Design of a voltage amplifier - Page 1
Forum section: electronics / projects

author: Totoro
author_rank: Contributor
timestamp: December 16, 2019, 03:07:34 pm
content: Hi all,I want to build a curve tracer front-end for an oscilloscope and would like to use a voltage amplifier for the collector sweep voltage. (The goal is to also be able to profile tubes--so, also plate voltage.) The transformer I'll use gives me +/- 450V. I found this circuit on the net and it would be pretty much exactly what I have in mind. (The PDF is also attached at the end.)There are some fairly obvious errors in this schematic such as the top and bottom MOSFETs not being connected to power and I think there's a missing connection at R24, 25, 26, 27. Also, the connection in the feedback at Q11, R23 shouldn't be there(?). Anyway, I've simulated this in LTSpice using FQP3P50 (P) and FQP3N80C (N) MOSFETs and can't get it to work. I'm no circuit analyst, true, but this looks kinda strange. And this may be kind of overkill anyway.Does anyone know of a circuit that would work or maybe what else is wrong with the pictured circuit? The supply probably doesn't have to go + and - as long as I can swap polarity (though that would be nice).Thanks for reading,Michael (aka Totoro)high-voltage amp circuit.pdf(119.87 kB - downloaded 103 times.)

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2019, 04:11:22 pm
content: The variousold Tektronix curve tracersare completely documented in their service manuals so ideas for the high voltage collector supply are available there.Linear Technology application note 18 has a couple of examples of lower voltage high voltage composite amplifiers with similar feedback configurations. Your example strikes me as unnecessarily complex but some of that is from trying to extract the maximum performance from such a slow cascode output stage.I would probably try to extend the configuration shown below if I did not find better inspiration in the Tektronix designs mentioned above. The major difference is that local feedback stabilizing the output stage is applied to the low impedance output of the operational amplifier and drive comes from the supply pins so the output stage of the operational amplifier is being used as a current feedback stage.What kind of output current are you expecting? 100s of milliamps for power tubes?

author: Totoro
author_rank: Contributor
timestamp: December 16, 2019, 05:11:24 pm
content: Thank you for your reply, David. I have the manual for the Tektronix 576 curve tracer. They cleverly use a multi-tapped transformer with resistor attenuation for the collector sweep. They have taps at 15, 75, 350 and 1500 volts. Getting such a transformer wouldn't be so easy (or cheap) but I thought about using multiple transformers, secondaries in series, to get various voltages. Heavy, literally, and not as flexible as a voltage amp, I thought. But that might be the way to go. HmmmI did take a look at AN-18 (thanks) and there is a promising circuit there...Between the circuit you posted and this one, maybe I can come up with something. I'm really more a software type and thought being able to control as much as possible with a micro-controller would make for an interesting project.Thanks again,Michael

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2019, 05:39:50 pm
content: I think now I see a problem in the design you asked about. The output stage has no bias control so operates as class-B with a big uncontrolled dead zone. A SPICE simulation could have trouble with that.In the example I gave, the top and bottom output transistors are current mirrors so always conduct and there is no dead zone.The example you give from AN-18 uses a more traditional class-AB emitter follower output stage. Nothing prevents extending it to higher voltages but cascodes need to be added to both the drive transistors and output transistors just like my example and your first example. High power audio amplifiers use emitter follower cascode stages.It would be interesting to replace the drive transistors with a pair of optocouplers for the high voltage level shift. In my example, the operational amplifier supply pins could drive a pair of optocouplers which then drive the output transistor stage differentially but there is not much advantage. It is not like the drive transistor cascode level shift is expensive and provisions for variation of optocoupler gain from unit to unit would need to be made. Optocouplers also sometimes have poor reliability when isolating high DC voltages.

author: Totoro
author_rank: Contributor
timestamp: December 16, 2019, 06:19:34 pm
content: Hi David,It's evening here now, I may look at these circuits later but more likely tomorrow.Thanks,Michael

author: Micke
author_rank: Regular Contributor
timestamp: December 17, 2019, 12:11:37 pm
content: To me it seems this thread has much in common:https://www.eevblog.com/forum/projects/op-supply-bootstrapping-substituting-the-p-mosfet-against-an-n-mosfet-possible/I have also stumbled before on those schematics earlier in this thread before I got the schematic of the all NPN bipolar version (schematic in the other thread). My prototype worked really well, opto isolated input that control AC/DC voltage on the output (I got +/- 1500V out on my prototype). I guess it is quite easy to scale down if such high voltage is not needed?