Deriving a bipolar supply

I have a variable 0-30V supply which I'd like to adapt to bipolar use for powering some breadboards with IC op-amps. I sketched the following. What do you think?



It seems straightforward enough to me, I but figured it wouldn't hurt to post it in case there's anything I overlooked.

The output looks more class B than AB because of the 10 ohm emitter Rs would that cause a deadband issue for the ground ref?

Will it be stable with the added EF buffer inside the feedback loop?


Reminds me of an old Marsh short writeup from the 80's?(maybe TAA) about using a channel of a power amp as an active power supply for another channel of a power amp

[quote author="NewYorkDave"]I have a variable 0-30V supply which I'd like to adapt to bipolar use for powering some breadboards with IC op-amps. I sketched the following. What do you think?



It seems straightforward enough to me, I but figured it wouldn't hurt to post it in case there's anything I overlooked.[/quote]

I've done similar tricks but usually with a known imbalance so I didn't have to be symmetrical.

I see little benefit from making it Class AB, IMO that's just wasting power. You could eliminate the bias string entirely. Just connect the bases together and to the opamp output. A few hundred ohm resistor from that output to junction of 10 ohms would provide for small fast changes.

I might be tempted to add some crude current limit with clamp diodes but if 30v bench supply is current limited you can ignore.

JR

[quote author="NewYorkDave"]
It seems straightforward enough to me, I but figured it wouldn't hurt to post it in case there's anything I overlooked.[/quote]

It should work. I've worked with power amp designs where a single winding transformer (no center tap) fed a bridge and the bridge fed two series-connected bulk 'lytics. The center tap of the caps was the load ground return, and the amplifier had a half-voltage dividet like your schematic. Hence there was no coupling cap to the load, and the amplifier enforced the voltage division mostly through the load.

At the end of the day I wasn't sure if the cost of the two bulk caps and the appearance of d.c. coupling to the load was a superior situation to the more conventional single supply approach with an output coupling cap, but it was interesting. It was not my design, since I was in the doghouse at the time. I do recall that there was an amusing issue with leakage from flux contamination, and since it was a stereo amp one amp was moving to compensate for the other---speaker cones were eerily moving slowly in contrary ways, and I was afraid that there was some fundamental and overlooked flaw in the design.

[quote author="mediatechnology"]Wouldn't it be easier to use a 7815 or LM317? Just tie the Vin of the 7815 to +30V and the 7815 Gnd pin to the 30V common. Use the output of your 7815 as "ground;" it sits +15 volts up from the 30 v common.[/quote]
Most conventional (non-LDO) three-terminal regulators have an emitter follower as the pass element, so they can only source current. The circuit you describe will go out of regulation if V+ draws more than V-, whereas NYD's should be fine.

JDB.
[checked the National 78xx/LM117 DS, suspect others operate similarly]

If I had known this could be done with so few parts, I wouldn't have bought one of these for a few dollars. The link is a PDF datasheet for a DC-DC converter.

[quote author="NewYorkDave"]

It seems straightforward enough to me, I but figured it wouldn't hurt to post it in case there's anything I overlooked.[/quote]
I haven't run the numbers, but what happens if you add decoupling caps to the +/-15V side ? At a first glance this looks like an opamp followed by a power buffer, and those can get mighty unstable when faced with capacitive loads. I may well be missing something here (sleep deprivation and all that), but wouldn't a servo-like structure such as Wayne uses in his capless mic pre be better ?

JDB.
[might be enough to only ever add decoupling between + and -, or add series resistors between +/- and the caps, or maybe I should shut up and go to bed]

Looks workable. Add some audio decoupling caps here and there to avoid Bipolar Disorder.

Depending on your current needs, you could use a circuit like the BB BUF634, which is a gain-of-one buffer and just the two reference resistors. This buffer will supply up to 250ma.

There is a good discussion of virtual ground reference circuits here:

http://tangentsoft.net/elec/vgrounds.html

Would there be and detrimental effects to a bipolar suppled amplifier which sees two different supply output impedances on the negative and positive rails (i.e. whatever NYD is powering with this circuit)? I could see this happening as the two transistors are basically changing supply ouput impedance to correct voltage differences. Someone mentioned oscilations when driving capacitave loads, but I could see adding a few large storage caps after this stage being benifital to lowering transient output impedance of this stage.

There seems to be a lot of class A power being wasted in this stage. Of course a tightly regulated switcher would be benifitial in this case as you can achieve control loop bandwidths well above the audible frequency range (especially current mode). Of course now, you have to deal with switching noise, but your efficiency would be much higher. You could implement a solution with two control loops (one for +, one for -) but this would be much higher parts count, and a probably significant amount of design time. With swithcing noise in the well above 100kHz range, i dont see how this could negatively effect your fidelity. Of course, I know we like to keep our supplies linear around here for the most part. Call me crazy, but I'm all about the efficiency.
-Mike