Voltage boosting an opamp - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: December 29, 2016, 09:25:27 pm
content: Hello!Just got me thinking... For some project I might need a precision DC voltage source controlled via a DAC. The problem is I need 4 quadrant operation - so opamp is the obvious solution. The voltage we are talking about might be say few +5V downto -50V, output current 1mA is more than enough, BW not critical, 1kHz very well sufficient.The problem is, for +5-50V output I'd need an opamp designed for at least 60V supply operation. I know that some high voltage opamps do exist, but mostly expensive. I'd like to try to stick with jellybean parts for this, not afraid the component count will be a little higher. Only if that fails, I'll buy a high voltage opamp for this :-)Do you have any suggestions, how to voltage boost the output of an opamp?One of my first ideas was to ground the output of the opamp through a rather small impedance and then to use a pair of current mirrors in the opamp supply rails, to control the output voltage.I have drawn my idea, please see below. Does it stand a chance to work? I think it just might, but I may well be mistaken.Thank you for ideas as always!Yansi//EDIT: Just foundthis voltage booster(the second image below). Almost exact schematic! Thats pleasing I have reinvented the wheel and it might work just fine. Also interesting to note the local feedback through R1 C1.Still I do not get the purpose of matching/pairing the trannies in here. What will the assymetry cause? I can't figure out.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: December 30, 2016, 06:30:58 am
content: http://www.linear.com/product/LTC6090is a bargain at $5 if you value your time at all

author: Yansi
author_rank: Super Contributor
timestamp: December 30, 2016, 01:59:51 pm
content: $5 is not bargain. I can make at least 5 of those circuits above for $5, with the same result. Sorry.And btw, the availability of LTC6090 at local major distributors is exactly zero.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 30, 2016, 02:37:22 pm
content: That second schematic makes a lot more sense - i was wondering where the opamp's output was going to (or how it was working at all), in the hand-drawn one

author: Yansi
author_rank: Super Contributor
timestamp: December 30, 2016, 04:25:26 pm
content: After a few (two or three?) hours of breadboarding, I would prefer using a common base gain stage used for this purpose, instead of this fiddly crazy supply current sensing topology. I couldn't manage it to work good enough if at all. The common base gain stage works more than good enough, however for AC signals, I'd be very afraid of significant crossover distortion.This time I just need to amplify a slowly varying DC signal.Now just only current limiting to be added to that circuit.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 30, 2016, 07:38:11 pm
content: The supply current configuration originates from some very old app note or manufacturers catalog. It's trouble-prone and Rod Elliot takes a shot at it on his site, suggesting any more standard topology is better. I had some success with it many years ago, probably with a simple 741 and some T05 devices, but also burned up a lot of parts!

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2016, 12:29:02 am
content: Thanks for the tip, I will try to look it up on Rod Eliott's website.Now that I have successfully built a prototype DC/DC converter for the project, I will try the above topology (common base gain stage) with the full voltage span (requirement for the output voltage is like +5V downto -50V, currently).The converter is based on LM5007, input voltage is +12V, outputs -56V up to 1W.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2016, 12:29:57 am
content: Quote from: KhronX on December 30, 2016, 02:37:22 pmEither works but the second one uses the operational amplifiers output as a current feedback input to better control the gain of the external transistor stage. When properly compensated, this is a great circuit.Quote from: Conrad Hoffman on December 30, 2016, 07:38:11 pmI have had very good results with three input and 1 or 2 supply line output operational amplifier configuration.I would like to see the application note where it originated. The oldest reference I have seen is in the Tektronix 7A29 vertical amplifier from 1979 where it was used with 741s.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: December 31, 2016, 01:56:37 am
content: single ended Class A with a 2 ma ccs at -50 V wouldn't have any crossover problem, use fewer Qyou really need a load spec - too much Cload and compensation becomes more "interesting" - would require some better modeling to recommend the shown feedback lead C and dominant pole integratorQuoteQuote

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2016, 02:05:47 am
content: Electronics is my hobby and 2 hours spent breadboarding something is more valuable, than trying to buy $5 opamp from LT website. Oh, you just forgot there is $250 minimum order limit and something like $50 shipping + VAT and dealing with customs. Really no thanks.I have tried the single ended class A with CCS few times. Didn't work as expected, stability problems. The CCS makes it really high gain stage, the opamp didn't like it. Still, class A with CCS (standard "discrete VAS" ) doesn't solve much, if both voltage polarities are required (how will you interface the opamp output to the class A transistor thats near the V+ or V- rail respectively)Can you show a schematic, how would you do it?

