Go Down

### Topic: Understanding Transistors (Read 5656 times)previous topic - next topic

#### afremont

#15
##### Mar 09, 2013, 06:13 pmLast Edit: Mar 09, 2013, 06:26 pm by afremont Reason: 1

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.

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?
Experience, it's what you get when you were expecting something else.

#### justjed

#16
##### Mar 09, 2013, 06:45 pm

Quote
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.

This thread was helpful: http://arduino.cc/forum/index.php/topic,105053.0.html -- just work with the common collector voltage regulator for starters.
... it is poor civic hygiene to install technologies that could someday
facilitate a police state. -- Bruce Schneier

#### JimboZA

#17
##### Mar 09, 2013, 07:05 pm
This site might help...
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### JChristensen

#18
##### Mar 09, 2013, 08:17 pmLast Edit: Mar 09, 2013, 08:33 pm by Jack Christensen Reason: 1

pop tart

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.

http://en.wikipedia.org/wiki/Electric_current#Conventions:
Quote
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.

#### runaway_pancake

#19
##### Mar 09, 2013, 09:08 pmLast Edit: Mar 09, 2013, 09:11 pm by Runaway Pancake Reason: 1

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".

> > >

So for that circuit, I have no idea where to start. haha.  I would find....the collector current first I think.

IC = The voltage across the "3.3k resistor" / 3300
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### JimboZA

#20
##### Mar 10, 2013, 05:24 am
Have to love Jack's definition:

Quote
holes (a place where an electron could be but isn't)

Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### funkyguy4000

#21
##### Mar 11, 2013, 01:29 am
It is just BJT, we aren't studying the PNP, yet at least.  In my course, we have studied the BJT and then we just moved on to the Darlington transistor.
MOSFET hasn't been mentioned at all.  I'd be surprised if any of my classmates have ever heard of it.

The level that confuses me is the analysis of the transistors behavior in different circuits.

There are so many currents and voltages and resistors for one component, that its overwhelming.
And at the same time, the transistor is the basis for all electronics so its intimidating as well.

Sorry, I don't mean to be difficult, its just I've tried to understand the transistor before this class and I've never been able to do it, and then after the unit in a formal class, I still don't get it so its just really frustrating.
Accelerate to 88 miles per hour.

#### funkyguy4000

#22
##### Mar 11, 2013, 01:38 am
Okay, a buddy just threw me this link and this is what I was looking for.
Anybody lookin for help like I had, bam.
http://www.ittc.ku.edu/~jstiles/312/handouts/Example%20DC%20Analysis%20of%20a%20BJT%20Circuit.pdf

The replies actually did help me get my head wrapped around it.
Thanks!
Accelerate to 88 miles per hour.

#### JChristensen

#23
##### Mar 11, 2013, 01:42 am

It is just BJT, we aren't studying the PNP, yet at least.

BJT refers to both NPN and PNP transistors. Analysis is the same, only the current direction differs.

Quote

There are so many currents and voltages and resistors for one component, that its overwhelming.
And at the same time, the transistor is the basis for all electronics so its intimidating as well.

Let it sink in a bit, it's not all that bad. For each of the three common configurations in linear circuits, there is a set of equations:

http://en.wikipedia.org/wiki/Common_emitter#Characteristics
http://en.wikipedia.org/wiki/Common_collector#Characteristics
http://en.wikipedia.org/wiki/Common_base#Low-frequency_characteristics

#### funkyguy4000

#24
##### Mar 11, 2013, 01:45 am

It is just BJT, we aren't studying the PNP, yet at least.

BJT refers to both NPN and PNP transistors. Analysis is the same, only the current direction differs.

Quote

There are so many currents and voltages and resistors for one component, that its overwhelming.
And at the same time, the transistor is the basis for all electronics so its intimidating as well.

Let it sink in a bit, it's not all that bad. For each of the three common configurations in linear circuits, there is a set of equations:

http://en.wikipedia.org/wiki/Common_emitter#Characteristics
http://en.wikipedia.org/wiki/Common_collector#Characteristics
http://en.wikipedia.org/wiki/Common_base#Low-frequency_characteristics

You rock
Accelerate to 88 miles per hour.

#### JChristensen

#25
##### Mar 11, 2013, 01:51 am

You rock

That definitely depends on who you talk to! Enjoy!

Go Up