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Old 29th August 2012, 03:36 PM   #491
gpapag is offline gpapag  Greece
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Quote:
Originally Posted by jneutron View Post
You spoke of hotter temperatures changing the thermal response/deviation of the circuit because of the lower ability to transfer heat outside the system. This of course impacts the absolute temp of the junctions, but not the differential between chips, which is the discussion.

I did point out that silicon's thermal resistance/conductivity is a function of temperature, as is BeO's. Other than that effect, the thermal response of the system is not dependent on the sink temp, because the heat sources are not.

jn
jh
I was talking of temp. differences that are building up between dynamically self heating (I*R) components which are inside the same IC chip. The same thing that you were referring too here:

Quote:
Originally Posted by jneutron View Post
2mv untrimmed...wait till you start finding thermal response issues in the 10 to 100 millisecond realm..then you'll come crying to the die people. Plastic just doesn't have a high diffusion velocity...
jn
I wanted to point out that these local delta T will increase when one decides to heat the whole IC in an attempt to control the DC drifts of the IC.
Is there an agreement on this?
(bcarso says yes )
Quote:
Originally Posted by bcarso View Post
Indeed, it is a seductive but misleading notion per se to suppose that running warmer will reduce the importance of fluctuations.

George
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Old 29th August 2012, 04:16 PM   #492
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Hitsware, you're not far of the mark. For many decades, bipolar transistors were considered BETA DRIVEN devices. I spent my first 5 years designing with that approach.
It it worked pretty well, because in many, many applications, the design WAS beta driven. It is also true, today, more than many realize.
As betas increased from let's say 20 to 100, then to 200 or more in many cases, then Vbe became significant, AND the real gain of the circuit was not limited by beta, but by the intrinsic Gm. The intrinsic Gm is .04mhos/ma. It goes up or down directly, depending on the quiescent current through the part.
So, 1A would have a Gm of 40 Siemens (or mhos)
In the early 70's the professors decided that beta was just a parasite and every attempt has been to ignore it.
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Old 29th August 2012, 04:21 PM   #493
bcarso is offline bcarso  United States
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Quote:
Originally Posted by hitsware View Post
I was being facetious. (I thought)
What little education I have
(USN Electronics Technician and Occupational electronics
(~ 30 years ago)) both used hfe as the main parameter
with bipolars. AND that xsistors are current controlled as
opposed to tubes being voltage controlled.
(I'm not arguing the point, only stating what I was taught)

hfe = delta Ic / delta Ib (IIRC)
re ~ 22 Ohm (small signal device)
input Z = re + (emitter resistor * hfe)
That re is at about 1mA Ic, typically, (I usually use 26 ohms as a rule of thumb [edit: Curl above, 25 ohms]) and reciprocal to that current, to a point. So if you want to have the series external emitter resistor dominate the effective transconductance, make re relatively small (or simply reduce the current, but this may affect distortion negatively).

BTW, the noise contribution from that "resistance" is due to shot noise in the current referred to the input as a voltage noise density. Since the transconductance goes up directly with current and the shot noise goes as the square root, for low source impedances the overall noise goes down with increasing current, to a point that, among other things, is limited by the real thermal noise of the base spreading resistance. The contribution of the shot noise/transconductance makes the expression for the re contribution look like about that of a real resistor half the value of re, which is nice.

It's funny to see people using very-low base resistance transistors at moderately low currents. The Marshall Leach-style common-base MC stepup circuit is an example --- but in his defense, he was also interested in prolonging battery life.
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Old 29th August 2012, 04:23 PM   #494
bcarso is offline bcarso  United States
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Quote:
Originally Posted by john curl View Post
Hitsware, you're not far of the mark. For many decades, bipolar transistors were considered BETA DRIVEN devices. I spent my first 5 years designing with that approach.
It it worked pretty well, because in many, many applications, the design WAS beta driven. It is also true, today, more than many realize.
As betas increased from let's say 20 to 100, then to 200 or more in many cases, then Vbe became significant, AND the real gain of the circuit was not limited by beta, but by the intrinsic Gm. The intrinsic Gm is .04mhos/ma. It goes up or down directly, depending on the quiescent current through the part.
So, 1A would have a Gm of 40 Siemens (or mhos)
In the early 70's the professors decided that beta was just a parasite and every attempt has been to ignore it.
JC, reminds me that Putzeys recently opined that triodes were the only real voltage-controlled voltage source
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Old 29th August 2012, 04:31 PM   #495
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Bcarso, I think you will find that the actual voltage noise derived from Gm is Re/2. The derivation is not obvious, but at least it does track Gm.
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Old 29th August 2012, 04:38 PM   #496
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Quote:
Originally Posted by hitsware View Post
47 Ohm or 47 kOhm ?
I already apologized for the typo. I meant 47 Ohm, but if to consider non - zero emitter resistance and non - infinite beta 26 Ohm indeed would be closer. But input resistance will be no higher than 10K, so of course you can't substitute directly in all cases.
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Old 29th August 2012, 05:27 PM   #497
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Quote:
Originally Posted by john curl View Post
In the early 70's the professors decided that beta was just a parasite and every attempt has been to ignore it.
Fairly quaint way of putting it. Translinear circuits work even at beta of 1, oh well we've been over this before no need to repeat.
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Old 29th August 2012, 05:30 PM   #498
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Originally Posted by gpapag View Post
I wanted to point out that these local delta T will increase when one decides to heat the whole IC in an attempt to control the DC drifts of the IC.
Is there an agreement on this?
(bcarso says yes )

George
I have hilted in red the salient point we disagree on. The local delta T will not increase as the entire widgit increases in temperature.

The local delta T will be dependent on the heat flow between the components of interest, not on the absolute temp. As I pointed out, if the thermal conductivity between the local chips changes as a result of absolute temperature, so will the delta T between them.


I believe bcarso and I are agreeing, and we think differently from you.

Perhaps when I get the time, I could draw it up, verbal descriptions are not easy to express nor follow at times.

cheers, jn
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Old 29th August 2012, 05:36 PM   #499
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Quote:
Originally Posted by ief View Post
-noob allert-

Today got a new pre-amp and that always makes me highly interested in what exactly I have but knowledge is almost non existant I'm afraid. Still I hope it would be ok for me to post this here since this thread seems a good way to see if I 'get' some of the stuff discussed. (perhaps some credit for going through the whole thread? )

If this is not wanted and/or to OT just ignore please.

!
The JFET and two bipolars are a very high input impedance unity gain buffer that probably works pretty well, but is necessarily single ended and needs AC coupling. The high impedances allow smaller caps and larger resistors to be used so at least they don't have to be big electrolytics everywhere.
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Old 29th August 2012, 05:41 PM   #500
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Quote:
Originally Posted by jneutron View Post
I have hilted in red the salient point we disagree on. The local delta T will not increase as the entire widgit increases in temperature.

The local delta T will be dependent on the heat flow between the components of interest, not on the absolute temp. As I pointed out, if the thermal conductivity between the local chips changes as a result of absolute temperature, so will the delta T between them.


I believe bcarso and I are agreeing, and we think differently from you.

Perhaps when I get the time, I could draw it up, verbal descriptions are not easy to express nor follow at times.

cheers, jn
George if you diagram it using (I know over simplified) the circuit analogy of input power as current, ThetaJ as a resistance, and temperature as voltage, you will see the point. Raising ambient ("ground") does not change the delta V's anywhere.
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