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: December 31, 2016, 02:34:37 am
content: spec the load, the supplies you expect to work from and I could sim up somethingQuotenot really, the common emitter output from the complementary Q collectors is just as high gain in your earlier circuit toobut I won't build/debug/compensate when there is a $5 solution - I expect to spend to achieve even hobby goals and as a comfortably retired US EE that threshold is a lot more than minimum wage

author: Marco
author_rank: Super Contributor
timestamp: December 31, 2016, 03:22:15 am
content: Quote from: Yansi on December 30, 2016, 04:25:26 pmWhy not just bias the whole thing to have quiescent current then? (Needs some more resistors and the first stage bases can no longer be connected to ground.)

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2016, 03:40:18 am
content: Quote from: Yansi on December 31, 2016, 02:05:47 amTake a look as how analog oscilloscope z-axis amplifiers work. They are discrete and very fast but if the single transistor error amplifier was replaced with an operational amplifier, then they would look like this. One side of a horizontal amplifier usually works the same way.They both generally use a constant current load and shunt feedback to make an inverting transimpedance (current to voltage) amplifier. Tailoring the output voltage range requires a level shift to drive the output transistor but that will not be required if the output transistor is in range of the operational amplifier.

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2016, 11:08:56 am
content: evb149: I know these supply voltage bootstrapping apnotes, but I don't like the idea do to the possible latchup problems (supply voltage sequencing) and other limitations.Yes, the schematic was cropped from that one (SNOA600B). Sure, can add other biased stages, but there is a lot of trannies extra, that I think the DC amplifier won't need. Would rather do whatMarcosuggests and that is to bias the complementary pair with the common base. I think for the simpliest, a pair of diodes in between the bases, biased with a pair of resistors to V+ and V-. Like this, please see the attachment.I will response to the rest after a lunch.

author: Marco
author_rank: Super Contributor
timestamp: December 31, 2016, 12:18:41 pm
content: If you use a string of 4 diodes instead and give all the transistors emitter resistors the biasing is more stable and you get current limiting for free (the transistor stage is actually a voltage to current amplifier, with those resistors the opamp swing limits the current swing).

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2016, 12:39:57 pm
content: Marco:Current limiting of what will I get? The current through the common base stage is limited by the 10k resistor. Moving the base up a diode or two won't change much, nor will it add current limit to the output stage. Or have I missed something?David Hess: The analog scopes I know and I have worked with, were mostly fully discrete and used cascode stages, using a small signal NPN on the bottom, and a high voltage one on top of it. I have never looked through the Z input stages, but have some schematics still printed out here, so will take a look.Here's what I have come up with. Added current limiting for the output stage. Little biasing for the common base stage to minimize the emitor voltage "wiggle" between the Q1 and Q2 opening. Also changed the feedback network by adding the 100ohm series resistor on the output - itshouldmake it stable into capacitive load, while maintaining precise DC voltage accross load. (that also is a response forf5r5e5dabout the load spec) What do you think about it?

author: Marco
author_rank: Super Contributor
timestamp: December 31, 2016, 01:38:31 pm
content: Quote from: Yansi on December 31, 2016, 12:39:57 pmOutput current. As I said, the transistor stage is a V to I converter. If you add emitter resistors it's a pretty linear V to I converter in fact, so with a limited voltage swing you get a limited output current swing. In the case of my pic you get about 10 mA per Volt.

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2016, 01:54:26 pm
content: In that case the current limit is NOT because of adding the diode bias. The output current is limited due to adding emitter resistors for the output transistors - which of course can be done also in my schematic, even in the cropped original one from the application note.But you haven't said anything about emitter resistors, did you? That's why I haven't understood you.But thanks for the tip, I can eliminate out the Q5 and Q6 from my last schematic and limit the output current by choosing a suitable resistors in the rest of the schematic.EDIT: Doh! Silly me! I read wrong. U saidgive all the transistors emitter resistors... then you were of course right!

