Need some capacitor advice with rail splitter/virtual ground

[mkaluza]

Hello guys!
I'm building a rail spliter for my project and I've come up with this so far (attachments)

I mostly followed this https://www.tubecad.com/2018/02/blog0412.htm and Horowitz,Hill Art of Electronics.

The circuit works for dc, but without any caps I get oscillations on the Vout of about 6MHz, 200-400mVpp when I load it with anything more than few mA. Those I put there fix things, but I don't really know why... Could you help me with placing right caps in right places?

To put this into perspective:
- I need the rail splitter for my Poor Man's Signal Generator that I'm building,
- It will be powered at 28-36V from a XL6009 boost converter board you can get from Aliexpress, which in turn is powered from whatever I have laying around - from laptop power brick to an usb phone charger.
- at some point I'd like it to be able to drive 50Ohm line, so it should be able to handle at least +/-150mA
- generator will probably output 1MHz max if that is important
- output will be DC coupled.

There will be also a buck converter (also cheap board from Ali) between those 28-36V and the virtual ground generating 5V for the logic part - display, serial and stm32f103 board (blue pill), which does the UI and feeds the DAC.

So I need this to have a ground reference for the cpu, dac and opamps (and output obviously).

I hope at least some of it makes sense...

Cheers,
Marcin

[fcb]

Post your LTSpice(?) model. Have you built it, or just modelling it at the moment?

Have you tried a Zobel network on the output?

edit: can you post the BD139/140 models as well.

[Zero999]

Just embed the models in the .asc file.

I'm not surprised it's oscillating, with the BJTs on the output., in common emitter configuration. Why not use emitter followers, which are mode stable?

[mkaluza]

@fcb didn't think of that, sorry. Don't have access to my pc now, but I used this http://ltwiki.org/index.php?title=Standard.bjt
Yes, I built it, but only on the breadboard. I saw those oscillations on the scope.

Haven't heard of Zobel network before - I'll have a look, thanks

@Zero999 I'll do that next time, thanks for the tip (that's my first contact with ltspice, so there's still much for me to learn)

The reason for adding that lat pair of transistors to the "diamond" circuit was that I saw something similar in Art of Electronics (and I needed more current/power on the output).