Alright so I've been studying for my degree for a while now and I've finally gotten to the point where we are learning about the transistor.
Only problem is, I'm just not getting it...at all. After one horrific exam, I've decided I need to devote a lot more time into studying this.
Whilst studying my textbooks and everything, I'm finding that I still just don't feel comfortable with my understanding of it. I figure I should just start at the beginning again. So I'm wondering if anybody knows of any good explanations about the transistor that would be used to explain it to a complete beginner.
the more advanced folk will laugh at this, but here goes..
a transistor is like a watering can, you need to take the handle and hold it at an angle to get water to come out of the spout.
top = collector (water goes in)
spout = emitter (water comes out)
handle = base (water output controller)
push some water (electrons) through the base and water will flow from collector to emitter.
stop the flow to the base and the water will stop flowing from collector to emitter.
note:there are different types of transistors but this general concept holds true.
The basics are easy, but they can get complicated. To start, you got your bipolar junction transistors and your MOSFETS...
The BJT's are called NPN and PNP, from the layers of substrate used to create the guts. There's more than enough info on that online if you care. They have an emitter, collector, and a base. When the transistor is "off", the emitter and collector are mostly isolated from each other. When there's voltage at the base, current flows from the base through the collector. This starts a reaction that causes the E-C junction to also start conducting, allowing current to pass through.
It's not a sudden threshold -- rather, as you pass more current through the base, there is a corresponding rise in the current that can flow through the E-C junction. When a the current passing through the base reaches a certain point, the transistor becomes "saturated" and conducts the load from emitter to collector about as much as the transistor can manage.
MOSFETs are similar... Instead of an emitter, collector, and base you have a drain, a source, and a gate. And, instead of the gate being activated by current flowing through it, it's activated by voltage. (Hardly any current flows -- the gate has a very high impedance.) The drain/source junction acts kinda like a voltage-controlled resistor. As the voltage at the gate goes up, the resistance between the drain and source goes down. Again, there's a point of saturation when the resistance between drain and source -- called Rds -- is pretty much as low as it can go.
Driving either a BJT or MOSFET to its saturation point allows the transistor to work as a switch. Driving it partially allows you to control the current flowing through the transistor. This is used to great affect to dim lights, lower fan speeds, or amplify analog signals (like audio). But, when the transistor is only partially conducting, some of the energy is wasted as heat. This is why larger loads require the transistor to have a heatsink. When you're just using the transistor as a switch, very little energy is lost. This is how switching power supplies and Class D amplifiers work at high efficiency, with little or no heatsinking.
I Don't think i'll ever understand them properly without actually being in a lab where i'm able to dope the silicon myself... no matter how many times I see how the electrons behave and which way electrons flow, the whole doping and psychical side of it remains a mystery.
So as far as I'm concerned, I Can drive a car but I don't know how it works, I can use a transistor be a PNP, NPN or even a Field Effect Transistor, including the gate, base, how to control the flow ... not a problem but until i get how doping really works i'm never really going to understand it, esp when we talk about holes ... wait it just failed to computer in my head again lol
But how to use a Transistor is a whole different story (EG, driving a car)
in short a transistor can be used in several different ways (and it took me years to get that)
It can be used as a Switch, eg switch on say 12v from 5v.
It can be used to Amplify a signal.
It can be used as a regulator (eg, using a Zener Diode on it's Base)
It can also be used to produce Logic Gates
It can be used to invert a signal.
*Many more uses, eg Hysteresis to stop runaway Oscillations.
So you wire up the transistor depending on what you need, the Base/Gate is simply to control the flow of electrons between Collect/Emitter (Source/Drain)
Do i understand how to use a Transistor, Yes, would I say i know one works... Do i need a chemistry degree? Where's Gordon Moore help me understand this!
j514:
the more advanced folk will laugh at this, but here goes..
a transistor is like a watering can, you need to take the handle and hold it at an angle to get water to come out of the spout.
top = collector (water goes in)
spout = emitter (water comes out)
handle = base (water output controller)
push some water (electrons) through the base and water will flow from collector to emitter.
stop the flow to the base and the water will stop flowing from collector to emitter.
note:there are different types of transistors but this general concept holds true.
hope that helps!
edit: better
I use a similar analogy when trying to explain them, but I use a faucet instead of a water can. The faucet handle is the base and with little effort I can regulate the current flow from the supply (collector) thru the emitter(outlet) and right into the ground. I obviously prefer the NPN model and I guess since I'm not pumping water into the handle, it must be an insulated gate thingamabog of some kind.
