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help w/ transistors Answered

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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. 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.
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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.

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.

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.

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...

Then what's the difference between a PNP and a NPN with C and E reversed?

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

Load can be connected to emitter of the npn transistor. In this case, voltage at emitter (voltage to "drive" load with) is Vb - Vbe (=voltage supplied to base - voltage different between base and emitter (check datasheet)). Vbe is usually about 0.65V for small transistors like bc547. If transistor is used this way, + of the load should be connected to transistor's emitter and - to the ground.

So if one want's to control 12V motor with 5V signal (for example from microcontroller) this isn't good way to do it, because voltage going to the motor would be just about 4.35V.
There are still many applications for connecting load to emitter of transistor, instead of collector.

This is usually termed an "emitter follower," which as you note have less than unity gain (output is < the input, I.E., amplification is < 1.) That's fine for some uses. Particularly since a "follower" doesn't invert the signal--the general rule for amplifying devices (transistors, tubes) is that each output stage is the opposite phase of the input. "Emitter followers" don't do that. They make great buffers, and generally have a lower output impedance than a voltage amplifier (collector or anode with a load resistor.) Rather than voltage gain, they have current gain.

i know that load between transistor and earth is the normal setup of pnp so maybe its better to ue pnp in here (maybe you have to invert the signal first i dont know)

The arrows can be a bit confusing, since they were created when it was originally believed that electricity flowed from positive to negative.

Voltage flows from negative to positive, and current from positive to negative IIRC. Thus, the confusing part. One thinks of Pos. and Neg in terms of voltage normally.

By effectively blocking voltage in one direction...up to it's break down voltage. The effect is that of an open circuit in one direction.

Last time I'm posting this link, it makes tranisistors make so much sense. I've like posted this link 5 times in the past week:

http://www.rason.org/Projects/transwit/transwit.htm

BTW you don't really need R2 in most of the examples, it's just there just incase, most transistors will work fine without it.