Biasing battery-powered opamps

For batteries, it's safer to use the backwards-MOSFET trick for polarity protection, i.e.

*-->>----------*----------0 | + |

----- | --- -----

----- -------- (to circuit) --- | A | | - | | | *-->>------* *-*---------0

(Fittingly, this was invented by Bob Pease.) It's sort of interesting: when the battery is in correctly, the FET turns on, and when it's in backwards, it turns off. The apparently-backwards connection of drain and source ensures that the drain-source diode is reverse biased in the off condition. (I use the old-fashioned MOSFET symbol because it's easier to remember the diode polarity that way.)

If you've got a big bypass cap, make sure to use a big enough FET, because otherwise the inrush may blow it up. The same bypass provides excellent protection to the MOSFET's gate.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics Electro-optics Photonics Analog Electronics

55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Except it wasn't "invented" by Bob Pease. Posted in an article by Bob, perhaps. But I did that in a battery charger controller for California Micro Devices almost 40 years ago. And I doubled-down... I used two FET's (back to back in the same tub) so I could stop current flow in either direction, upon command. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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I love to cook with wine.     Sometimes I even put it in the food.

It seems to have been invented more than once (as you'd expect, since it's simple and useful). Did your version have the body diode?

Hooking it up back-to-front so that the body diode doesn't interfere is really the essence of this trick. It was patented in 1988, so it's free as air now anyway:

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics Electro-optics Photonics Analog Electronics

55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

[snip]

Yes. My usage was source-to-source (two devices) so flow in either direction could be blocked: battery control operation, limiting discharge and stopping charging at a selected point.

As in...

and...

That's why so many patents are worthless. I tend toward keeping careful documentation of _private_ art. Thus, if anyone challenges my use, I point out usage X years ago ;-) The new proposed patent scheme, first to apply, is going to be a disaster :-( ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

How sensitive is this arrangement (or the corresponding p-channel on the other leg) to going pfut! from handling while replacing the battery? A tens of uF cap on the (to circuit) side feels like it ought to be enough but I cringe at the vision of random sparky fingers touching the battery connectors.

--
Rich Webb     Norfolk, VA

And so did I, some 15 years ago, without knowing that it had all been done before. And it was in a battery charger controller too! What a coincidence. But I may not have recalled it if you and Phil H hadn't brought it up.

Some time after I replied to Phil Allison's cautionary note, I'd started recalling a technique I thought up and used a few years ago (probably also an independent re-invention). It's related to the MOSFET circuit but uses a PNP transistor.

Just put a resistor in series with the gate... most any _discrete_ MOSFET will have enough gate capacitance to prevent a punch-through.

Even in my microchip stuff, the foundry calls only for 433 Ohms. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

The worst case is when you first turn it on, or first plug in the battery. In that case the bypasses all get charged up from zero, _right_now_, and that can easily blow up the FET if you aren't careful.

I like to use fairly high voltage FETs anyway, so their RDS_on is usually enough to make them safe. You do have to calculate it, though!

Some folks do it all fancy, with a series resistor and shunt diode to protect the gate. If you pick the RC time constant on the gate properly, that can limit the inrush too, as long as the capacitor gets discharged nice and fast when the battery is removed. If it doesn't, it's liable to be vulnerable, you're right.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics Electro-optics Photonics Analog Electronics

55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

It depends on what you're trying to minimize.

If I go with what you've described as needing NE5532 specs, then similar speced but lower current consumption would be Linear LTC624HV, LT1678 or National LM8261 would be good choices. A lower current consumption will be in most cases more valuable because the battery voltage will hold up for longer, but these amps also work to lower voltages than the NE5532 as well - making the specification of minimum supply voltage moot. So same quality and 2 or 3 times the battery life. (Needing 8 volts the LM833 is not going to be a good choice)

If I go on the spec of needing better quality than PA but not cutting edge audiophile quality (so say under 1% distortion as acceptable) and the need for a long established product then I would look at TL-072 , TL-062 (for duals. TL-0x4 for quads) - both will work (incompletely speced) to +/3.5. Supply current of 1.4ma per amp for the 07x or 0.2ma for the 06x.

To try and get the best of all worlds using the older parts I would try using TL-06x amps all through except for the final output stage where the NE5532 has much better drive capability. If I had a genuine problem with generated noise I would use a TL-07x for the first gain stage only. Or if your filtering stage was extra demanding then again substituting TL-07x for 06x.

Back of the envelope figures suggest the mixed configuration would run about 3 times longer on a 9v battery than an all 5532 configuration and would probably meet your quality requirements.

