Current Boost for Op Amps

I've been using the ADA4661 recently and it fits what you're going for. Probably a little "nicer / more expensive" than what you need... $1.50 in quantities of 1500 at Digikey. Dual, 18V, 3mm QFN with thermal pad.

That's assuming you don't have room for the traditional solution for current boost: add an NPN-PNP push/pull output, and take the op amp feedback from the output of the push/pull.

Probably a little "nicer / more expensive" than what you need... $1.50 in q uantities of 1500 at Digikey. Dual, 18V, 3mm QFN with thermal pad.

ent boost: add an NPN-PNP push/pull output, and take the op amp feedback fr om the output of the push/pull.

It looks like a great part, but for some reason when I plugged it into my s imulation it was clipping badly at a point the LM8272 was just beginning to clip. There is one number in the data sheet I don't get.

Continuous Output Current IOUT Dropout voltage = 1 V 40mA

This is the same rating for 10 and 18 volts Vsy so clearly 12 volt operatio n would be the same. The typical graphs show the output reaching within 2 volts of the high rail at 90mA, but in the simulation the output clips hard at that voltage at less than 66 mA.

It also doesn't handle capacitive loads so I'd have to deal with that. Thi s circuit drives cables. The LM8272 is rated to be stable for all capaciti ve loads.

At least they had simulation models in LTspice so it was easy to check.

Thanks for the suggestion. It does look like a nice part.

Probably a little "nicer / more expensive" than what you need... $1.50 in q uantities of 1500 at Digikey. Dual, 18V, 3mm QFN with thermal pad.

ent boost: add an NPN-PNP push/pull output, and take the op amp feedback fr om the output of the push/pull.

Oh yeah, you are right that I don't have space on the board for output tran sistors. I also don't know how easy it would be to work those into my circ uit. I guess they would just be on the op amp output before any of the fee dback and the output resistor. But this board is so tight, it would be a r eal pain (in the literal sense of the word) to try to squeeze in 8 more par ts, even tiny ones. This is already 5 kg of circuit in a 2.5 kg board.

I use a resistor from the op-amp output to the load, sized for 20 to 50mA at 0.6 volts, and add npn and pnp emitter followers around the resistor. Small sot-89 parts can do the job. E.g., dnls350y and dpls350y. You can put these three parts on the bottom.

I've already said I don't have space on the board. I'm not clear on your description "around the resistor". I may not be able to use the circuit on this board, but I'd like to understand it.

Pure class-B, without the crossover-killer resistor, is counter-intuitive and uncomfortable but seems to usually work.

In addition to the standard npn + pnp emitter followers, add a resistor bypassing them, straight to the output. This provides current for light loads. Select it for 600mV drop at nicely below the opamp's maximum current.

BTW, checkout small six-pin packages with one each npn and pnp transistor. And the resistor is optional.

Are you using the very smallest packages available for each part type?

With tiny packages, the op amp + NPN/PNP could work.

e.g. NPN/PNP in a 2mm DFN with thermal pad: PBSS4112PANP,115

Thanks for the suggestiongs, but DFNs are not magic in fact, they can be le ss space efficient than leaded parts like MSOPs. An IC doesn't just take u p space from other parts, it takes space away from routing and vias, those oh so essential vias. A part like a QFN or DFN typically are a solid block of obstruction in PCB layout. Nothing can go where the part is, no routin g can go under the part and no vias can go under the part. An MSOP allows routes and vias to be run under the package with only the pads actually tak ing up board space. If you have a similar sized part it can go opposite an d the two parts share the same space on both sides of the board with no fur ther routing restrictions.

The MSOP pads use about 4 sq mm or 8 sq mm if you add in a 10 mil clearance . The DFN approach would use more like 20 sq mm as the MSOP is a dual and the DFNs are singles.

The current circuit is working well. We've sold something like 10,000 unit s over the last decade or so. I just thought there might be a better op am p for this application by now.

The slightly extreme specs come from the IRIG-B120 application. Seems ther e is no spec for the electrical interface and there are systems that use up to 10 Vpp and there are 50 ohm systems. Not an easy range to drive, espec ially if you want both at the same time. lol I didn't make 10 Vpp, but I did manage to get 7 Vpp driving a 50 ohm load presenting a 50 ohm driving i mpedance with low distortion (14 Vpp differential into 100 ohms). Two chan nels of this on a board about 4.5 inches by 0.85 inches along with an RS-42

I wouldn't mind getting the output voltage range into 50 ohms higher, but t hat isn't a big deal at this point. I don't think they sell many for IRIG these days. Mostly they are sold for 600 ohm audio interfaces.

That opamp (and many others) is available in SC-70. Likewise many npn+pnp combos, although you may need the heat-removal pad.

And the op amp with the transistor is still half again the size of the exis ting design even after accounting for the space between the rows of pins. The SC-70 is small enough that while I can run two traces between the rows of pads, I can't put a via there. Pads require a 10 mil clearance to anyth ing a solder bridge might form to.

There is a reason why I used dual op amp packages. Eliminating two pins pe r package saves space and the MSOP package is routing friendly. I see the same issues between QFP and QFN packages. The QFP seems bigger, but all th at space underneath can be used for vias and routes. The QFP is a solid b lock with the thermal pad. So even though it is "smaller", it is no easier to use on a board.

Most likely I won't do anything with this design. The customer is asking a bout some changes in the input circuit that will just be resistor value cha nges. I was just looking to see if there was an easy way to get more volta ge swing into the minimum load. Looks not not so much.

To be fair, I checked my art work and it looks like I shaved the length of my MSOP8 pads a small amount. But I would not be comfortable doing that wi th an SC-70.

Does the crossover distortion show up in distortion tests? This unit it tr ying to stay close to "CD quality" as much as possible.