because probe potential tracking is planned for this case. More important is more current in later versions up to 0.4A.I tried simulation Leonid Ridico's design with ngspice but as soon as I add the negative rail there is no convergence at all. Maybe ngspice is not capable for simulating these circuits. I will try to build a prototype because mosfet stacking is easy as I showed in my previous design.I had a look into the EDN atricle about boostrapped opamps. The problem I see is that I need a operational amplifier which is capable for that current. Let's say 15V supply voltage for the floating opamp which means 1.5W power dissipation. For later versions this would be even more. this could be solved by using a current buffer wich could be low voltage because all is floating.The Transistors to let the opamp float should be stacked in that case to be operated in a safe area with that voltages.

author: mawyatt
author_rank: Super Contributor
timestamp: June 07, 2024, 01:16:14 pm
content: If the load is allowed to be floating wrt to ground, a differential drive (H Bridge) can reduce the voltage levels across the output stages.Best,

author: magic
author_rank: Super Contributor
timestamp: June 07, 2024, 10:33:03 pm
content: Quote from: David Hess on June 07, 2024, 12:50:30 pmOne would need to build this to be 100% sure, but I don't see the problem. Miller compensation is easy and well behaved, output stage gain variation is hidden behind feedback through C1, the stage is a transimpedance amp with frequency response determined by that one capacitor. It only needs a certain minimum bias to maintain sufficient gain and bandwidth inside that local loop, and in this particular topology it only needs bias on either one of the two transistors at a time, because base voltage swings are applied to both transistors equally.It simulates OK, also with steps to other voltages (and resulting load currents), including zero.

author: GN89
author_rank: Contributor
timestamp: June 10, 2024, 05:34:19 pm
content: I made one simple version for a converter which uses the PA97 high voltage operational amplifier. The PA97 can provide 10mA which which is enough to drive an output buffer of stacked mosfets. I attached the schematic, transient, dc and ac simulation. This one simulates well up to 1kHz with driving a resistive load of 400 ohm.What is missing is the gate protection diodes for the mosfets. Could somebody look into that if there is some possible optimization for a bit more bandwidth and if the PA97 needs more protection because that is the expensive part (about 150 USD).

author: magic
author_rank: Super Contributor
timestamp: June 10, 2024, 07:35:17 pm
content: Blown FETs may result in blown chip, so it's a good idea to protect them. Problem is, overcurrent protection means limiting gate-source voltage to a few volts in the event of output short. Since we have no control over source voltage (it's shorted), the usual solutions sink current from the gate to source to reduce Vgs. Unfortunately, your PA97 will try to fight it and source more current into the gate until it blows up, because it has no overcurrent protection. You would need to increase your gate resistors orders of magnitude to get worst case output current under 10mA.If I were desperate to really make it work with this opamp, I would try to add overcurrent protection to it by putting zeners on pins 4 and 6. But YMMV and it's 150 bucks down the drain if something goes wrong. Not even 100% sure if the schematic is complete and fully accurate.

author: BrianHG
author_rank: Super Contributor
timestamp: June 12, 2024, 12:27:34 am
content: You can try modeling and making your own bipolar op-amp, a simple one's design and function is illustrated here:https://www.eevblog.com/forum/chat/the-history-of-the-invention-of-the-op-amp/msg5538773/#msg5538773Use HV npn/pnp transistors to make this one. The basic design illustrated makes up many simplistic audio amplifiers with +80v designs, so, I do not know how hot driving such a design with a ~ +/-260v rail will make the differential input and class A stage heat up unless you are operating at low current at those stages and also use heat-sinks where needed.For extra precision, you can make it a compound opamp by adding a normal +/-15v opamp in front driving your home made one with a larger 20vpp signal.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2024, 08:52:38 am
content: Quote from: BrianHG on June 12, 2024, 12:27:34 amEven in lower voltage designs, the input stage and VAS may have cascodes to reduce the voltage and power dissipation requirements. High voltage transistors usually compromise other characteristics and combining a low voltage transistor with a high voltage cascode gives better performance.

author: magic
author_rank: Super Contributor
timestamp: June 13, 2024, 03:57:04 pm
content: Quote from: BrianHG on June 12, 2024, 12:27:34 amAs shown, it's too simple for its own good, lacking essential protectionsIt also needs more HV transistors than strictly necessary, including a 500V complementary pair. The "boosted opamp" circuit posted by David Hess uses only a pair of 250V transistors (besides the output FETs). Note that doubled collector voltage also means doubled power dissipation and this becomes particularly important if it's a difference between running with or without a heatsink.BTW, the IXTH10P60 seems to have enough SOA not to need stacking for 500V operation. The IXTH24N50 doesn't, but I would consider upgrading to a higher voltage part instead of stacking. It's less hassle and may end up being cheaper, too.