EDIT: On second thought, the faucet is more of a high side setup, so maybe it's PNP. The faucet in the driveway leaks out the knob when you turn it on, so I guess it's PNP.
j514:
the more advanced folk will laugh at this, but here goes..
a transistor is like a watering can, you need to take the handle and hold it at an angle to get water to come out of the spout.
top = collector (water goes in)
spout = emitter (water comes out)
handle = base (water output controller)
push some water (electrons) through the base and water will flow from collector to emitter.
stop the flow to the base and the water will stop flowing from collector to emitter.
note:there are different types of transistors but this general concept holds true.
hope that helps!
edit: better
I use a similar analogy when trying to explain them, but I use a faucet instead of a water can. The faucet handle is the base and with little effort I can regulate the current flow from the supply (collector) thru the emitter(outlet) and right into the ground. I obviously prefer the NPN model and I guess since I'm not pumping water into the handle, it must be an insulated gate thingamabog of some kind.
EDIT: On second thought, the faucet is more of a high side setup, so maybe it's PNP. The faucet in the driveway leaks out the knob when you turn it on, so I guess it's PNP.
The main problem is that the analogy of a water faucet is an oversimplification as it (either as a pnp or npn) lacks a load wired to the transistor and the load is essential to understanding the interaction of the transistor to the circuit as a whole. This applies if the transistor is being used as a simple switch or as a linear amplifier.
afremont:
I use a similar analogy when trying to explain them, but I use a faucet instead of a water can. The faucet handle is the base and with little effort I can regulate the current flow from the supply (collector) thru the emitter(outlet) and right into the ground. I obviously prefer the NPN model and I guess since I'm not pumping water into the handle, it must be an insulated gate thingamabog of some kind.
EDIT: On second thought, the faucet is more of a high side setup, so maybe it's PNP. The faucet in the driveway leaks out the knob when you turn it on, so I guess it's PNP.
The main problem is that the analogy of a water faucet is an oversimplification as it (either as a pnp or npn) lacks a load wired to the transistor and the load is essential to understanding the interaction of the transistor to the circuit as a whole. This applies if the transistor is being used as a simple switch or as a linear amplifier.
Lefty
Well we are just trying to get a basic explanation across to a laymen now, not teaching them to design. Would a hose and sprinkler do as a load? After all, the longer the hose, the less pressure at the sprinkler due to resistance in the hose. Evaporation is like dissipation or other circuit losses and the rest flows to ground. I know it's not a perfect analogy, but we're using water to represent charge flow anyway. Even the "truth" is a fraud since "electrons" actually flow from ground to Vcc, yet we all think of it in the opposite fashion when start talking about current flowing from positive to negative. Shouldn't they burn all that malarky and start teaching people the truth?
Even the "truth" is a fraud since "electrons" actually flow from ground to Vcc, yet we all think of it in the opposite fashion when start talking about current flowing from positive to negative. Shouldn't they burn all that malarky and start teaching people the truth?
Actually in the military electronics training they taught us using 'electron flow' (negative to positive) but did explain that classic EE training used 'current flow' theory (positive to negative). In the big picture either way can be used as long as you are consistent.
Again trying to explain basic transistor theory without the context of the circuit it is used in is more likely to just confuse the student rather then enlighten him/her. Water valve analogy works ok for explaining some of basic DC ohm's law where utility water pressure = voltage potential, water value position = resistance, and output water flow = current flow.
Well lets say we have a very simple BJT NPN transistor circuit:
a 6v DC source with a 5k resistor on the base,
a 4v DC source on the collector with 3.3k resistor,
and a 100 ohm resistor on the emitter.
We are studying the analysis of BJT transistors.
So for that circuit, I have no idea where to start. haha. I would find....the collector current first I think.
Just analyzing them is very difficult for me.
As the replies indicate, there are several things to understand. What level gives you the problem?
The basic concept for me was easy because I started out learning about vacuum tubes and I think the operational concept should start there. A diode was a cathode and anode with electrons jumping between them, always in the same direction. Next came the triode. Since like charges repel you could stick a screen into the diode and control the flow of electrons with a smaller charge, hence amplification. The closer you could get the control screen to the anode the higher your gain because the like charge on the screen took more to over come.
When I took the electronics class guys were getting hung up on holes, depletion zone whatever and having a hard time with the overall concept of what a diode and transistor actually did.
Petroleum being the "Energy in the form of combustion"
Take a car's accelerate pedal... imagine this pedal to be the base pin, voltage/pressure, harder you press the more voltage that goes into the transistors base terminal (or in this case the car analogy more Oxygen to burn) The Emitter, this would be petroleum gets fed (or electrons) and the Collector Terminal would be the "Bang" but instead let's call it electrons flowing from the emitter to the collector and into the device to power, eg an LED.
in reality, there's a depletion region and all kinds of crazy things going on that makes a transistor work.. Shockley i'll grant you you're a genius, but you're still an asshole.
