I agree that most discrete circuits are simple things, and that one normally thinks about beta more than about delta-Vbe.
I still say that the transconductance mechanism is more fundamental, because you can explain the current amplification by the transconductance (with recombination), but you can't explain the transconductance by the current amplification. The reason that beta varies all over the place is that it's the reciprocal of a small loss term that depends on a lot of fine processing details.
The OP's confusion seemed to be that he didn't understand the voltage feedback mechanism in an emitter follower, though, and seemed to think that current amplification alone was an adequate basis for understand a BJT.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
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hobbs at electrooptical dot net
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On a sunny day (Mon, 19 Aug 2013 12:07:44 -0400) it happened Phil Hobbs wrote in :
After I posted that (sure eat your popocorn Spehro) I thought it is a bit of chicken and egg, or religious thing too. What was first, the voltage or the current? We are talking electons.... little balls.... that move (never mind the holes and golfing).
Beta is a variable, depending on many things, but it is an essential design parameter. The Vbe is _also_ depending on many things. But if you want any kind of linearity, you need to drive the transistor (look at the transistor) as current amplifier. Driving a sine voltage into the base emitter junction will not produce a sine wave form in the collector current. Modulation Ib with a sine _will_ much more result in sine in Ic.
Yes, yes. A classical one is the emitter follower cable driver...
This always bothered me: + | c video -------- b NPN
I don't agree at all. Apart from I_S, which is a more-or-less constant device parameter, V_BE depends only on fundamental constants and the junction temperature, and at lowish currents, it follows the Ebers-Moll curve to absurd accuracy. Beta depends on everything including what the silicon crystal-puller had for lunch.
You can explain the current gain by the transconductance mechanism (electrons being emitted over top of the potential barrier between base and emitter, a few recombining in the base), but you can't explain the transconductance from the current gain.
That's one approach, but there are lots of others, e.g. current mirrors, negative feedback, and translinear things like using junctions for collector loads.
For the same output swing, transconductance works fine. If you want better performance, put in an emitter resistor. An emitter-degenerated transconductance amp is far more predictable than a beta-dependent circuit, for one thing, and for another, the beta linearity of a BJT isn't necessarily anything nice. To do a good job, you're going to need feedback components either way.
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.
Cheers
Phil Hobbs
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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
hobbs at electrooptical dot net
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On a sunny day (Mon, 19 Aug 2013 14:40:52 -0400) it happened Phil Hobbs wrote in :
I though: "Now he will come up with bandgap reference..."
What you say is correct, I look at it maybe from an aplication POV.
And you more from the scientific POV.
Yes, but junctions for collector load is also driving current, Zc is [almost] infinite impedance.
I am not convinced about that, first putting in an emitter series resistor to get better linearity is IMNSHO not a very nice solution, but it will work of course.
In the same way you can put a collector-base resistor as feedback (I do that a lot to get the DC working point, correct Vc).
That depends, take an RF amp, AGC done by changing Ic, signals are small, no feedback.
I will tell you a story, told it before here years ago, I was designing (1968 or so) a pre-amp for a vidicon camera. This is interesting, as you have the giga Ohm fetish, so maybe it will make you think.
The usual way they do this (industrial espionage) is make a very high impedance amplifier, and then use feedback. A vidicon target electrode is a very high impedance, really (so voltage drive).
I tried that, but had endles problems with interference pickup from the scan coils.. Experimenting I accidently shorted the vidicon target to the base of the second amplifier stage, a normal BJT, and saw a perfect picture for a moment. Curious I just removed the high impedance input stage and used the target electrode as currect feed into the base of that transistor. Absolute gain is not that important (light level changes constantly), but the low impedance presented by the base shorted, got rid of, all the interference, current drive! No feedback! Amazing picture!, Shorted the target and wiring capacitance out too, good resolution. I left it that way...
For those who have a GOhm fetish...
The 75 Ohm on the left,.. somehow you need to match the cable impedance.
But that's a transimpedance effect, not a current-gain effect. If beta is constant, changing I_C doesn't change the gain of a current-mode amplifier.
A base resistor functions a lot like emitter degeneration, except it isn't as predictable because beta varies orders of magnitude more than g_M for a given collector current.
Good news.
I don't like using huge resistors like that, for all the reasons you give, plus the fact that they make everything slow. Even using all sorts of bootstrap-driven shields and stuff, it's still slow.
The only reason to do it is if there's no other way to get the signal-to-noise ratio you need. Even with picoamp things, I try to use noninverting bootstraps rather than giga- or teraohm resistors wherever possible. With photodiodes, for instance, if you make a sufficiently good bootstrap, you often don't need a TIA at all--just take the output from the bootstrap. (We can argue over a beer sometime whether that's actually any different from a plain-Jane TIA. I claim that it is, but the other side is also arguable.)
