Transistors-Am I slow or was this hard for other people too?

[Ethan]

I took a break from trying to understand this a little while ago, but recently took it up again. I understand the explanations of some of the behavior but not the result: (i.e., an NPN transistor behaving like two diodes connected at the anodes thus, the base-collector is reverse biased and the base-emitter is forward biased. I understand the concept that the action that makes the transistor so great is the ability to control a larger curent with a smaller. Where I get lost is HOW it amplifies.

An analogy I've often seen is to think of the base as a lever standing in the way of the collector-emitter. The more force is used to push on the base "lever" more current flows between collector-emitter...

So for NPN... Electrons passing through the base-emitter junction (forward biased) looking for holes, saturate the base. When the collector voltage is higher than the emitter and the base, those electrons flow through the collector-emitter circuit. Generally speaking is that right? Once again, I'm at a loss as to how this amplifies... Could anyone offer an easy to understand explanation. Every text I've searched loses me right there.

 

[bcarso]

Ethan, you are ambitious to attempt an understanding of bipolars (especially). Most people only use them as black boxes with terminal equations and some knowledge of what bias voltages are required for the equations to be fairly valid. And frankly that's enough for a huge amount of good design work.

I'll try to find a mid-level account on the web. In the meantime familiarize yourself if you haven't already with the way a diode works and the concepts of diffusion and drift. Bipolars work mostly by diffusion. Another thing to remember is that if it weren't for recombination in the base region, there would be no base current after the initial bit---all the charge carriers would make it from emitter to collector.

It took the inventors of the bipolar a while to realize that they could use the base as the control electrode---they started off just looking for the power gain associated with current going in at a low voltage and coming out into a circuit with a high voltage, that is, a so-called common-base stage.

Again, you don't really need to know this stuff to do a lot of work!

 

[Ian MacGregor]

Ethan, here is a website for two classes I finished this year: http://my.ece.ucsb.edu/bobsclass/

The lecture notes are close the the top of the page, 137A is the class that deals with basic transistor biasing, operation, etc. Having someone explain everything to you face-to-face is invaluable, but maybe the lecture notes could help you out. The transistor equivalent models really helped my understand the operation of various devices.

As bcarso said, knowing the actual physical operation is not really necessary for some design work (though still good to know!!). I've found that after learning the physics, I had to force myself to "forget" it while learning the small signal models, or else I ended up really confused.

Ian

 

[Joe]

The N-type emitter has free electrons. The P-type base lacks electrons. The input voltage alone is enough to attract large numbers of electrons into the base. Since the input resistance is high, only a few can exit there.

The postive voltage has depleted the N-type (collector) area near the base of extra electrons, so naturally any extras in the base will go there. (They can't go back to the emitter, since it's full and they repel away.) There lies the action of amplification.

Because the base has a fixed size, and is very thin, only a certain number of electrons will fit within it's cross-section. So to increase electron throughput to the collector, base/emitter electron velocity (current) must increase proportionally.

In a high-gain transistor the collector is "wrapped" around the emitter/base junction, like a semi-circle of sorts. The curved collector surface takes advantage of electron repulsion, increasing gain substantially.

Hope that makes sense. (1st post)
-Joe

 

[bcarso]

Welcome Joe!

 

[Ethan]

Thanks guys that actually helps a lot. The equations for gain are simple enough. It just bugged me that I didn't INTIMATELY understand exactly what was going on. I guess just "trusting" that the trasistor amplifies and following the equations might be enough to get by, but I'm that anal type that wants to know how things operate at the lowest level possible.

It just frustrates AND motivates me at the same time how you guys can just look at circuits and assess what's going on or what SHOULD be going on. This is all part of the journey I suppose :thumb: Your help is invaluable to me--THANK YOU!!!

 

[PRR]

> Am I slow or...

Took me about a decade to "get it", and I will never believe the BS that the physicists talk about, just accept it as Black Magic and know the standard spells.

> for NPN... Electrons passing through the base-emitter junction (forward biased) looking for holes,

No, there are no electrons, no holes neither. When you look at a river, do you think about water molecules? There is CURRENT, which you measure in Buckets Per Minute, and you may want to know which way it is flowing.

> transistor behaving like two diodes connected at the anodes thus, the base-collector is reverse biased and the base-emitter is forward biased.

There are three other conditions: B-E reverse-biased, C-E forward-biased, or both. These don't do very interesting things (like amplify) so you only need to be aware of them and have a rough idea what happens in those cases. Ebbers-Moll concisely tells exactly what happens, but mostly you can pretend the junctions act like independent diodes.

In the interesting case, B-E forward and C-E not forward: B-E acts (almost) like a simple diode. Less than ~0.6V, no current. More than ~0.6V, infinite current. The range is really 0.3V for very small current and 1V for melt-down current, but for most practical purposes the B-E voltage is nearly constant.

The C-B junction has to be not-forward (not necessarily "reverse") to stay in the interesting quadrant of the four cases. On the face of it, that implies zero current flow, if this were an ordinary diode. But it isn't. One end is mixed up with the B-E junction. If B-E current is zero, C-B current is zero; but if B-E is passing current, this screws-up the C-B junction so it passes current even when "it shouldn't".

