@jonk So the circuit has to stay exactly the same, as it is, but the component values can be changed to whatever is suitable to reach the x12 gain. I have explained the steps I went about finding the orginal calculations in the question if that helps? I can provide more calculations I made if need be. 56K for R4 is also fixed which should be worth noting.

@Bimpelrekkie I appreciate the advice on actually changing things, but I have tried that. I was hoping someone could explain some actual formula that can be applied to get there. Because changing the combination of values has so far been no use for me. I have succesfully completed 15V-0V supply CE amps with no issues. It is just the issue of supply which is throwing me off.

@jonk Yes, this is a teaching excerise. I am a student, however, what is really frustrating is no one can get this circuit to work. I have spent a couple of days on this now and have been left with no real answers.

@ElliotAlderson Fair comment given the usual do your homework business here... I have added the theory I orignally applied. However, as I explain I did not find the results I hoped. The circuit was then altered by someone else but they could not tell me how they got the values, nor how to reach the x12 gain.

Thank you very much. This is very useful! The reason I am using such an old driver is because I bought the VFDs on a pcb with the drivers on. I wasn’t going to use them but thought I may as well try…

@piojo yeah they’re exactly that, being used as a linear regulator. The issue with this new technology is actually the voltages. Current sense is made using current sense resistors and a feedback circuit. But the voltage sense is done by an ADC, which as you may know are notoriously inaccurate (unless extremely precise components are used) and even more so at low voltages which batteries can be. If you can come up with an ADC that’s precise with a large bandwidth then you’re laughing. The guy that makes them says they’re fine but my flukes tell me otherwise ;) sorry bit off tangent 😆

@piojo oh yes, I missed that! That is indeed correct. There are around 20 used within our chargers, so I guess fast pulsing and multiple of them does the job... when they come up for calibration I’ll try and borrow one. Get a scope on the gate and see the pulse width

@piojo did you use fig.1 on the datasheet? It doesn’t seem to show the full Vds curve anywhere so I don’t think you would have seen the full soa, although I might be mistaken. It was purely an example. Everyone is always after a linear regulator for battery charging, unfortunately batteries can be very volatile and smps, for example, is a poor choice for charging unless very well designed. As you say, heat is the issue, but if correctly designed you won’t have any problems. Yes, mosfets and microcontrollers are currently the biggest upcoming developments. Primarily for automation.

Great! I’ve used these before on ac circuitry, I’ll have a look at some. Didn’t even think about them. Thank you