Of course I did recently build a transistor tester box with a 1-Tohm resistor in it (and two each of 100G, 10G, 1G, etc, down to 100 ohms), so you had room for reasonable doubt about the gigohm enthusiasm. (And besides, your average physicist or chemist thinks nothing of putting a
1G resistor on the end of a cable.) ;)
Cheers
Phil Hobbs
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.
Cheers
Phil Hobbs
--
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
hobbs at electrooptical dot net
http://electrooptical.net
True. I think that's a small effect in practical devices, though, since it wouldn't vary much from device to device whereas beta is all over the map.
I'm not sure I'm getting your point here. The OP was thinking of the BJT solely as a current-gain device. That idea doesn't in itself constrain the base-emitter voltage at all, whereas thinking about it in terms of transconductance does.
Cheers
Phil Hobbs
--
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
I don't know how gm helps you in this case. For purposes of determining the transistor loading on the zener, most people agree it will be (beta +1) x Rload. If this estimate is all over the map because of beta, there's nothin g you can do to improve on it by using gm, the loading will be still be all over the map. Manufacturers help by providing beta variation with Ic and m in and max values, things that help bound the variation.
That's a second-order effect, though. If all you have is beta, you have no way of knowing what the offset voltage between base and emitter is. Six tenths of a volt? a hundred volts?
Another way of looking at it is that although (as you say) you can compute the sag at the base lead due to a change in emitter current, you have no idea what if any resistance exists between there and the junction.
A BJT with a beta of 100 still has a beta of 100 if you put 10k in series with its base. Or another zener, for that matter.
Cheers
Phil Hobbs
--
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
On a sunny day (Mon, 19 Aug 2013 16:50:36 -0400) it happened Phil Hobbs wrote in :
I was always under the impression that beta depends on Ic??? Confirms your own statement that beta depends on a lot of things :-)
Well, it is early morning, I will save the word salad for dinner.
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.
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...
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.
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. (*)
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.
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
Also some curves from previously posted questions...
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I love to cook with wine. Sometimes I even put it in the food.
On a sunny day (Tue, 20 Aug 2013 11:21:41 -0400) it happened Phil Hobbs wrote in :
It is not hard to understand, my error, I was not 100% awake yet...
As to beta versus Ic, I will have to look up some transistor specs, RF transistor specs... I see for example the BFR92 has a flat curent gain versus IC, while the BFR96 falls of below 40 mA, making it more suitable for AGC. Both are 5GHz wideband small signal NPN... But I would probably use a dual gate MOSFET for AGC...
Nice, good price too.
I have ordered a Sony super HAL CCD 'starlight' camera module. what it can do:
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0.001 lux... ebay:
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I had almost pinged you for help trying to make sense of the specs... It is supposed to give a clear picture with just starlight illumination (I mean at 30 fps). On board is a DSP too that reduces contrast so you can get contrast from areas while in in other areas there is strong light, Will go in my 'drone' alias 160 km/h SDcard trafficing .. This is to bypass NSA (Hello) of course. It is faster than internet...
Good, yes, usually I do this:
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this gives 2 x gain, to compensate for 50% loss due to cable termination.
I have never done chip design, I did read some papers, but I think I am not ready for that ;-)
Chopamps usually are shooting healthy packets of charge out of both input pins. Offset voltage and bias current can depend on what impedances the input pins see, especially whether they see capacitance or resistance.
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Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
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Multichannel arbitrary waveform generators
Models do, and variations among devices on the same chip are relatively small. However, things like defect density and variation of doping vs. depth vary from wafer to wafer, and those affect the shape of the beta vs I_C curve, do they not?
Besides, the context was Jan's claim that his BJT AGC amps work primarily by controlling beta via I_C, whereas I claim that it's a transconductance effect. A purely beta-controlled amplifier might easily have to use a 100:1 ratio of I_C to get a 10 dB gain range, and the available range would depend a lot on the selection of the individual device.
OTOH in a transconductance amp it's only a nice repeatable factor of sqrt(10), corresponding to a delta-V_BE= kT/e*ln(sqrt(10)) ~30 mV (single-ended).
Would you build a beta-controlled AGC amp?
Cheers
Phil Hobbs
--
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
I feel your pain. CCD datasheets are uniformly horrible.
Cheers
Phil Hobbs
--
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
Some seem to be a lot better than others, e.g. the OPA378 doesn't seem nearly as bad as the OPA2188.
Cheers
Phil Hobbs
--
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
The usual way to solve it is by the Victorian technique: "Add mass until nothing breaks." ;)
That is, crank up the zener current or use a reference with internal feedback.
My claim isn't that knowing beta is useless, far from it. We all use it all the time for exactly this sort of calculation. But it isn't enough on its own, because all you have is beta, you can calculate how far the zener will sag, but not how far the emitter will sag. Putting a 10k resistor in series with the base inside the transistor package won't change beta, but it sure will trash the load regulation.
If you know that the transconductance g_m = eI_C/kT, and that the base current is small, you can do a lot better than if all you know is beta.
Which is why these sorts of regulators use BJTs and not FETs. ;)
Cheers
Phil Hobbs
--
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
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