Transistor question-biasing

yungman

I tried to explain the feedback in post#2 but I don't think I did a good job and don't know how to explain it better. Remember it is the voltage between the base and the emitter that govern the collector current, not just the base voltage. The emitter resistor serve as the feedback to stabilize the transistor so it does not drift that much. Hope someone else can write it better to explain to you.

BTW, nobody claim this 3 legged animal is easy!!! Take your time, keep asking question here and people will try to answer. One book that I studied this subject is by Malvino. It is the easiest and the best book on transistor I've seen.

Bassalisk

You really made me laugh with this 3 legged animal hahahahah.

Don't worry. I truly appreciate yours, and any other expert help that I can get here. Because, to be honest, today there are few people that are willing to even try to explain something to you.

People like You, Jim and countless others, make the students life more easy, where new concepts are not that naive.

I will definitely check out that book, thank you for recommendation.

es1

Slight change in 'forward gain' due to early voltage. This changes both the collector and the base current technically. If we assume the feedback mechanism hold Ie constant and the biasing network was correctly picked to hold Vb constant.

Ie is actually the controlled variable here. As the emitter and collector current are very close in a forward active high gain NPN usually we just consider them to be the same.

This link does a good job of running through the math.
http://www.stanford.edu/class/ee122/...Notes_Rev0.pdf
Start at section 6.

Google for "common emitter feedback" and you'll get a ton more links which explain it in various ways.

The above calculations assume an ideal NPN and therefore B is constant but it is pretty easy to see from their math what would happen if B was reduced due to early effect.

So I think the answer to the question you were really trying to ask, what breaks as I increase the pot's resistance, is B is reduced. Or, the NPN becomes a poorer amplifier even though it is still forward active.

Bassalisk

So my hunch about Early effect was actually right? wow :D

es1

Yup. I thought your original understanding of the NPN operation was right.

The feedback concept in this circuit is something else.

I edited my post a bit to try to keep the wording clearer. There are a number of moving pieces and as one adds details things get murkier so I wanted to make sure to state my assumptions. Hopefully the newer version, and the new link, is better.

Bassalisk

Already reading it. Many thanks.

yungman

That little 3 legged animal can be as complicated as you want. The place where I find hard to explain is exactly what ES1 said, the collector current change due to the collector voltage as you change the collector resistor with the base voltage constant must mean there must be some change to the base current and the gain of the transistor. If you take into consideration of all these, it will never end. That is the reason I mentioned about the Malvino book. It present a very simplistic view of the transistor..........and surprisingly it is good enough to design very advanced circuits.

You really chase to the bottom of this, you'll find it very long and unnecessary hard. From the question you ask, I think the Melvino book is perfect for you. Not only that, this book get the best introduction of op-amp I ever seen including Bode Plot.

Baby steps, learn the basic of transistors and then go back and dig into it. It can fill up a whole thick just on this little animal. You are not going to get any simple answer on this.

Read the book, ask questions here. If I have a chance, I'll try to look for the book on Amazon and see whether you can take a peek inside the section on DC biasing and AC equivalent of NPN.

Bassalisk

Can you give me the title of the book?

yungman

I was trying to see whether Amazon will have "Look inside this book" option on this book, sadly they don't. Here is what I think is the book, mine is a very old edition 30 years back, it look nothing like this. I just remember the name of the author. Try this one, it is only about $4 used, so it would not be too much money if this is not the one. In fact I just bought one for myself.

http://www.amazon.com/Electronic-Pri...9040280&sr=1-4

Look for the DC bias and AC equivalent section. It give you a lot of approximation to make it easy to learn. It does not even get into the early voltage which you don't really need. I am currently designing some electronics used in guitar and I am using the knowledge gained in that book. Just hope it's the right one.

Bassalisk

I just LOVE good books. Might sound geeky but between google and a good book, I always go for a good book.

Thank you for the research, I will surely buy it.

yungman

I hope you have not order that yet, I received the book, it is the exercise book for the text book, not the text book!!! No wonder it is so cheap!!!!

jim hardy

i'd recommend the ancient Texas Instruments book "Transistor Circuit Design"

a google showed it in a lot of used bookstores.

While on the subject their wonderful "OpAmps for Everyone" is available as a download
from TI's library

search for slod006b.pdf and add it to your library...

Bassalisk

No I didn't order from there, because I don't think Amazon can send to my country. I found it on ebay.

http://www.ebay.co.uk/itm/Electronic...item2a14dafa8b

Bassalisk

Already did. Thank you. I am learning waaaay more than I need to pass my exam. This course I am taking is very narrow, although it should be since Im studying telecommunications. But I cannot just accept things like, negative feedback, differential amplifiers and so on. Need to have a nice explanation for it :)

yungman

It is the correct author, make sure you get the text book, not the experiments book that I bought.

Bassalisk

Btw. the book, Electronic Principles by Malvino, is just out of this world. I just wish I had it when I started learning transistors. Its making my life just 10 times more easy.

Thank you very much.

yungman

I am glad you like it. Make sure you study the opamp and Bode Plot. It is very very good. It is so good that I even tried to buy one and get the wrong one.

Don't think this is too simple to be useful, I designed heavy duty transistors circuits and integrated circuits for years, the stuff works. that little r'e and Vt that is 25ohm for 1mA and 25mV and room temperature is all you need to know and to scale the r'e with current.