author: Brutte
author_rank: Frequent Contributor
timestamp: December 31, 2016, 02:08:51 pm
content: Quote from: Yansi on December 29, 2016, 09:25:27 pmwith single power supply.The voltage sink/source always has only two terminals. So to build 4-quadrant one you either need a +5V:-50V push-pull tied to global GND or you can use two push-pulls not referenced to GND (floating, supplied from separate DC source). The advantage of the latter is that the voltage effectively doubles but the supply must not be GND referenced (your op-amp power supply must not be earth'ed). Then a raw LM224 would do.If #1 outputs 0V and #2 outputs 30V then you get +30V and when #1 outputs 30V and #2 outputs 0V then you get -30V. Just add a fixed -25V offset between GNDs and instead -30:+30 you get -55V:+5VThe 35V limit only applies to Vcc-Vss

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2016, 02:34:39 pm
content: Quote from: Brutte on December 31, 2016, 02:08:51 pmSingle or not, +5-55V is the same 60V accross opamp as +-30V or whatever else combination.Floating load is not an option, I need the output referenced to ground. Floating the power supply is also not an option, because the design as a whole will need at least two more floating supplies for some other circuitry, really not a big fan of making stuff with too much complex power supplies.I do not understand the rest of your comment, as all circuits posted above will work in 4 quadrants, both polarities of voltage and current.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2016, 02:46:24 pm
content: Quote from: Yansi on December 31, 2016, 12:39:57 pmThe cascodes are added to the current source and output transistor to allow high voltage operation with fast transistors and eliminate the effect of Miller capacitance which would otherwise limit high frequency performance. They could be added to Q3 and Q4 for the same reason but with such a slow error amplifier, they will not be necessary.In your circuit, I would add small value emitter resistors to Q1 and Q2 to better control the idle current of the common base input stage whether 2 or 4 diodes are used.

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2016, 03:48:15 pm
content: Sure, that is also one of the clever methods of improvement some members suggested me when I was designing the regulated high voltage power supply - ie not to drive the high voltage power mosfet gate directly from an opamp, but to use a cascode with a smaller suitable low voltage BJT.Yeah, shall add small emitter resistors there to be on the safe side.

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2016, 05:24:55 pm
content: Quote from: Marco on December 31, 2016, 01:38:31 pmVery good, except it doesn't have any voltage gain!Increase the values of R4 & R3.Yes it does. It's just the common base stage doesn't. The preceding common emitter stage does have plenty of gain.The circuit only needs to be able to provide 1mA so all the resistor values can be higher.As the output voltage swing is +5V & -50V, the bandwidth is low and linearity isn't important, then the gain could even be asymmetrical, with more gain on the negative side than the positive.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2017, 10:57:11 pm
content: The power supply doesn't have to be +/-55V. +15V & -15V will do.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: January 02, 2017, 01:24:18 am
content: why complicate the output?you really only need SE Class A with 2 Q and simple bias:the Q1 LED biased ccs on the negative railthe Q2 folded cascode on the positive end, which is just another simply biased from Vsource(s) ccs and a R to the op amp output that "steals" some currentmore "interesting" is how to compensate which really, really needs a load Z spec - with the cascaded gains of op amp and any V boost stage there will always be some C_load that will be unstablemy sim compensation is totally eyeball guesstimation and a few repeated sims looking at overshoot, 1p to 1n Cload - with a load spec simplified loop equations more optimum compensation could be found

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2017, 02:44:25 am
content: f5r5e5d, that is like an oscilloscope z-axis or horizontal amplifier. The major difference is that they use a common emitter output configuration; the inversion allows a small amount of AC coupling between the output of the error amplifier and the current source to produce a symmetrical transient response.But an error amplifier using an operational amplifier is too slow to require or output cascodes or coupling to the current source making for a less complex circuit.

