Vbe stuff

Well, I did explain it and as simply as I can. I must bow out, now. Best of luck.

Jon

Assuming T is fixed is ignoring the elephant in the room. T is the most significant variable by far, that is why diode connected transistors or Vbe multipliers make such good temperature sensors, especially if you cancel out the poorly controlled Is with the two current method which has been discussed here extensively.

Pease just threw about some typical values for near room temp, entirely correct as far as he went but not even close to a complete analysis. I see no reason to expect to duplicate his approximate typical values with any accuracy, and I am sure he did not say you could. If you want the complete analysis, read the book, Barrie Gilbert knows the subject fairly well. Vbe is in fact very well behaved and predictable within well understood limits, I have used the effect many times in 6-decade log amplifiers, multipliers and temperature sensors with no difficulty. But then I read the book :-). IMO a few SED posts are not a good subsitute for reading a comprehensive reference book written by experts if you really want to understand the subject.

Robert, Post your lash-up. You must be doing something terribly wrong.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Are you referring to these Pease articles:

What's All This Logarithmic Stuff, Anyhow? Bob Pease | ED Online ID #6068 | June 14, 1999

What's All This Logarithmic Stuff, Anyhow? (Part 2) Bob Pease | ED Online ID #11301 | November 7, 2005

Just curious if we are discussing the same articles here.

Glen

Methinks Robert is soldering on resistors to set current ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Several things come to mind in your comment, Glen. (1) is the fact that the T in the equation provided above actually isn't the only effect that leads to using these junctions for temperature -- Is itself is a function of temperature, enough so that it actually reverses the sign of the rate of change. So the above equation doesn't tell you the picture there. I can post the Is equation, if you'd like (already done it before, here.) (2) at Vbe of a few tenths of a volt and currents in the tens of microamps, self heating of the die shouldn't be a problem, yes? And we are starting at around 300K, after all, which means even for dT of 20K, we are talking about 6-7%. (3) Is has a huge effect when comparing Vd of diodes and transistors. (4) 'n' varies widely in diodes -- very widely.

So, would you please elaborate why you say that T is the most significant variable?

Jon

Because it's in an exponential function that defines IS ?:-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Might as well post it:

Is(T) = Is(Tnom)*(T/Tnom)^3*e^[-(Eg/k)*(1/T-1/Tnom)]

The derivative of this with respect to T, combined with the kT/q effect results in the -2mV/K net slope circa T=300K, yes?

So yes, there is a 3rd power there and a power of the inverse kelvins. Good catch!

Still, we are talking about such little power for self-heating and the likelihood of ambient temp changes that are beneath the (87-59)/2 or

14K difference needed. On the other hand, it's possible that handling the parts by hand could do that. 37C internal body temps might, through the fingers, create a useful difference between measurements I suppose.

More to the point, though, this still doesn't deal with the basic Vd differences in absolute magnitide, only the rate of change. It's almost dead certain, from my limited experience, that this is due to Is differences. Diodes very often have much larger Is than BJTs, from my meager experiences.

So Jim, do you really imagine this is the most important explanation in this case? My own bet is on the huge differences in Is and the likely difference in 'n', possibly combined to a lesser degree with T (granted.)

No matter what, I'll learn something from this discussion.

Jon

Just to make sure we are talking about the same thing, the question as I understood it is, for a given (forward) I through a diode at "room temperature", what is the resulting voltage, Vbe? I will take "room temperature" to be between about 65 and 85 F, (my winter/summer thermostat settings), or 291 to 302 K. Since we are talking about Vbe as a hopefully useful and repeatable function of I we should restrict the discussion to diode connected small signal transistors having decent gain over the range of currents considered.

Considering the approximation

I = Is(e^qVbe/kT - 1)

I = is the collector current. Is= Emitter saturation current, typically well below 10^-13 amps in a well made silicon transistor. Very temperature dependent. q = The unit-charge constant, 1.602?*10^-19 coulombs. Vbe = Base-emitter voltage. k = Boltzmann's Constant, 1.38062*10~-23 joules/deg K. T = T-absolute, K (273.15 at 0C).

Then (neglecting the -1):

Vbe = kT/q ln(I/Is)

Vbe varies directly with T, at approx 2 mV per degree K, or about 22 mV over "room temperature"

Leaving the question, how much does Is cause Vbe to vary. Is is not included in many transistor data sheets, but from my notes I have a few data points at 25 C for some transistors commonly used for logging (source unremembered):

Tr Vbe @ 1 mA Is MAT01 .63 2.2 * 10^-14 MAT03 (PNP) .585 1.3 * 10^-13 LM194/394 .58 1.6 * 10^-13 MAT02/04 .57 2.3 * 10^-13

So, with around 60 mV variation due to Is it looks like you were right! But IME with the transistors I have used for temp measurement such as 2n2222a or 2n3906 the part to part variation due to Is has been much less than the variation over the range of "room temperature"

- I think the MAT01 comes closest to your typical high gain small signal transistor; the others listed have relatively large dies for low bulk resistance resulting in better log conformance at higher currents.

But any way you look at it, T cannot be ignored even over the range of "room temperature".

Glen

I do not think you said why the discrepancy.

All well and good, but he stated a value and used the word "fact". Maybe i should re-do the measurement with the collector open...

As far as using a transistor with Vcb=0, with care and selection, one can approach 11 decades of useage.

Very crude; the base and collector leads were twisted together and poked into the "sideways: hole in the banana connector which then was cinched up; the emitter lead was treated in a similar manner to the other banana connector of an HP5326B in DVM mode. An adhustable HP6228B supply in series with a fairly large resistor and current meter supplied the current to the lash-up at the banana connector plugin holes. When i have time, i will try with the collector open.

Look at:

Where did you get that figure?

11 decades is 1pA to 100mA or 10pA to 1A.

1pA is dangerously close to Is for good linearity.

100mA, let alone 1A, will get you into trouble with self heating and current crowding.

--
Thanks,
Fred.

Thanks for the proper article reference. Two things come to mind as possibilities: Pease did say he was discussing the characteristics of a particular transistor ("This transistor ..."), and in the 5th from last para discussing "what other factors" he mentions a Vbe difference of "with monolithic npn .. only 5 or 10 mV", where variations in Is would be the primary cause of differences as I now see it, and the differences would be larger between devices not on the same die - up to 60 mV just between a few transistors specifically designed for logging, see data in another post. Secondly, your leads are fairly well thermally connected to the power dissipating and presumably somewhat warm HP5326B; I think you need to thermally isolate and preferably closely control or at least accurately measure the temp of your DUT.

Don't do that, you will further confuse yourself ;-)

What is it that is still troubling you?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Aha! The most important part, for you at least, is in the first few paragraphs... "IS" is NOT a constant, but is a rather hairy function of temperature.

The rest of it is typical Pease-eisms... off the wall bull-crap. He should stick to talking about mountain climbing ;-)

The only important thing to remember is that DELTA VBE is well-defined.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Pease knows that but he has to simplify things for the masses of bull-crap people...