Well lets say we have a very simple BJT NPN transistor circuit:
a 6v DC source with a 5k resistor on the base,
a 4v DC source on the collector with 3.3k resistor,
and a 100 ohm resistor on the emitter.
We are studying the analysis of BJT transistors.
So for that circuit, I have no idea where to start. haha. I would find....the collector current first I think.
Just analyzing them is very difficult for me.
I hear what you are saying. Most people actively involved in electronics professionally obtained their basic electronics education & knowledge through some kind of formal training with professional instructors using vetted lessons plans, supported with hands on experiments, etc. A beginner trying to 'teach themselves' using question and answers from posters trying to help is always going to be hit and miss in my opinion. So many beginners start off wanting to know first how leds or transistors work in their projects without first even mastering ohm's law theory. There will likely always be gaps in their knowledge and they will probably never reach a level where they are able to teach themselves new things by building off what they have already learned. Too much rule based instructions given around here, do this, never do this, that way won't work, kind of thing. Too bad as I'm sure there are lots of good links to structured lessons that follow the classic training steps used in teaching electronics fundamentals.
Well lets say we have a very simple BJT NPN transistor circuit:
a 6v DC source with a 5k resistor on the base,
a 4v DC source on the collector with 3.3k resistor,
and a 100 ohm resistor on the emitter.
We are studying the analysis of BJT transistors.
So for that circuit, I have no idea where to start. haha. I would find....the collector current first I think.
Just analyzing them is very difficult for me.
You might want to start with the base current since no collector current can flow until you have base current flowing. Then you can attempt to use hfe (beta or gain) to calculate the collector current and consequently the voltages and currents at the collector and emitter. Then you should build the circuit and measure how different the real world is from prediction.
EDIT: Does this seem like an odd setup to anyone? That collector supply voltage and resistor just don't seem right given the base current. I see a transistor turned on to the point that the collector voltage is 0. The base current is as large as the collector current and this makes no sense to me. What am I missing?
Even the "truth" is a fraud since "electrons" actually flow from ground to Vcc, yet we all think of it in the opposite fashion when start talking about current flowing from positive to negative. Shouldn't they burn all that malarky and start teaching people the truth?
Actually in the military electronics training they taught us using 'electron flow' (negative to positive) but did explain that classic EE training used 'current flow' theory (positive to negative). In the big picture either way can be used as long as you are consistent.
This, combined with some other issues, makes transistors give me fits. That and the lack of a concentrated, formal education (I mean concentrated as opposed to 'learn as I go when I'm fiddling with some idea', not as in accelerated night classes). At some point, quite early in life, I developed my own visualization of electron current flow. So I have to think harder about positive flow, though I've gotten better at it with practice. But the arrow is always pointing the wrong way for me.
The #1 thing, though, which made my initial grasp of transistors maddening was the terminology. Nothing I read explained that the terms 'emitter' and 'collector' are with reference to the base, not the circuit. So even simple circuit drawings gave me fits, because I was trying to interpret the device (transistor) with respect to the circuit, just because made sense using English words as I understood them. (And the arrow thing didn't help either.) At some point, somebody here pointed to an article which cleared that up, by explaining that the terminology was inherited from tubes, where those terms referred to the role of nodes (?) in a tube, relative to the plate. If I'd learned tubes first, I'd have an easier time. But Grob, and other books, go into semiconductors before going into tubes, if they go into tubes at all.
afremont:
Even the "truth" is a fraud since "electrons" actually flow from ground to Vcc, yet we all think of it in the opposite fashion when start talking about current flowing from positive to negative. Shouldn't they burn all that malarky and start teaching people the truth?
They do teach the truth, there is no fraud. The difference between electron flow and "conventional current flow" is taught the first day or two in Circuits 101.
A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention for the direction of current which is independent of the type of charge carriers is needed. The direction of conventional current is defined arbitrarily to be the direction of the flow of positive charges.
Since the discussion here is about transistors, note that electrons are the majority current carrier in N-type silicon, and holes (a place where an electron could be but isn't) are the majority carrier in P-type silicon.
funkyguy4000:
Okay Well lets say we have a very simple BJT NPN transistor circuit:
a 6v DC source with a 5k resistor on the base,
a 4v DC source on the collector with 3.3k resistor,
and a 100 ohm resistor on the emitter.
That would be too hard to lay out on a breadboard?
[As you've described it, nothing bad will happen.]
You don't have a Voltmeter (DMM)?
Make it a little easier on yourself and use 5V in place of both the "6V" and the "4V".
funkyguy4000:
So for that circuit, I have no idea where to start. haha. I would find....the collector current first I think.
I**C** = The voltage across the "3.3k resistor" / 3300
To start, you got your bipolar junction transistors and your MOSFETS...