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2024, 05:53:11 pm
content: I would be cautious about voltage ratings. P-channel or PNP devices have lower maximum voltage ratings, so will place a lower limit on the number of devices required in series. I would further derate the maximum voltages to 66% or even 50% for each device.

author: magic
author_rank: Super Contributor
timestamp: June 13, 2024, 09:47:51 pm
content: These devices have full FBSOA specifications down to DC, one may hope this means something. While the N-ch part chosen by the OP is only speced to stand 500V for up to 10ms and demands 400V derating at DC, the P-ch does not.Since IXYS offers 1000V rated parts, 50% derating is a possibility too.

author: GN89
author_rank: Contributor
timestamp: June 15, 2024, 10:47:39 am
content: Thank you for the Answers and suggestions so far. I made a completely different design without the expensive PA97 and built a fully discrete operational amplifier. Q1-Q9 form a difference amplifier. Feedback from the output comes from R16,R17. Q5 is a current source which supplies the difference amplifier current. In this case it is a high voltage pnp but it could be replaced with a low voltage one (Am I right?).Q2,Q3,Q8 and Q9 are cascode but stacked to keep the vce voltage as low as possible. These are low capacitance PNP types. Q4 and Q6 are the load for the difference amplifier. Maybe one of you could help me with protection. When there is no vcc voltage Q1 and Q7 vbe voltage could be too high. is it sufficient to use zener diodes for protection?Q10, Q11 form a current source for the VAS. Again these are stacked to keep them in safe operation. Maybe I will use a two transistor Mosfet current source for that to prevent secondary beakdown at all cost.Q12 is for biasing the output stage.Q13,Q14 are stacked and used as VAS transistors. Q15, Q16, Q17 and Q18 are the actual output stage mosfets and stacked. The simulation works and is attached but I have some questions for you. I attached transient simulation with 10kHz 5V input signal and AC analysis.The VAS current source needs to be quite high at 8mA to have a sine wave at the output which is OK for me.I need some quiescent current at the output to minimize prevent crossover distortion. In this case the quiescent current depends on the voltage across vg1 and vg2. How about thermal drift of the Q12 and the impact on qiescent current. I want the quescent current as low as possible. DO you have any ideas how to set this current as presice as possible?How would I extend the output stage to allow 4 quadrant operation.Is that a good amplifier design in your opinion and how to protect that circuit as strong as possible to prevent is from blowing up when I build a first prototype.Offset voltage is quite bad but I yould use an oprational amplifier in negative amplifier operation to prevent that. Is that a good idea?

author: David Hess
author_rank: Super Contributor
timestamp: June 16, 2024, 02:52:57 pm
content: Quote from: GN89 on June 15, 2024, 10:47:39 am4-quadrant operation means that it can sink or source current over its entire output range, which your design can already do.QuoteVbe current limiting can use the emitter resistors at the output for fast short circuit protection.

author: GN89
author_rank: Contributor
timestamp: June 30, 2024, 09:12:10 am
content: I did some work on the amplifier and used different transistors for stacking to stay in the SOA of the mosfets. There is a current limitation which is done by Q19,Q20, R18,R35 and R41. R41 is important to make current limit possible in the negative half wave.Slew rate is limited due to the high impedance drive of the mosfets by the upper current source Q11, Q12, Q13, Q14. The quiescent current setting of the output stage depends on R29 and is very temperature dependent. Do you have any suggestions how to improve the output stage to have better control over quiescent current over temperature and increase the slew rate?

author: MrAl
author_rank: Super Contributor
timestamp: June 30, 2024, 01:05:36 pm
content: Quote from: GN89 on May 31, 2024, 11:50:34 amHello,Do you really have to use MOSFETs for this or are Bipolars ok too?