"Phil Hobbs"

** Yaaawwnnnnnnnnnnn....

Not true when it is a 9 volt type an the terminals will simply not connect to the snap the wrong way round.

A 1N4004 only handles the odd accidental reverse touch event.

... Phil

I've already mentioned the LM833 as being unsuitable for just that reason. The LTC6242HV and LT1679 (dual LT1678) look good technically. I'd prefer TH types but my main objection is cost. At the only online shop accessible to me, they cost the equivalent of US$5-6 apiece whereas I can get the NE5532 OTC for about $0.25-0.50. Noise is actually somewhat worse with the LTC/LT types than with the NE5532.

I'm aiming for 0.1% max THD. The TL07x JFETs are my workhorse opamps, but they're too noisy for the input stages and the TL062 is still too noisy even for subsequent stages. I don't have their specs for minimum Vs. Nice to know that they can work down to

+/-3.5V.

I haven't completely rejected your suggestion of the LT/LTC types. This isn't a one-off project for my own use, which is why I'm carefully considering trade-offs between the various factors. Perhaps a combination of LTC6242HVs and NE5532s may be a good compromise.

You might want to look at TI for op amps too (I didn't - TI is more "difficult" for me)

There is something wrong here. TL-071's have been used many times for Hi-Fi pre amps and mic amp. They are low noise and even lower distortion. The GBW of the TL-061 says it is useful for modest levels of gain at audio frequency and it won't slew rate distort at 3.5 v/usec.

Thanks for your continued interest. Terms like 'low' and 'high' are relative. As I said, the TL07x series is one of my standard workhorse opamps, along with the LM358 & LM324, and I've often used them for audio. But this application is for pro use with low level signals, and while the THD figure is great, the noise is not. Typical input noise levels at 1kHz for the opamps we're discussing are, in nV/Hz^0.5 --

NE5532 -> 5 TL07x -> 18 TL06x -> 42 LM8261 -> 15 LT1678 -> 5.5 LTC6242 -> 7

"Pimpom"

** Either of the above op-amps is more than good enough for professional audio use, with any signal that has gone past the mic preamp stage.

Gilding the Lily is common practice in pro audio.

Cos it is cheap to do and fools love playing numbers games.

.... Phil

Ah, but this *is* for the input stage of a mic preamp. I'd considered and rejected the superb INA217 for two reasons: cost (~$8.50 US each over here) and the fact that it needs a minimum supply of 9V. I'm considering it again, thinking I might as well go for 2x9V batteries as the power supply.

"Pimpom" "

** For use with a dynamic or condenser mic?

It makes a HUGE difference - cos the latter have incorporated pre-amps.

Neither the above ICs are much good with a dynamic mic - no matter how you do the circuit topology - UNLESS you use an input matching transformer.

... Phil

Another trick which can be used, is to use a separate low-noise front end circuit to get your first 10 dB or so of gain, and feed this into an op amp for further gain and drive capability. This would reduce the relative contribution of the noise of the op-amp stage, as it can run at a lower gain. If I recall correctly, there are a few op amps which are particularly friendly to this approach, in that you can bypass their own normal input stage and inject signals into a later part of the circuit (e.g. via a compensation input pin) and still close the feedback loop around the op amp.

One "trick of the trade" in audio, a while back, was to use the common

2N4401/2N4403 switching transistors as low-noise audio amps - although they are not spec'ed for this, they turn out to have a low base spreading resistance and are very quiet in linear applications. I've seen them used in this way in moving-coil phono cartridge pre-preamps and similar circuits. I'm sure there are better parts available these days, but these work very well in many such applications and are common and inexpensive.

Last year I built a preamp for a dynamic mic cartridge (on a communications headset) to use the mic with radios that normally need the higher output level of an electret cartridge. Like you, I considered (and bought) but then rejected the INA217, as I wasn't sure it would work reliably on a single 9-volt battery.

Instead I built a classic differential amplifier with a current-source tail... three 2N4401s and some passive parts. Works beautifully on a single 9-volt battery, and the balanced/differential input topology should give it very good hum resistance and (with a few ferrite beads included) good RF resistance. I've used it only a foot or so from my

5-watt portable radio's transmit antenna, with no sign of RF getting back into the audio.

--
Dave Platt                                    AE6EO
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18 nV/Hz^0.5 is low noise. In any sane configuration you won't hear the noise of the op amp - after that it makes no sense to claim that "this" one has less than half the noise of "that" one who's noise can't be heard. In most sane configurations you won't hear the noise of the TL-061 either but you need to apply it with a bit more care.