help w/ transistors

Does anyone know a good experiment to teach what transistors do? I have read everything I can find about them and I still don’t understand what they do and how they do it. Can anyone help?

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NachoMahma8 years ago
. In a nutshell, a transistor is an electron "valve." The voltage/current applied to the base/gate controls how much current can flow between the emitter/source and collector/drain. Much like adjusting the handle on a water valve varies the flow.
PS: I've been working with electricity/electronics for over 40 years and I still don't grok capacitors. I can parrot a lot of info and formulæ, but I don't fully understand how/why they work as they do.
. Ie, don't worry if you don't understand all the details. If you can plug the right numbers into the right formula, you'll be OK.
kendallickes (author)  NachoMahma8 years ago
that is very helpful, I understand it a little better. What are the arrows for that define a PNP and a NPN?
. Indicates polarity, as with a diode. . IIRC, we used Not Pointing iN as a memory aid.
kendallickes (author)  NachoMahma8 years ago
so on a PNP the current goes + into the Emitter and - out the Collector right? if thats so does the NPN current go in + at the collector and out - at the Emitter flowing backwards? is so how is that different that switching the leads on the collector and emitter?
nonono... Collector is always positive and emitter is always negative. NPN makes a connection when you apply power to the base, PNP breaks the connection.
gmoon yourcat8 years ago
Naw. A PNP emitter is positive. In electronic double-speak, they are said to emit "electron holes," because generally positively-charged single particles don't move (but ions can move in some circumstances, and current flow can be in either direction.) In general conductors, it's usually electrons that "jump." The "holes" concept is applied more commonly to semiconductors and things like plasma. An aside: there's no such thing as a PNP vacuum tube, since they are constructed with electrons in mind. Theoretically, a plasma tube might conduct via a "holes" mechanism. NachoM uses a PNP mnemonic; personally I just remember the arrow is the emitter, and always points toward the negative...
yourcat gmoon8 years ago
Then what's the difference between a PNP and a NPN with C and E reversed?
gmoon yourcat8 years ago
The collector and emitter don't serve the same purpose (see the discussion in this thread about loads on emitters, vs. collectors), so you can't just reverse them.

You won't get any voltage gain (amplification) from the emitter, for instance.

The most important difference between PNP and NPN is just polarity (also, PNPs aren't as quick or efficient as NPNs, since they are "hole carriers" as noted above.)

The polarity difference just means that a PNP could be a more convenient choice for a particular application. It's common, for instance, to use a complementary NPN + PNP pair for a push-pull amplifier output stage, which would be simpler than two NPN transistors.

This is perhaps a good place to mention some characteristics of transistors not discussed before:

-- Bipolar transistors are current-in / current-out devices. This has several ramifications:

::: The amount of current being controlled (switched or amplified) depends on the amount of current input (on the base.) So the greater the output current required, the more must be applied: that gives bipolar transistors a variable input impedance, and often a low input impedance (takes more current to do the work.)

::: The load is what converts the current output into voltage gain. It's an Ohms Law thing: if a fixed resistor is placed between the collector and the current source, and we vary the current, the voltage has to change. Without the load, there's no voltage amplification.

::: Therefore, the larger the load resistor, the greater the voltage gain. Yes, you read that right: greater resistance equals more voltage. However, there's a cost: less current available to do work at that higher voltage. As the voltage rises, available current falls, and then it's said to have a high output impedance.

::: A high output impedance doesn't have enough current available to drive a low input impedance, which points out some of the limitations present when coupling transistor stages together.

All this load and impedance stuff is difficult to visualize, but if you can get a handle on it, you'll be able to look at a transistor circuit and have some idea what's happening...
. The transistors are physically and electrically different. PNP is P-type/N-type/P-type material. I'll leave the NPN structure as an exercise for the reader (Google is your friend).
an npn (arrow points out) is on the - side of the load the load is allways connected to + the - of the load goes to transistor C the E of transistor goes to - of main circuit to switch the npn on you give it + to the B thru a relatively large resistor i never really dealt with pnps
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