author: David Hess
author_rank: Super Contributor
timestamp: June 30, 2024, 02:13:11 pm
content: Quote from: GN89 on June 30, 2024, 09:12:10 amThermal feedback from the output transistors need to be applied to the Vbe multiplier, either with a PN junction or a thermister.I calculated that the VAS current is about 12 milliamps which is all that will be available to drive the output positive. That driven into the input capacitance of the output stage will be a problem. Amplifiers with big output transistors may buffer the drive into Q22 and Q23 using emitter/source followers or class-AB buffers.Your topology closely follows a standard class-AB amplifier with input stage, VAS, and output stage. Compensation is controlled by Miller capacitor C7 and the transconductance reduction from R10 and R12, but I wonder if drive to the output stage is so weak that the output stage cutoff frequency is so low that it is limiting performance.

author: magic
author_rank: Super Contributor
timestamp: June 30, 2024, 02:28:51 pm
content: Did you check if slewing doesn't modulate Q12 current due to negative feedback to its gate through C10-C12? Such modulation would act to reduce slew rate unnecessarily.Quote from: MrAl on June 30, 2024, 01:05:36 pmDo you know power BJTs good for 100mA at a few hundred volts?It's my impression than BJTs generally have poor SOA at high voltage, and there aren't very many HV BJTs to begin with, particularly PNPs.

author: MrAl
author_rank: Super Contributor
timestamp: July 01, 2024, 09:06:43 pm
content: Quote from: magic on June 30, 2024, 02:28:51 pmHi,Why don't you do a search.But really I just wondered if it mattered to you.PBHV3160ZXSTN9360

author: langwadt
author_rank: Super Contributor
timestamp: July 01, 2024, 09:17:39 pm
content: Quote from: MrAl on July 01, 2024, 09:06:43 pmPBHV3160ZX: datasheet doesn't even have an SOA graphSTN9360: SOA graph shows it's good for about 8mA DC at 200V...

author: magic
author_rank: Super Contributor
timestamp: July 01, 2024, 09:20:48 pm
content: Exactly, these are switching transistors. And one shouldn't even need to read the datasheet to know that SOT223 can't handle the power dissipation of an HV output stage...

author: MrAl
author_rank: Super Contributor
timestamp: July 02, 2024, 07:04:01 pm
content: Quote from: magic on July 01, 2024, 09:20:48 pmWell my question was not if the two transistors were suitable, my question was can you use bipolars.Apparently you do not think it is possible.I believe that those are probably not useable for linear operation with any significant power, so you'd have to do your own search and make your own conclusions. It also depends how much output power you can live with.It is probably possible to use NPN transistors, again if you can find any that are suitable. If you do not like that then you can probably convert the output stages to PNP+NPN drivers.It's entirely up to you, you can use four rusty nails if you want to and can get it to work

author: David Hess
author_rank: Super Contributor
timestamp: July 02, 2024, 09:18:43 pm
content: The KSA1381/KSC3503 are my current go-to transistors for high voltage applications, but they are only 7 watts and 300 volts, limiting current to perhaps 20 milliamps in the real world. Their high speed also makes them more likely to oscillate.For a 500 volt and 100 milliamp design, that leads to maybe 2 or 3 in series and 6 or 8 in parallel, which is not quite as bad as it seems because parallel operation only requires adding emitter ballasting resistors. That is about $30 or $40 worth of transistors, but still less expensive than the power MOSFET solution, and performance would be higher.I wonder what high voltage high current PNP transistors used to be available. The old 2N6214 could sustain 40 milliamps at 200 volts.The modern MJE5852G ring emitter audio transistor can sustain 50 milliamps at 250 volts, so only two are needed in parallel.

author: MrAl
author_rank: Super Contributor
timestamp: July 03, 2024, 07:11:04 am
content: Hi,You reminded me of the old 2N6250 transistor which is a high power device, although it is an NPN.I used something like 5 of these in 'series' to test an old CRT television set where the flyback transistor died and I wanted to test it to see if it would work with a new transistor that I would have had to purchase at a premium cost. I already had several 2N6250's so I rigged it up to test the TV. It turned out it worked so I purchased the right transistor and fixed the thing.We used these back in the 1980's to drive huge bipolar transistors for use in high power (1000Watts to 30kWatts) converters. Those transistors were over $100 USD each back then and the big stud mount case was made of copper with a 1/2 inch bolt stud also copper. They must have weighed in at least a pound each. Pretty amazing transistors. After that we went to the big rectangular type which were cheaper but also very high power (100 amps). After that we started going to MOSFETs for their higher switching speeds.I have a feeling not all the transistors still available out there can be found on regular outlet sites like Digikey and Mouser.

