Pete Lefferts LED current source

I'd just do a BandGap ;-)

...Jim Thompson

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accorded

Me too. I'll take well rewarded vile obscurity any day :)

somthing like this maybe? United States Patent US3947753

-Lasse

Bandgaps need 20 dB of gain to get to the same voltage as the LED trick, which is why they're so noisy.

Cheers,

Phil Hobbs

Bandgaps need 20 dB of gain to get to the same voltage as the LED trick, which is why they're so noisy.

Cheers,

Phil Hobbs

as

For those playing along at home:

Dave.

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That doesn't have the temperature compensation feature--its drift will be the _sum_ of the LED's and the transistor's, because the reference is V_diode+V_be.

Cheers,

Phil Hobbs

Germanium? Use the Vf of a tunnel diode!

John

An excellent point, thank you Mr. Hobbs.

A second point is that the referenced patent (US3947753) itself references the Lefferts article of this thread in ED, from 1975.

Walt Jung

The Vf of a tunnel diode won't have the right temperature coefficient, I don't think, because the physics is different. I'm not a semiconductor expert by a very long way, but metal-insulator-metal junctions are almost temperature-independent.

With barriers of more than about 25 kT, the tunnelling probability is a weak function of the energy, so the contribution of the extra electrons above the Fermi level is pretty well cancelled out by the missing ones below. (If you're interested,

Cheers,

Phil Hobbs

What do you know of the physics of "buried zeners"?

...Jim Thompson

About zilch, I'm afraid.

Cheers,

Phil Hobbs

I was thinking of a classic germanium TD, the Easki diode. It's just a heavily doped, super-abrupt PN junction. I think they smashed an indium-wire mesh into a slab of germanium and started etching until just one tiny mesa was left.

John

Tunnel diodes are cool, I agree. When I was about 11 I was given a book of circuits, called something like "109 electronic projects for experimenters", that had an FM wireless mike that used a tunnel diode. I always wanted to build that, but never had the money for parts, which is why (somewhat later) I built a lot of tube stuff from old TVs, even though this was the 1970s and that was _not_ cool.

The reason the tempcos are so different is that the physics of tunnelling is different from forward conduction, and the thermal broadening of the Fermi level affects it differently. I did a bit of a web search and found the very low tempco of Esaki tunnel diodes mentioned---though no actual numbers. From the point of view of temperature coefficient it doesn't matter a great deal whether the diodes are made of semiconductor or metals and insulators.

Cheers,

Phil Hobbs

Cheers,

Phil Hobbs

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=A0 =A0 =A0 ...Jim Thompson

Phil Hobbs wrote,

" Noise is usually much more important to me than small amounts of drift, so I like using forward-biased LEDs for voltage references. You get a free pilot light out of the deal, too. Forward-biased diodes have a noise temperature of 150K at room temperature, so if you drive it reasonably hard, you can make a really quiet voltage reference this way. (I'm sure there'll be some raised eyebrows in this group over a 'voltage reference' with 150 uV/K drift. Take it up with Maxwell's Demon.)"

Darn, You mean I can make a quiter voltage reference? To get a 1nV/ rtHz voltage reference I've been filtering the piss out of the standard IC voltage references (buried Zeners) These have a great TempCo's but 100nV/rtHz or greater noise.

Say, I tried to "derive" the stated 150uV/K drift but I came up with numbers an order of magnitude bigger. Is this the difference between the drift of the BJT and LED? The Maxwell's Demon comment makes me think it is something more fundemental.

George Herold

PS. thanks all for the nice discussion. I think I "got" about half of it. The rest can be mulled over at my leisure G

Yes, you can make a quieter low-voltage reference--a good 20 dB quieter than a bandgap, if you're willing to spend some current. The noise model of a PN junction is full shot noise (sqrt(2eI) in 1 Hz) in parallel with the differential resistance of the junction, which for an ideal diode (e.g. a diode-connected transistor) is kT/(eI). When you combine those two formulas, and compare it with the Johnson noise formula, you get the useful result that T_noise = T_j/2.

Real diodes are a bit noisier than this because although they're reasonably exponential, the constant in the exponent isn't kT/e (25 mV at room temperature) but a bit higher, 35-50 mV in most small signal devices. That means roughly that if you spend a milliamp in the transistor and a milliamp in the LED, you can make a 1V reference whose noise is on the order of 1.5 nV in 1 Hz, including both the diode and transistor noise.

Bandgaps work by adding a proportional-to-absolute-temperature (PTAT) voltage to a V_BE drop, and adjusting the total voltage until the drift cancels, which happens at about 1.22 V, the zero-temperature band gap of silicon (hence the name). (IC guys like Jim and Walt know lots more about bandgaps, including stuff like higher-order corrections for the curvature of the V(T) curve, but I'm a noise guy.) The PTAT voltage comes from the DeltaV_BE between two transistors running at roughly 10X different collector current densities, which comes out to roughly 60 mV at 300K. That DeltaV_BE has to be amplified by about 10 times before being added to the V_BE drop to make the 1.22V output voltage, and that's where the problem is.

The absolute noise level on the 60 mV is reasonably low, assuming that the collector currents are the same (i.e. the current density ratio comes from scaling the device area rather than the collector current). The V_BE is quiet too, but unfortunately you have to apply about 20 dB gain to the DeltaV_BE portion, which makes it very noisy. Bandgap designers also work under a lot of pressure to reduce operating current, which doesn't help the noise one bit.

On the other hand, you can get far better temperature stability with a bandgap, and if you really need to, you can combine 10 of them to reduce the noise.

The 150 uV/K number is a SWAG for how closely you can expect the two temperature coefficients to track each other. You can adjust it a bit by changing the diode current--lower current equals higher drift. In general the LED/emitter follower trick is great for most things except A/D references.

I haven't done a 1/f noise comparison between the two kinds of devices, but V_BEs generally have very very low 1/f corners.

Cheers,

Phil Hobbs

nts

Thanks Phil! I now have a new circuit to try some day. I wish there was a means to "buy you a beer" via the web.

The PTAT and diode discussion "speaks" to me. I'm using a diode connected transistor (2N3094) as a temperature sensor. This has two modes of operation. First one can run a constant current (10uA typ.) through the diode and measure the voltage drop. You calibrate it at a few known temperatures and interpolate in between. You can then also look at the forward voltage drop at two different currents (at the same temperature), the "slope" is given by the thermal voltage kT/ e. The sensor is almost self calibrating. For the 2N3904 I find the error is a little less than 1%. With the predicted temperature one or two degrees higher than what is measured by some other means. I assume this is the non-ideality factor of the transitor. I wonder what causes this non-ideality and if there is a different transistor (or diode) that is more ideal?

George Herold

A 1% error might be coming from beta nonlinearity--it's I_C that's exponential, not I_E. You might get better results with something like an MPSA18, which has a huge beta, or a 2N5086, which IIRC has amazing beta linearity. It might also be self-heating or the extrinsic emitter resistance, if you're running too much current. The sweet spot for that sort of measurement is around 100 uA or a little bit below. There are circuit things you can do as well, e.g. drive the base from a follower and measure V_BE rather than V_CE.

Cheers,

Phil Hobbs