Transistors

[Terry]

When I first went to college after starting work, one of our lecturers insisted on referring to Practical Wireless and Practical Television as "Camm's comics", which I thought was a bit unfair, considering the wide range of abilities these magazines (successfully) addressed.

As for those wartime radios, the diode function was achieved in unusual ways such as a wire pressing lightly on the surface of a rusty razor blade. Army bed-springs were also found to function well as aerials ...

'It took centuries to discover you can use electric power to transmit AF'? Don't you think that this was more like natural evolution? Electricity was known about for a very long time before ways were found to generate it reliably in efficient ways and there was also the battery evolution, from Volta's piles upwards to find reliable ways to produce electricity where mechanical generation was not viable or desirable. Then, of course, it took further evolution in the form of generating and using radio waves which were followed, finally, by the invention of valves and then types capable of amplification. To suggest that all this took place before anybody attempted to describe how all these mechanisms worked is, quite frankly, bordering on the ludicrous!

[Nowhere-Man]

Well one example: The superhet was discovered by TRF receivers going into oscillation and a new frequency created. Nobody worked it out in a class. OK, I grant Einstein did create the Theory of Relativity purely by calculation but this was uncommon. If TRF listeners were creating new frequencies from that you get the clue to an IF (and the 455 KHz was arrived at by trial and error). So, books are great to learn from but I think application is the best way. I try not to credit the textbook approach as paramount but I read them in moments of spare time.

[Hartley118]

AFAIR, even Einstein's theory of relativity didn't arise out of mathematical thin air. It was the Michelson-Morley experiment in 1885 that set Einstein thinking by demonstrating that the speed of light appeared constant irrespective of the expected velocity of its 'aether' transmission medium. From this surprising observation flowed Einstein's mathematical proofs of his even more surprising theories of relativity.

Martin

[Kalee20]

Getting back to transistors, in common-emitter mode we have the internal 're' as approximately 26/Ie ohms (where Ie = emitter current in milliamps). Then gm = 1/re = 38 x Ie numerically in mA/V. Adding a collector load, you get a voltage gain of gm x Rl = 38 x Ie x Rl... and since Ic is approximately equal to Ie (alpha > 0.95 even for really crummy transistors!), you can see:

Voltage gain = 38 x volts dropped across collector load resistor.

So - transistor currents really aren't important, nor is the collector load, for working out gain. All you need is to calculate (or measure) the no-signal voltage across the load resistor, and the gain immediately follows!

Of course, this is a gross simplification - nobody is going to operate their transistor at an amp of current with a load of a few ohms, because battery life would be horrendous. And it also turns out that for low-noise applications, there is a broad optimum operating current. And the '26' and '38' numbers are temperature-dependent, too. But it does illustrate the relative insensitivity to details that exists.

[Nowhere-Man]

I'll share my attempt to make sense of the tutorial here
http://www.learningaboutelectronics.com/...transistor

I thought it was strange to just publish the formulae in complex digits with no clarification. So I just plunged in and tried to get the figures to add up.
The base voltage is just a regular voltage divider and it's 3 volts so I won't dwell on that.
Below I figured you are meant to multiply 10K by 30 K and then divide that figure into 40 K (the sum of the 10 and 30). That gives the 7500 cited below.
Then I found if you divide Beta into 7500 it gives 75 which I assumed you add to 0.5 K. So 575.
3 V - 0.6 gives 2.4 so divide 575 into that yields the 0.004.
At this point I can only hope I am following this as per plan as I found the whole thing vague.
1500 times 0.004 gives 6 volts. So hopefully I did this the right way and not some coincidence.
Does anyone feel this is a good or a bad tutorial? Is it pointless? Is it too complicated?

[Mark Hennessy]

The base voltage is just a regular voltage divider and it's 3 volts so I won't dwell on that.
Below I figured you are meant to multiply 10K by 30 K and then divide that figure into 40 K (the sum of the 10 and 30). That gives the 7500 cited below.

This part is Thévenin in action. In other words, replace that potential divider with a 3V voltage source and a 7k5 resistance.

I've mentioned this before - if you haven't already (and I'm guessing you haven't because you didn't spot it here) - please make the effort to learn it. It is far more fundamental than all this transistor stuff, and actually far more useful.

I won't say what I think of that tutorial because this is a family-friendly site. Similar comments apply to just about every other article I've read on there. Unfortunately, because it's quite well presented and search-engine friendly, a lot of beginners cite pages from that site when really they shouldn't.

You didn't reply to my post recommending text books a few days back. I thought that was a bit rude, though hopefully it was just an oversight. There is a lot of badly-written stuff out there, and you're not yet able to tell that apart from the good stuff - but that's where we can help. You don't have to follow just one method, as you said, but at least stop following the bad ones. I've lost count of the number of times I've recommended other books, only to see post after post talking about ARRL and other poor references. Please respect our experience here - or at least make it clearer to us that you respect our experience - or I'll have to question how things continue. We have been extremely patient and tolerant here.

[Nowhere-Man]

"You didn't reply to my post recommending text books a few days back. I thought that was a bit rude, though hopefully it was just an oversight."

An oversight I guess. The last 3 days I've been offside due to dehydration. Working on the boat in the current heat has been tough but I have a deadline to finish. Yesterday was serious heat to work in and I got very dehydrated. I'll be glad of more normal temps. List the books and I'm happy to look at whatever is available.

[Terry]

 List the books and I'm happy to look at whatever is available.

As Mark said, he already has!

Take a look at post #60 in this thread ...

[Kalee20]

I'll share my attempt to make sense of the tutorial here
http://www.learningaboutelectronics.com/...transistor

I thought it was strange to just publish the formulae in complex digits with no clarification...

Does anyone feel this is a good or a bad tutorial? Is it pointless? Is it too complicated?

I've just had a look - it gives the formulae but doesn't explain the WHY bit. So you plug the numbers in, without any insight.

It's not even right: Vce = Vcc - Ieq(Rc + Re) is wrong. Not massively so, but should be Vce = Vcc - Ieq(αRc + Re). Because collector current is less than emitter current by factor α.

[Nowhere-Man]

Ah, found it. The Art Of Electronics. "someone here might well have a copy going cheap (in return for a promise to stop referring to that damn ARRL book)."
To date I have no transistor reference book. The dreaded ARRL puts most of the emphasis on valve.
It's a 1960 edition.