Re: Pete Lefferts LED current source

On Oct 23, 5:06 pm, Phil Hobbs
<pcdhSpamMeSensel...@xxxxxxxxxxxxxxxxxx> wrote:
ggher...@xxxxxxxxx wrote:
On Oct 21, 8:07 pm, Phil Hobbs
<pcdhSpamMeSensel...@xxxxxxxxxxxxxxxxxx> wrote:
I've used the LED/BJT follower trick often--I first saw it in probably
1980, in an app note describing a really quiet battery-powered mic
preamp.   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.)

Cheers,

Phil Hobbs- Hide quoted text -

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- Hide quoted text -

- Show quoted text -

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
.

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