author: Yansi
author_rank: Super Contributor
timestamp: January 02, 2017, 02:52:31 am
content: Just a quick reply, the rest later:Simple SE is a powerwaster. If I need to deliver up to 10mA, it has to run with 10 or more mA of the CCS. I like the previous solution more, due to not wasting power continuously. 1W is not a significant problem, but well... why? If it can be solved by adding a trannie or two. Even if they will be unbiased, the resulting crossover distortion is not a problem for my application, as I need mostly DC operation. I'd like to try lowering the idle consumption of the whole device, as it already is wasting a lot power in other power supplies.About the load Z spec: As I mentioned earlier. See how I did the feedback loop for it to be stable at any C load. (or I think it will be stable). Couldn't find a better image:http://i.cmpnet.com/eetimes/news/08/09/1540chart_pg48.gifBut like that. I think it should make it stable into any sensible capacitive loading, isn't it? But I might be wrong.Also, as it will be used as a general purpose control voltage supply, one can connect anything to it, so specifying the load is not that easy here.BTW, what purpose does the R6 C2 serve in your schematic? Such small capacitance value there.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: January 02, 2017, 03:01:52 am
content: moving the goal post?you said you needed 1 ma, I give a circuit showing how to do that - with to-92 Q no heat sinkI wanted to show simple, don't think you can get much simpler than just 2 output Q without wasting even more power in bigger negative V with a pull down Rthe folded cascode drive removes the sign inversion of the output stage allowing both loops negative feedback to sum at the op amp inverting input

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2017, 12:50:49 pm
content: I redesigned it, disregarding all the other schematics posted here. I believe this is the most basic approach.It should be stable, without any additional compensation capacitor because op-amp has a gain of 10 and the booster gain stage has a gain of just under 3.2, thanks R4 & R5.It can sink 10mA but only source around 1mA. If it doesn't need to sink so much current, then Q2 & D1 can be removed. If it needs to source more current, then an additional transistor is required.R6 could be replaced with a current sink which would improve both the linearity and reduce the current consumption when the output voltage is higher.

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2017, 01:50:10 pm
content: Quote from: Yansi on January 02, 2017, 02:52:31 amThis will source more than 10mA and sink 10mA.I've implemented the current sink to reduce the power consumption.It will have crossover distortion though, which can be removed, at the expense of more components and power consumption.

author: John Heath
author_rank: Frequent Contributor
timestamp: January 04, 2017, 02:00:01 pm
content: Quote from: Yansi on December 29, 2016, 09:25:27 pmI built a sub woofer for my computer based on your same design. However in my case the op-amp rails went to two darlington NPN / PNP outputs through a resister to base then resister to emitter with both collectors joined for output to speaker sub woofer. It ended up with + and - 30 volts and an average of 10 volts across the op-amp. For thermal protection you can place the op-amp close to the output heat-sink. This way bias temperature corrected current built into the op-amp will temperature correct the output darlingtons. It worker for a few years. I think I still have it around somewhere. In summary I had good luck with your design and found it could be simplified by resistively going from op-amp rails to darlington bases. An added benefit is it looks like it can not possibly work on paper with op-amp output going to ground and + input as negative feedback due to output stage inversion. Lots of fun showing off the diagram to onlookers scratching their heads , ha.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2017, 04:51:26 pm
content: Quote from: John Heath on January 04, 2017, 02:00:01 pmIt becomes fairly obvious how it works, once it's pictured, showing the internal schematic of the op-amp.It's been discussed before:https://www.eevblog.com/forum/beginners/non-standard-op-amp-configuration/

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 04, 2017, 05:16:23 pm
content: I have a suggestion:You can boost the output range of a jelly-bean op-amp like the 741 by sliding the operating voltage of the amp up or down within a higher voltage than it is rated for. See the first circuit below. The output of the 741 determines what voltage is applied to its own power pins. The resistor dividers connected to the transistors bases set the fraction of the total supply range is applied. It can be shown mathematically that the voltage between the + and - pins of the op-amp never changes regardless of what the output voltage is. The operating power for the amp just slides up and down between the rails as needed by the signal.I have used this a couple of times and it works well for not-so-large bandwidths and reasonably low loads. Don't push the drive specs of the op-amp. The only worry is that the op-amp becomes slightly less stable at higher frequencies than normal and may oscillate if driving capacitive loads or long lines. In that case stick a few hundred ohms at the output pin of the 741 and all is well.The first attachment below shows this arrangement. But if the combined voltage tolerance of the op-amp and two transistor is not enough then the transistors can be stacked to give even more range to the amp. The second attachment shows this with a 741 operating with close to 115V output range.One thing you cannot do with this arrangement is use it to get large gain at the same time as that might require the inputs to be a large voltage away from the output.

author: Yansi
author_rank: Super Contributor
timestamp: January 04, 2017, 08:06:53 pm
content: I think I have already mentioned that and this supply boosting topology has some serious disadvantages, against any other topologies posted here. Please see this paper.http://joebrown.org.uk/images/DualPSU/BootstrappingOpAmps.pdf(Also note you have only a buffer, I need voltage amplifier.)

author: David Hess
author_rank: Super Contributor
timestamp: January 05, 2017, 02:18:06 am
content: Quote from: Hero999 on January 04, 2017, 04:51:26 pmIt works even better if the output of the operational amplifier is used as a current feedback input to stabilize the voltage gain of the output stage.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2017, 07:44:36 pm
content: Quote from: David Hess on January 05, 2017, 02:18:06 amLike this?

author: David Hess
author_rank: Super Contributor
timestamp: January 05, 2017, 11:35:03 pm
content: Quote from: Hero999 on January 05, 2017, 07:44:36 pmYes, exactly like that. Some implementations just use AC coupling (capacitor in series with R12) back into the output. Even if you do not use feedback, the series resistor on the output (R13) should be used to trim the output stage frequency response.The output stage feedback is especially important when the load impedance is not well defined making the voltage gain at the collectors of Q11 and Q12 vary.A high voltage implementation adds either zener diodes or cascodes in series with the operational amplifier's supply pins.

author: Yansi
author_rank: Super Contributor
timestamp: November 09, 2019, 03:38:21 pm
content: After loooong loong time, I finally got back to work a bit on this project.So this is what I came up with for a prototype to test:The idea was to have at least a bit of bias flowing through the power stage - in this case 1mA. That equals to about 0.7V across R3 (R4), requiring about 850uA of current through T1, T2. Diodes D1 and D2 shall do just that, as about 1.2V is left across R14+R15 (resulting in about 850uA. Of course, it all depends on the exact voltage drops across all junctions, but I think as a ballpark figure it should work.Resistor R7 was calculated such that under any circumstances it can produce a maximum of 2mA through T1 (T2), resulting in about 10mA maximum output current.T1 and T2 are purposely biased from 0 and 5V, which is the same voltage as the opamp has, so that with a zero output voltage, the opamp output should sit just in the middle around 2.5V.What do you think about that? Will it work or do you see there any major flaws? Thank you for any hints. If all goes well, I will test the circuit tomorrow.

author: dom0
author_rank: Super Contributor
timestamp: November 09, 2019, 04:47:20 pm
content: Looks fine on first glance. Since the MCP6001 is easily outpaced by a melting glacier, you might even get away without C2. Circuits that put gain or extra poles into the feedback loop of an op amp will often need at least some tweaking in practice, outside the simulator. The faster the OP, the more tweaking.

author: Yansi
author_rank: Super Contributor
timestamp: November 09, 2019, 04:57:55 pm
content: Haha, melting glacier... funny.Fortunately in this case, I don't need more bandwidth than say 100Hz, so 6001 opamp will be I think fine in this application.So let's have some fun doing a test PCB for it.

author: dom0
author_rank: Super Contributor
timestamp: November 09, 2019, 05:11:47 pm
content: Quote from: David Hess on December 31, 2016, 12:29:57 amI looked at the 7A29 and I believe you are referring to the current sources feeding the input limiting bridge, A2, the circuitry surrounding U100?The idea of that circuit is imho a bit different from the power/voltage extenders, because it seems to be mostly used to work around the limited output swing of the 741, which wouldn't allow for the ~10-11 V or so at the node of U300 pin 3 if you add the output follower that it would need to achieve that. So you can't use a follower directly, so we either have to shift level from the OP's output to get to a follower or we use a common emitter amplifier. But that we can't control anyway with the OP, because it never reaches its rails. So shifting that through the power rail makes sense. RT109/R109/R107/R113 configure U111/Q114 for a gain of -0.75 or so for ~11 V at U100 pin 3, and U121/Q124 is just a straight inverter working off that node for -11 V at pin 13 of U100; diode bridge current is then set by the resistors internal to U100.Hm, guess I do agree after all, same idea, just a bit of a different application.

author: Yansi
author_rank: Super Contributor
timestamp: November 09, 2019, 05:51:01 pm
content: Could you please post the part of Tek schematic you're talking about?...meanwhile, I made the PCB. So let's clear the mess on my bench so I can solder it.

author: dom0
author_rank: Super Contributor
timestamp: November 09, 2019, 06:00:02 pm
content: [ Specified attachment is not available ]

author: Yansi
author_rank: Super Contributor
timestamp: November 09, 2019, 06:52:41 pm
content: Now if it would just work at least a bit how it was intended...Time got spared, so I am working like no tomorrow. Testing within a moment.//Unfortunately, I could not find any brand name BCP53. Of course, I found those from ONsemi right after I soldered this chinese junk on the pcb.

