# Where's my degree

Discussion in 'Electronic Design' started by Fred Bartoli, Apr 16, 2007.

1. ### Fred BartoliGuest

Ok, this is probably a fraction of a degree.

Hunting for a supposed thermal offset in a circuit I set up this little
contraption to get/verify some figures but I fail to see anything...

+40V--+-------+----.
| | |
.-. |/ .-.
clamp | | |> | |
level '-' | '-' 100n
| Q3 | | ||
GND '----+-------||---+---->
| || | 100uV/div
|/ .-. scope input
+25.7V >--| Q2 | |
|> | |1G
| '-'
| |
.-. GND
Rc| |10K
| |or short
'-'
|
|
|/ Q1 = TUT
+5.7V >--|
|> 20K
| ___
+---|___|-.
| R2 |
R1.-. |
2K| | +-||
| | ->||
'-' +-||---< pulse
| |
GND GND

Q1 is the TUT (transistor under test), Q3 is there to clamp the signal
amplitude and avoid thermal distorsion in the scope stages.

With Rc=short Q1 dissipates 5mW or 5.5mW depending on the pulse input state.

Tested Q1 are some TO92 small signal devices, like BC548-9, 2N3904,
2222, ...

Pulse has been varied to check for thermal effects.
Say Tp= 50ms, duty ratio = 20%.
Looking at some moto datasheet for the BC558 we see the transient
thermal resistance to be 0.25*200K/W = 50K/W
500uW*50K/W*2mV/K = 50uV.

I should see it, but nada. Down to the uV level.
Strange...

2. ### john jardineGuest

"Fred Bartoli"
How's that clamp transistor work?. I can see the need for one due the
big -0.5V step but can't get my head round how it's actually clamping.

3. ### WinfieldGuest

Nice drawing. Can you run some numbers for the lazy reader? We
see a very small change in power dissipation, from 5.0 to 5.5mW,
which we imagine won't cause much temperature rise in a device
that can handle 500mW or more (is it 0.1 or 0.2C?). And we see
a current sink with 5V across the CS-setting resistor(s), and a
by-comparison microscopic change in Vbe from the aforesaid small
change in temperature (about 0.1m/5V = 25ppm?). Just how much

There's gotta be a better way!

Thanks for the diversion, hav'ta get back to the taxes!

4. ### Tony WilliamsGuest

Snip diagram.
20V and 2.5mA is 50mW?
I could not find a data sheet for the BC458 (npn) with
a transient thermal resistance graph, so is this why
you used the BC558 (matching pnp) data sheet?

Looking at the Semtech data sheet for the BC558,
with a tp of 50mS and a tp/T of 50/250mS, rth-A looks
So it looks more like a 5mW step and 180 K/W.

Would it be better to step the dissipation by
stepping the 25.7V on Q2's base (by 2V or so)?

5. ### Fred BartoliGuest

Tony Williams a écrit :
Arghh. You're right, and 250uA.20V is 5mW, so it's even worse than I
thought.
I used:
http://www.onsemi.com/pub/Collateral/BC556B-D.PDF

The way they phrase it isn't very clear, but I think the abscisse time
is the pulse duration, not the repetition rate (which wouldn't make
sense for single pulse BTW).

Also, here we are not interested in peak temperature, rather in
temperature excursion, so we have to use the single pulse curve, not the
ones with duty ratio as parameter. This temperature excursion will ride
on an elevated average temperature.

From this, I read the 'excursion Rth' to be Rth_ex = 0.25*Rth = 50K/W

I make it 5mW and 50K/W, so it's now 500uV that are missing.

Don't know why it should be better, but it'd be less simple.

6. ### Tony WilliamsGuest

Thanks.... might as well sing off the same songsheet.

[snip]
Because you are currently looking for a small slope
after a relatively large current step (10%), plus
the change in Q1 current must produce a small change
in Q1's Vbe,

Stepping the 25.7V can produce the same change in
dissipation without having any Q1 current step, maybe
a small front end transient only.

It's not that difficult. Put a 2222 ohm resistor
in series with the base of Q2, and then move the top
of the 20k to the base. That should step Vb(Q2)
from 25.7 to 23.13, or by -10%.

7. ### James ArthurGuest

But only during the pulse. The measurement, AIUI, is made
afterwards. Stepping Vce would be cleaner then, since it doesn't
directly affect Vbe, but the error from briefly stepping i(e) isn't
great.
Since he's trying to measure Vbe(Q1), why not take the 'scope signal
directly from Q1's emitter? That avoids the need for clamping (not
knowing the clamp level, Q3 bugs me).

And, if he then steps Vce as you suggest, Q1's emitter gives the
error signal directly.

Best regards,
James Arthur

8. ### Tony WilliamsGuest

Buggerit. You've just trashed my next post.
Let's connect Q1's base to +0.7V (using a Vbe at 2.5mA)
and the bottom end of the 2K to -5V.

9. ### John LarkinGuest

Yeah, that's clean:

Vc
|
|
|
c
+5---r1---b
e
|
|
|
+------- scope
|
r2
|
gnd

Step Vc from, say, +10 to +20. That will change power dissipation and
the delta-Vbe waveform should say a lot about the junction thermal
time constants. R1 is maybe 33 ohms, just to keep it from oscillating.

John