I was always under the impression that beta depends on Ic???
Confirms your own statement that beta depends on a lot of things
Not in a predictable way, it doesn't. It does droop at very high I_C
due to high level injection, but that's all that's really predictable.
At lower current, different devices of the same type can have beta that
goes up with I_C, goes down with I_C, or is flat to within 10% over two
orders of magnitude of I_C.
Well, it is early morning, I will save the word salad for dinner.
What's so hard to understand? A resistor R in series with the base
provides the same negative feedback as R/beta in series with the
emitter, except for the effects of shunt capacitance and beta
nonlinearity. (*)
Yes, that is what made me say that..
For reasons you already mentioned I try to stay clear of GOhms.
Once at an university designing electronics for some experiments I did notice
that the average? chemist does not have any fears of let's say orders of magnitude...
I give you you are doing beautiful work at the 'edge' of what can be done.
I did study your transistor tester but I still have fear building things with that high resistors,
especially if it needs to be precise.
If normally somebody would show me such a thing for a design review I would suggest looking
if there was an other way..
Moisture, high humidity, you'd need a sealed box too.
Oh, I'm with you there. I certainly wouldn't do it for anything but a
lab one-off, because it takes too much babysitting.
I was trying to measure both log conformance and beta linearity in one
go, to pretty good accuracy. The usual trick of using tee networks in
the feedback loop plus really really low offset amplifiers wouldn't work
because chopamps all seem to have about 200 pA of input bias current. I
could have used a charge dispensing loop, but it's a lot easier to store
samples of a voltage. The right approach would have been to use more
complicated MUXing and a bunch of online calibration, but I'm not as
fast at MCU stuff as you are, and since the job was for a university
group with limited funds, I really wanted to keep the hours down. It
works fine as long as it's me driving it.
Probably most of us have a bunch of those one-offs lying around that
we've built over the years, and they come in really handy sometimes.
(Of course I also have a Keithley 405 Micro-Microammeter that I got for
$5 on eBay--it has a 100 fA FS range, but takes a good two hours' warmup
to get down that low. It's really fun to use, though--electrometer
tubes are actually pretty amazing.)
I bought a Keithley 602 for very cheap, mostly for the meter, range
switch, and box. All solid-state, several years newer, 100 times less
sensitive. I'll put new guts in it one of these days when I'm motivated.
+ | c video -------- b NPN +1 to +2V e | [ ] 75
75 Ohm coax 10 meter
|----------==============================================-----------||---
amp | | |
| [ ] 1k /// /// [ ]
75 | |
/// ///
You don't even need the resistors on the BJT side--it'll drive the
coax fine with all of the DC load at the far end.
The 75 Ohm on the left,.. somehow you need to match the cable
impedance.
Nah, as long as the far end is properly terminated, the near end looks
like a 75-ohm resistor anyway. It's the dual of
series-termination--short circuit on one end, Z0 on the other. Not a
lot of circuit protection, of course.
Will have to play with that is spice one day,
did it but only for frequency sweep, that was OK (used this several times as video output stage).
But when I had to design a big thing to drive RGB into long cables I decided to use very expensive opamps
with huge bandwidth that did drive symmetrically... just to be on the safe side.
No LTspice in those days, IBM PC was a new thing...
IIRC Horowitz & Hill suggest this method, with another resistor in the
collector to provide current limiting. I don't think I've ever actually
used it, but series-termination is dead useful. One good thing is that
it won't oscillate with an unterminated or short-circuited cable,
because the transistor won't be in normal bias.
Cheers
Phil Hobbs
(*) Yes, it's technically R/(1+beta), but up at frequencies where the 1
matters, beta has a serious phase shift, so it's rarely needed.
Designing accurate current mirrors is one of those rare places.
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net