author: Yansi
author_rank: Super Contributor
timestamp: November 09, 2019, 08:39:29 pm
content: So, taking exactly the schematic in post #38 and testing it on a prototype pcb, yields very promising results:Bias current through the final stage settled to 1.25mA (I'd say close enough to my design goal of 1mA).Even though a glacier melts faster than MCP6001T moves its ass, I get 140kHz (-3dB) bandwidth, test from full swing. (test pcb supplied with +-20V and +5V for the opamp). Which I think is quite impressive, considering no tweaking was done whatsoever.So I think it will be good enough for 100Hz//EDIT: I will test the output short circuit current later. I will also change the supply voltages to the desired +12 and -55V to see what it'll do.//EDIT: Great! current limits to about 12.5mA. I have used a smaller R7, so das stimmt.

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2019, 11:14:15 pm
content: The buffer will improve the slew rate because it's much faster than the op-amp. The oo-amp's output only has to change a little for the buffer's output to change much more. Look at the op-amp's output voltage.

author: Yansi
author_rank: Super Contributor
timestamp: November 10, 2019, 01:38:49 pm
content: ^ That of course makes sense.Currently thinking about those 4 diodes in series, biasing the whole amplifier.What would happen, if I'd replace those 4 diodes with a NPN Vbe multiplier (NPN with Rbe, Rcb resistors)? That would also give a bit more flexibility in selecting the bias voltage.Just wondering, how could I get more stable biasing. Huh.What would be the dis/advantages of using 4 diodes vs. Vbe multiplier?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 10, 2019, 03:39:25 pm
content: Quote from: Yansi on December 29, 2016, 09:25:27 pmDefine expensive. How many units are you going to build? Are a few dollars going to really matter? A couple references: OPA452, ADA4700-1...And do you think the added parts, time to design, and extra PCB area for a discrete version will really yield a lower cost solution? Unless you go for large quantities, I even kinda doubt it.

author: Yansi
author_rank: Super Contributor
timestamp: November 10, 2019, 03:50:49 pm
content: Yes, OPA454 is exactly what I mean by EXPENSIVE NONSENSE. OPA454 is about $7 + VAT at single quantities from Mouser.MCP6001T is below $0.5 in single counts, and those 4 jellybean PNP/NPN transistors cost next to nothing. So say about $2 maximum per discrete amplifier unit?Does not pay off to use OPA454 even at single quantities. Every idiot can find a ready made solution given amount of $$$. Sure, time is valuable, but knowledge even more so. I value knowledge more, than the time spent getting the knowledge.Next time I will need a HV amplifier, I know sure how to make one, cheap.Now, don't you have anything better to do, then question the obvious from over 2 years old posts?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 10, 2019, 04:16:47 pm
content: Learning things is interesting, but the choice here is absolutely NOT obvious. Is gaining say $5 (and again you didn't count the extra PCB area needed and time to design, so it's likely less than that) worth the trouble? The added part count also means a larger BOM, which does not always pay off either. If it floats your boat, fine, but saying using HV opamp is idiotic doesn't make sense in general.And I didn't notice it was initially 2 years old, as there are a number of recent posts discussing it. You're apparently still working on it 2 years after judging from your above posts? If you still think it was worth the trouble, fine. Just saying.

