Basics of Transistors

Question

something has been bothering me for awhile. When I look at a circuit involving anything more complicated than RLC components (and perhaps op-amps) I struggle to figure out what it's doing unless its a configuration I've seen before.

In contrast, I feel pretty confident that no matter how complex an RLC circuit I'm given I could eventually figure it out.

Now when I'm analyzing an RLC circuit my tools are basically

• \$V = IR\$

• \$I = C \frac{dv}{dt}\$

• \$V = L\frac{di}{dt}\$

• Parallel and Series combinations of those components (I guess this isn't really separate from Kirchoff's laws but...)

• Kirchoff's Laws

So what I'm asking is what tools am I lacking for analyzing more complex circuits? Mainly I want to know how to analyze circuits involving BJTs and FETs. It seems like there are so many modes of operation for transistors its hard to keep them all straight. Anybody know a good website that lays out everything?

Thanks

EDIT I also want to mention that in practice there are things like \$V \neq IR\$ when temperature changes. I don't care about that for now, I agree with stevenvh that simulation is needed, but I want to be able to have the concepts down well enough to design a circuit which I can then tweak with a simulation etc.

Answer 1

Transistors are not hard to understand at the first approximation, and that is good enough to at least understand what's going on in many circuits.

Think of a NPN transistor this way: You put a little current thru B-E, and that allows a lot of current thru C-E. The ratio of a lot to a little is the transistor gain, sometimes known as beta and sometimes hFE. One minor wrinkle is that the B-E path looks like a silicon diode, so will usually drop about 500-700mV. The C-E path can go down to about 200mV when it would allow more current than the external circuit is providing. The details go on and on, but you can get a lot done with that simple view of a NPN transistor.

A PNP is the same thing with the polarities flipped around. The emitter is at the high voltage instead of low. The control current goes out of the base instead of into it, and the collector current goes out of the collector instead of into it.

Let's stick to bipolar transistors for a bit and understand them first, since that seems to be what you're asking about more. FETs are equally simple to understand at first approximation, but I don't want to confuse things at this point.

While the model above is useful for understanding most transistor circuits, it suggests a lot of ways transistors can be used that may not be obvious. The conceptually obvious way to use a NPN is to connect the emitter to ground and the collector to the positive supply with a resistor in series. Now a little change in base current can cause a large change in the collector voltage.

The tricky part is not in understanding how the transistor works, but to imagine all the cool things you can do with a device that works like that. Getting into all those would be way too much for a post here. I suggest you think about the simple model I described above, then look up some common transistor circuit topologies and think how the simple properties of the transistor are utilized to do useful things.

Things to specifically look up and analize according to the simple model are:

• Common emitter configuration. This is the basic amplifier. A particular issue is how to keep the transistor in the middle of its range to use its amplification capability effectively. This is called "biasing".

• Emitter follower. Gain is not just making a higher voltage. In this case you get slightly less voltage but higher current and lower impedance.

• Now look at some multi-transistor circuits and try to follow what they are doing, how the transistor is used to advantage, but also what trouble the designer had to go thru to run the transitor in a way to be useful.

• When you feel more comfortable, look at more unusual configurations like common base. Its not often used, but has its specific advantages.

Answer 2

What makes transistors difficult to work with is that you have to be aware of lots of different parameters which influence each other, and none of which are linear. Therefore it's not easy to exactly model their behavior, and that's why we use simulation tools like SPICE. You still have to know what you're doing to design a circuit, but SPICE will help you check your design/calculations, in which you sometimes have to simplify.
I'm not sure websites will be comprehensive on this. I think a good textbook will give you better information. Maybe others can recommend some.

Learning from repeated exposure is not a bad way to learn things. You'll get real practical knowledge and learn what are typical circuits to solve typical problems.

Answer 3

The thing with transistors is that they aren't linear devices, so there won't be any simple equations that apply under virtually all conditions, like the ones you have for passives. The usual approach is to recognize that at any given moment, a transistor operates in one of a few characteristic ways - cut off, active, saturated. Within any one of those modes, you may apply some approximations to analyze transistor circuits, but it has to be understood that the approximations only hold within limits.

For example, if you first establish that a transistor will be operating in its active mode, you can then draw up the small-signal AC equivalent circuit, in which the transistor is replaced (in the simplest model) by a resistor and a current-dependent current source. You can then use your linear equations to good effect on the equivalent circuit. Why is it called the small signal AC equivalent though? Because if you apply a large enough signal, you will break the limits of the model; large signal inputs may drive the transistor into cut-off or saturation, invalidating the model.

Answer 4

The more elaborate the model, the more accurate the response you calculate. However, sticking with basic Common Emitter NPN:

1. Two resistors on the base, act as a voltage divider. Generally, they are ABOUT the same value, making base about half of supply voltage.

2. The emitter is about 0.6V below the base. If there is a resistor on the emitter, you can now work out the current through it.

3. Emitter current goes through collector, too. If there is a resistor on the collector, you can now work out the voltage across it.

That's it for DC.

For AC, a few millivolts change on the base can become several volts on the collector. If the emitter current (and / or the collector resistor) is too big, or base bias is odd, you get saturation or cutoff - which distort the signal you put in. This is not always a bad thing (think: guitar distortion effects).

Answer 5

you can consider transistor nothing more than a device which help you in controlling the parameters or, say, circuit 2 with the help of circuit 1(just a rough estimation) if transistor is joining the two circuit. For eg. as in digital electronics there is a clock pulse and say you want to do something when the clock is at a particular level, similar is the case with transistor, you can model the transistor so that at the operating point when the voltage at the base reaches a particular level then you can turn the device on and so the current can flow in the ckt2, or you can think of it as a relay, or a switch, not only this transistor is an amplifier.

for designing purpose just keep in mind that transistor helps you in controlling the parameters of the circuit 2 with the help of ckt 1, so for determining the operating point you can use any model. Don't get confused with the different models available for solving transistor these models are just for you convinience, it is easier to use the re model as it facilitates easy computation, h-parameter(hybrid) model is the most versatile and is considered the best in solving any transistor, but T-model is also good. to get a basic feel of what a circuit is doing you can approximate using the approximation like the Vbe = 0.7 and all these all approximations lead to easy computation.

i know two very good books on studying transistor 1) electronic devices and circuits, boylestad , a very good book, but it uses a lot of approximation and is good for somewhat approximate analysis but if you want to model the transistor in detail like you want to know the exact parameters and all then there is a better book 2) microelectronics circuits, sedra smith. this you can call a bible, super book, but i would advice to first read the book 1, then move on to 2, otherwise you won't be able to learn much and you will just bury yourself in complex mathematics.

for learning how to solve how to analyse circuit study as many circuits as possible and then with the passage of time you will come to know how can you use transistor in many different ways

for learning this you can refer to books written by forest m. mims they contain just circuits. and you can analyse them.

FET is not much different from BJT, its just FET are mainly used for making of amplifier because of its very high input impedence but the output impedence is almost comparable, it is also small in size, but on the contrary BJT has high switching power so if your application has to do something with switching BJT would be a great choice.

at last i would again say, if you want to learn transistor then study a lot of circuit may be you can look into the construction of op-amp as they are nothing but 4 stage differential amplifier and through that you can also learn..

have a nice time learning TRANSISTOR !!!