Re: Bipolar transistor transconductance

On Sat, 28 Jul 2007 18:15:33 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 28 Jul 2007 17:51:33 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Sat, 28 Jul 2007 17:33:21 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 28 Jul 2007 17:19:25 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Sat, 28 Jul 2007 17:09:11 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 28 Jul 2007 16:52:12 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Sat, 28 Jul 2007 16:29:06 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

[snip]

For a fairly ideal transistor, Gm = 40 * Ic.

So it's a pretty big number, and temperature-dependent?

Fraction of a mho typically, and it varies with temperature only if
the dc collector current depends on temperature.

Certain parties will predictably nit-pick.

John


John, Are you going to call me a nit-picker if I point out that...

"40" = q/(kT)

So your temperature-independence statement is dead wrong. You get
flat gm if IC _varies_linearly_ with temperature.

...Jim Thompson

You mean with absolute tempearture? -273 is a long distance away, and
transistor betas vary by 5:1 or more on the datasheets, so it hardly
matters. Calcs like this don't have to be right by even 2:1, unless
you are designing ICs, which I don't think we are.

If I'd filled up a screen with equations, I'd be "right" but wouldn't
convey a lot of understanding. Having designed electronics for 40
years or so, transistor math at this level is plenty good enough.

OK, how about "hardly varies with temperature"?

John

BS, John! You're wrong! Beta doesn't have anything to do with gm
except in a very miniscule way...

gm = q/(kT)*ß/(ß+1)

I just posted an explanation of why beta doesn't change Gm. b/(b+1) =
1 for all practical purposes. Nit picking.

Who the hell is going to design something that needs precise beta *or*
Gm? I do most transistor calculations in my head, to 10% or so, and
everything just works.


For some reason all of my TVG's in sonar's and ultra-sounds have bias
current varying with temperature... and have beta correction as well.
I posted one for Fred once when he was bragging about his analysis
skills. I've yet to see his analysis.

Doesn't matter. Even if he can do analysis, he can't design, so he has
nothing to analyze.


The ratio between -40°C and +140°C is (273+140)/(273-40) = 1.773, not
inconsequential for REAL designers ;-)

Inconsequential when beta and temperature-tolerant design is done.

Again, we're not designing ICs. The level of approximation that I use
is plenty adequate for discrete circuit design, and I have many other
fish to fry. I suppose the level you use is adequate for IC design.
Neither is correct.

John

I design those sensitivities out because I can... you have to live
with it, and do ugly things like have to use uP's to do
auto-calibrations ;-)

...Jim Thompson


Last delay generator I did, I measured the pcb temperature with an
LM71 SPI temp sensor. The uP does three different temperature
compensations, two linear and one polynomial, and drives 10 different
dacs. Among other things, the through-the-box delay tc was reduced
from 40 ps/K, mediocre, to something like 3, hard to measure.

CMOS has a rotten delay tempco. I suppose one could program Vcc versus
temp and help a lot. I'll have to try that some day.

Really, all this uP stuff is liberating. I can poke all these
compensation coefficients into a calibration table that's saved in
eeprom, and change them any time it might be needed. That sure
pipelines the design process, sort of like zener-zapping on steroids.

The downside, of course, is that eventually you have to write a bunch
of damned code, which is what I'm supposed to be doing this particular
instant.

John


I know, I be yanking your chain (as if you haven't been yanking mine
;-).

I just finished a chip that features auto-cal/auto-zero before _every_
measurement. It doesn't use a µP, just a timing chain to sequence
analog events.

I even find myself offering clients digital solutions to their analog
problems ;-)

Auto-cal, auto-zero, and auto-DC-restore can be really nice features
;-)

...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 |

America: Land of the Free, Because of the Brave
.

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