author: Yansi
author_rank: Super Contributor
timestamp: November 10, 2019, 04:47:01 pm
content: It is not your issue to question what is or isn't viable for others. That kind of questioning is what indeed pisses me off. Either stay on topic and discuss the amplifier topology, or go away please.This circuit is part of a much more complex project, where shaving $5 out of a BOM really counts. Even if only 10 pieces of it would be made (I guess probably more will), $50 spared just on one amplifier is not an insignificant amount for a homegamer.PCB area is a stupid argument anyway, as if space would be the constraint, I would probably not be going this route in the first place.I have a lot more things to do, than just this amplifier that currently will not earn me any $, so please forgive me I still work on this project for more than 2 years.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 10, 2019, 05:23:08 pm
content: Jeez, just relax. If you don't want other people's opinions, don't use a public forum. This forum is not your playground where you set your own rules. When you open a thread, all kinds of people are going to give various opinions, and that's actually what makes it interesting and fruitful. If you're not interested in one opinion, you can just ignore it. And again *I* did not revive this thread, it got active again, and I reacted to it (was not even on the forum when it first started.) It's every user's right, as long as they are not insulting anyone. I don't think I did. But you did with your "stupid" words.Threads in a public forum are not just a private discussion. If you wanted it to be private, just use private messages. Many other people looking for a similar solution can be interested in reading various options and opinions. Without counter-arguments, some people after reading it may conclude that using HV opamps is stupid whatever the context - which is obviously silly. Threads are shared discussions, not just a private and free tech support tool.As to the time it took, I don't doubt (or care, it's your business) you had many other things to do, but it just shows designing a discrete version was definitely not as simple as it first looked, so considering other options was definitely not stupid engineering-wise. So I consider my point (which was short enough not to require hours to read) to be relevant. Again, take it or leave it, but it could help others deciding what's the best option for their own project.

author: Yansi
author_rank: Super Contributor
timestamp: November 10, 2019, 05:42:50 pm
content: Your solution is the same type, as if you would just say "buy a complete ready test instrument for the purpose", why making one. No thanks.Now I politely ask you to leave this thread, as you did not contribute anything useful, rather just being provocative and nitpicking about what is or isn't expensive, which is not the topic this thread is about. Neither are you entitled to evaluate how much work something required to make. This is my thread and I make the rules.Thank you for making mess out of this thread, no-one will be willing to read. Please leave. Bye.

author: magic
author_rank: Super Contributor
timestamp: November 10, 2019, 07:46:28 pm
content: Quote from: SiliconWizard on November 10, 2019, 05:23:08 pmHV ICs is boutique stuff, only available at big distributors and pricey. I feel like people use them either because they are space/power constrained or because the knowledge how to whack together a simple analog circuit has largely been forgotten in the mainstream, due to the huge success of low voltage ICs which are indeed highly competitive both on performance and cost. I think I have seen examples of the latter.Discrete still lives in some commercial power analog applications like audio.Here'sone of the highest performance speaker amps out there. I can see a few ICs but they are in a sea of transistors, I think it's safe bet most of the high voltage signal path is discrete. Go tell those guys they should be using opamps

author: magic
author_rank: Super Contributor
timestamp: November 11, 2019, 07:07:24 am
content: And speaking of the actual circuit...Quote from: Yansi on November 10, 2019, 01:38:49 pmI'm not sure what bias instability you have, but if it decreases under heat, which is what it probably does, you should perhaps replace one pair of diodes with 680Ω resistors to match the tempco of the parallel branch.

author: Yansi
author_rank: Super Contributor
timestamp: November 11, 2019, 01:09:47 pm
content: Not saying it was unstable, but it just got me thinking how or if it could be improved right away.Your suggestion does make sense, that way the tempco of those two Vbe get compensated. However what about those two voltage controlled current sources? There are still those -2mV/K, i.e. 0.002V/100ohm = 20uA/K drift. Which if I understand correctly will make a change of 1mA in bias @50K temperature difference, which is not that nice.Hopefully the calculation can be done that way - taking the tempco of the PN junction divided by the Re of the current source - considering constant voltage at the base. (decreased Vbe means increased Re voltage, hence increased current).

author: magic
author_rank: Super Contributor
timestamp: November 11, 2019, 07:44:20 pm
content: Almost correct, although in principle you should also account for intrinsic emitter resistance. That's the inverse of transconductance: how many mV ΔVbe per 1mA ΔIc. In a VCCS circuit, rE adds to your emitter resistor, for relatively obvious reasons.That being said, transcondutance at collector current Ic is approx Ic/30mV, depending on die temperature. So your rE is a few ohms, quite insignificant.

author: macboy
author_rank: Super Contributor
timestamp: November 12, 2019, 01:29:34 am
content: Quote from: Yansi on November 10, 2019, 05:42:50 pmIt most certainly is notyourthread just because you started it, andyoudo not make any rules in this forum.

author: Simon
author_rank: Unknown
timestamp: November 12, 2019, 07:50:55 am
content: Quote from: Yansi on November 10, 2019, 05:42:50 pmNow I will politely ask you to leave this forum if this is the way you intend to conduct yourself! you don't own the thread oh powerful one so I have locked it, SO THERE!