author: langwadt
author_rank: Super Contributor
timestamp: July 03, 2024, 07:14:27 am
content: Quote from: David Hess on July 02, 2024, 09:18:43 pmFJPF5021 seems to be in that range too,

author: David Hess
author_rank: Super Contributor
timestamp: July 03, 2024, 12:18:49 pm
content: Quote from: langwadt on July 03, 2024, 07:14:27 amI was only looking for PNP candidates since they are less common. There are plenty of NPN candidates and audio output transistors have complements.

author: langwadt
author_rank: Super Contributor
timestamp: July 03, 2024, 12:33:44 pm
content: Quote from: David Hess on July 03, 2024, 12:18:49 pmmaybe something like this from when PNP power transistors were less common,https://sound-au.com/articles/dcp-f6.gif

author: David Hess
author_rank: Super Contributor
timestamp: July 03, 2024, 03:03:10 pm
content: Quote from: langwadt on July 03, 2024, 12:33:44 pmThere is nothing wrong with the quasi-complementary pair, however the PNP still has to sustain the same high voltage across its collector. The advantage is that the PNP is lower power. It would be more suitable if a higher current or higher power PNP was not available.Did IXYS make linear p-channel MOSFETs? I cannot find any reference to them. Anyway, their standard 600 volt IXTH10P60 is suppose to support 350 milliamps at 500 volts if you believe their datasheet, and can find it in stock. Pair it with a IXTH20N60 I guess, but of course a high voltage driver will still be needed to drive their input capacitance.

author: magic
author_rank: Super Contributor
timestamp: July 03, 2024, 03:37:18 pm
content: Driving the output FETs really becomes simple in the "earthed source and floating HV supplies" configuration, where all drive electronics work at ground-referenced low voltage and high current is no issue.Otherwise, one might consider a "rail to rail" common emitter output stage with FET cascodes. The problematic, high voltage Cgd is now driven by the power FET itself and Cgs is driven by the common emitter/source transistor but it only sees low voltage so it can be any standard part with low power rating. The common emitter transistors should also be controllable by means of fairly low current, so the preceding stage can operate at low power and use simpler and cheaper transistors. And they only need to handle one rail worth of collector voltage.

author: amyk
author_rank: Super Contributor
timestamp: July 04, 2024, 03:01:34 am
content: Am I the only one here who thinks a vacuum tube design would easily be able to handle this sort of voltage and current?

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2024, 07:12:11 am
content: Quote from: amyk on July 04, 2024, 03:01:34 amA vacuum tube design is very feasible, but without a PNP or p-channel equivalent, I think it is limited to class-a operation. (1) If you went this route, then the same topology will also work with only high voltage NPN or n-channel transistors and there are lots of options there.A vacuum tube design will also require floating heater voltages that can sustain 500 volt swings, so some custom transformers.How fast was that optically coupled design? Did it use all n-channel or NPN output devices?(1) A complementary class-ab design with p-channel vacuum tubes made of antimatter is left as an exercise for the reader.

author: magic
author_rank: Super Contributor
timestamp: July 04, 2024, 09:31:38 am
content: Couldn't this output stage work in class AB with tubes? You may not even need I5 thanks to R7 and negative grid bias on Q7. Connect R8 to a stacked negative rail whose value is twice the desired idle grid voltage and you have 100% freedom in selecting standing current, while both outputs can be driven up to zero grid voltage.editTo stay more on topic, this is also a possible way of avoiding P-ch power MOSFETs

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2024, 01:05:51 pm
content: Quote from: magic on July 04, 2024, 09:31:38 amYes, but see below.QuoteThe input stage is depending on 4 p-channel JFETs.

author: GN89
author_rank: Contributor
timestamp: July 12, 2024, 06:52:29 am
content: Thats a lot of input from you guys. Tank you. I continues working on this so I show the results so far. What I changed is to get rid of the biasing circuit and add a new stage for driving the output transistors which will also make quiescent current setting more easy (Q18-Q24).Slew rate is still below the limit I want to make sure 20kHz works with full swing. The stacking of the VAS current source is not ideal but there are no BJTs with such SOA even Mosfets need stacking.