From that fact alone, you can build a voltage amp. Very small changes in B-E voltage produce huge changes in C-B and C-E current. C-B and C-E voltage have (almost) no effect on C-E current. So if you stick a resistor in series with a power supply and the collector, you can get very large voltage swings at the collector from small voltage swings at the base. We have voltage amplification.

It gets better. The C-B current is greater than the B-E current. About 20 to 500 times greater, depending on the device. I like to think of "tour guides". One person entering at the base (tour guide entrance) can guide 50 people entering at the collector (public entrance). No tour guides, no tours; many tour guides, many people tour. So we also have current amplification.

> It took the inventors of the bipolar a while to realize that they could use the base

That seems strange now. But their devices had Alpha greater than 1!!! In all modern (say post-1957) devices, Alpha is 0.95, 0.99, maybe 0.995, but never 1 and certainly never greater than 1 (ignoring leakage at high temperature). They had Alphas well over 1, which looked like early vacuum triodes with Mu of only 2 or 3, which improved with development.

 

[Ethan]

OK, no more talk of electrons, holes and crystal lattices (whew!)
So what happens when the collector and emitter of a PNP are at the same potential? Or if the collector is at a tiny bit lower potential than the emitter?

 

[bcarso]

"So what happens when the collector and emitter of a PNP are at the same potential?"

Not much. You still have to have some bias voltage to get current to flow out the collector. However the collector can be "closer" to the emitter than the base and still get current, to a point. When there is ample base current available and you feed a current to the collector, the resulting voltage from collector to emitter is called the saturation voltage, and may be as low as a few 10's of millivolts. This is good for switching things efficiently but incurs the penalty of sluggish turnoff behavior due to stored charge. That's why many times one tries to avoid this condition via various means, including various kinds of diode clamps.

The inverted configuration of the biploar used to be fairly popular, especially beofre decent JFETs et al. got available, for low level switching, since the voltage drop gets quite small at low currents. However the reverse beta can be only a few, unless the device is made expressly for the application.

 

[PRR]

Nothing special happens until the C-B junction is forward biased. And if the B-E junction is forward biased (our favorite condition) then by inspection the C-B junction can't forward bias until the collector gets down to emitter voltage.

That assumes identical junctions and currents. In real life, as bcarso says, you can pull down to a few dozen milliVolts and still have good transistor action.

That also assumes parasitic resistance is small. Transistors have maximum current at at that maximum current the C-E path will usually drop about 1V, 1,000mV. So if you can stay down to 1/10th or 1/100th of the maximum current, the resistance drop will be "small".

And as bcarso says, the transistor gets slow. The C-B junction is, in effect, two conductors separated by a non-conductor, which is a capacitor. We can leave junction width modulation to the physicists: all semiconductor junctions act like voltage-variable capacitors, voltage pushes the plates further apart, so at low voltage they have maximum capacitance. Most 40V transistors will lose speed when Vce is less than around 5V. This may or may not be a problem. It is one reason why very stable power amplifiers show bursts of oscillation when pushed to clipping.

 

[WJS]

try reading this, once you get past the authors opinions there is some good info
http://www.amasci.com/amateur/transis.html
dont forget the second page!!

 

[bcarso]

Not a promising start with some of the misspellings etc., but...

Hey, that is pretty good! You could argue that alternative interpretations still make sense, but he has a point. As I remarked in this thread, if it weren't for recombination, beta would be unlimited.

I like his insistence that charge flows, although again it is somewhat semantic.

At least you can't step in the same dog poo twice, unless you really work at it.

Barrie Gilbert told me that AD have some experimental heterojunction bipolars with betas of order 100k. He was unwilling to give a ~simple explanation and sent me back to Sze (Physics of Semiconductor Devices) which I thought was a bit cavalier---but maybe he was having a bad day ;-).

Now Dick Feynman would have never responded that way---but would have not only explained it, but gotten some new insights along the way. Damn I wish he had not been called to Washington in '65 when I was a student at the Summer Science Program and he was scheduled to come and hang with us. There was truly a bright guy, hangups notwithstanding. His daughter hosted a chamber music concert in what had been his house in Altadena the other day, and I had the privilege to attend. There was an oil portrait of him on the wall from late in life that seemed to convey what I imagine to have been the essence of the man late in life, when he knew he would not live much longer and thus not be around for the physics breakthroughs to come.

 

[Sammas]

I found the "stepping on hose" analegy useful in high school... i think it was in reference to a PNP transistor... other than that i have no idea.

I understand the workings behind semiconducting material thanks to chemistry though...

 

[Svart]

For some reason, FETs were so much easier for me to start with.. even though they are fairly related..

For some reason, the whole current drive part seems to screw people up royal. I suggest studying FETs of all kinds and then going back to BJTs and the other assorted Transistor types.

Hell, I truly agree with PRR and Bcarso in that sometimes it's better to just trust the math and make the part work rather than trying to understand exactly how... BUT that isn't DIY is it? :green:

Good luck, and if you can figure it out, write it down and send it my way. :thumb: