uA741 rail splitter with complementary Darlington emitter follower

[Zero999]

Here's a schematic showing a hypothetical op-amp with external pass transistors (Q1 & Q2).

Highly hypothetical, because it doesn't work
(Qcm2 is forced into deep saturation, and even that may be not enough to turn off Qvamp.)

Anyway, I agree that these are irrelevant details. It's the output stage that matters, in this case.

I missed off an emitter follower. It'll work now. Thanks.

[wobbly]

Hi guys,

Thank you for your continuing interest in my humble newbie project.  I appreciate your guidance.

I've been working on it a bit more and added a few bits of house keeping such as a PFET on the input power connection for reverse voltage protection and a fuse and a 10-turn pot for trimming the virtual GND point.  I've also added an RC filter to inhibit a slight tendency to oscillate, which seems to have worked quite well.  Circuit now looks more like the attachment below.  Still a work in progress though.

I'll build this with actual TO-220 Darlingtons on my breadboard and then re-do it on a small perfboard for the final unit.

I also changed over to using Texas Instruments LF411 op-amps (mainly because H&H said they are OK ).

Honestly, having a circuit on my bench that a year ago would have bewildered me, but which I now (kind of) understand is a huge win!

[Zero999]

That will work, but it has a couple of issues.

When the current direction changes, the op-amp's output voltage will have to change by four V_BE drops (two for each Darlington pair), about 2.4V, very quickly, causing the output to become high impedance for a short length of time. The capacitors will help to absorb that, but then they also increase the risk of oscillation. Adding a resistor, say 330R between the op-amp's output and the 0V node will help to soften this to some extent. It might also be necessary to introduce some series resistance to the capacitors, to reduce the oscillation.

I could simulate this, but it's really something you need to build and measure with an oscilloscope. A good way to test this is to add a MOSFET switching a load resistor to the output. The MOSFET should be turned on by a square wave, from a signal generator or perhaps a 555 circuit. The voltages on the +V an -V rails should change as little as possible and there shouldn't be any ringing (oscillation).

[magic]

Also, R5 and R6 are doing nothing obviously useful

[wobbly]

Also, R5 and R6 are doing nothing obviously useful

I don't know either.  I have seen a lot of circuits of this general family and some of them have these resistors, and the values very wildly (I've seen as low as 150 Ohm).  I removed the pair of 4K7s I was using and it made no difference to the circuit whatsoever, so you're right.  Thanks for the tip. 

I've been building it on a bit of perfboard of suitable size (pic below).  Not quite finished, still need to add the output caps but the thing does work nicely.  Reverse protection PFET does its job and the virtual ground is stable with small loads.  I'll try loading it harder tomorrow.

I think I've soldered the potentiometer in backwards though but I can live with that

Don't you love it when a circuit looks like its own schematic?

[Zero999]

Also, R5 and R6 are doing nothing obviously useful

I don't know either.  I have seen a lot of circuits of this general family and some of them have these resistors, and the values very wildly (I've seen as low as 150 Ohm).  I removed the pair of 4K7s I was using and it made no difference to the circuit whatsoever, so you're right.  Thanks for the tip. 

The resistors are required, when there are diodes in series with the op-amp's output. This was the case for the schematic in your original post, when the resistors pulled the base towards either the negative or positive rail, with the op-amp pulling in the other direction, in order to turn the respective transistor off. In your most recent schematic, the op-amp turns the transistors on, so the resaistors are unnecessary.

[wobbly]

I'm going to call this little beginner project a success.  At least by my low standards.

If I understand correctly (I'm explaining this to myself more than anyone else  )...

When the load on each rail is equal, the circuit basically just keeps the virtual GND point steady, neither sourcing nor sinking current.
In that case, the upstream PSU (e.g. my bench power supply) will provide all the power to both loads, and very little power will be dissipated in my Darlington circuit.

But when one rail is subject to a greater load than the other, the op-amp compensates for this imbalance and begins to source or sink current accordingly and keeps VGND as well as it can.

So I think the main limiting factor to how hard I can push this design is down to what load imbalances are likely to occur in use.

Studying it with a DC load on one rail or the other and watching the Darlingtons' temperature carefully with a thermal camera, this design can sustain a load imbalance of about 2 Watts continuously.  This looks quite symmetrical (+/-) and the small heat sink tops out at about 70°C at room temp after an hour or so.

The VGND holds it's set point better at higher input voltages.  With an input of 30V (the maximum), and a load of 130mA on one of the output rails, that rail only sags by about 40mV, which is awesome as far as I'm concerned!

I put this together as a simple PSU project to power my future learning of op-amps and analog electronics in general, so I think it's unlikely that the rail load imbalance will get to anything like that kind of extreme (unless I connect something wrong - and that never happens obviously).

Seriously though, sincere thanks to all of you for your kind advice.  After being scared of analog electronics for so long, I'm finally loving it.

Cheers!

[Zero999]

Good, you've got it working so well?

Have you done and transient/AC analysis/tests?

[wobbly]

No, I haven't.  Don't know what those are.  "Beginners" forum right?

I haven't sent this up into LEO on a cubesat either, before you ask   Just kidding.

I know what you mean though.  The proof of this "design" (which I basically copied and pasted from 10 different sources on the internet) will come when I make some new circuits that take their power from it.

I'm not there yet.

[Zero999]

No, I haven't.  Don't know what those are.  "Beginners" forum right?

I haven't sent this up into LEO on a cubesat either, before you ask   Just kidding.

I know what you mean though.  The proof of this "design" (which I basically copied and pasted from 10 different sources on the internet) will come when I make some new circuits that take their power from it.

I'm not there yet.

The idea is to test what will happen when the current through the 0V output changes.

Here's an example of a test circuit.

Normally the rail splitter sources 400mA into R2. When M1 is turned on, R1 is connected, which draws 800mA from the positive rail, with 400mA going through R2, the rail splitter has to now change from sourcing  to sinking 400mA. Look at  +V and -V with an oscilloscope, which can be set to AC coupled. The voltages should change as little as possible, without any oscillations.

The resistors obviously need to be able to dissipate the power. You might want to start with higher values, then work downwards. Keep R1 = ^R2/₂ then it'll sink and source the same current.

[Andy Chee]

FWIW, this amp design shares a similar topology to your rail splitter. 

Note D5,D6 which your circuit lacks the equivalent.  These diodes add bias to the output transistors, thus minimising crossover distortion.  This may or may not be important in your final application of your rail splitter.

Also note C1,R1 which your circuit lacks the equivalent.  These components form a low pass filter which rolls off the high frequency response of the amplifier.  It is extremely difficult to get a circuit to respond equally from DC to GHz, so set yourself a realistic design goal, say 100kHz, then filter off everything above it.