Transistor Biasing.

[rockgardenlove]

I'm looking for a good resource on transistor biasing.
I understand why transistors need to be biased, and basically how it works, I just don't quite understand how to determine what actual parts values to use in the various biasing schemes.  I was wondering if anybody knew of any good sites regarding this stuff?

Thanks!

[Dragonfly]

From the "Useful lLinks" thread...

http://www.101science.com/transistormenu.htm

this is a great PDF on it...http://www.ibiblio.org/obp/books/socratic/output/bjtbias.pdf
enjoy !

AC

[petemoore]

  http://www.google.com/search?hl=en&q=transistor+biasing&btnG=Google+Search
 Googled...transistor biasing...<

[rockgardenlove]

Already tried Google.  The problem is that with stuff like that I get totally lost in all the various abbreviations and letters...

Dragonfly, what's the PDF?  You posted the same URL twice. 

Thanks!

[Dragonfly]

Dragonfly, what's the PDF?  You posted the same URL twice. 

Thanks!

Fixed...sorry 'bout that !

AC

[rockgardenlove]

Good link...thanks...
Is there an answer key anywhere?  That would be immensely helpful.

[R.G.]

I've typed in the way to do this several times. Search for messages with me as author and containing "bipolar" and "biasing".

The secret is to decide ahead of time what proportion of the power supply voltage you want across the collector and emitter resistors versus the collector-emitter of the transistor, then to remember that the base emitter voltage is always 0.5 to 0.7V. Once you get there, you're 90% done.

[gez]

For those who don't already know (and this included me until a few days ago), if you click on search then 'advanced search' you can search threads written by one author - RG for example.  It means less wading thru everything if you know who wrote the info you're interested in and also means you can find your own threads quicker if you want to link to them.

[puretube]

ooooh - yer late, gez!

(but you do know you can narrow down the search to e.g. the "Lounge" only...)

[gez]

ooooh - yer late, gez!

(but you do know you can narrow down the search to e.g. the "Lounge" only...)

[zjokka]

I've typed in the way to do this several times. Search for messages with me as author and containing "bipolar" and "biasing".

The secret is to decide ahead of time what proportion of the power supply voltage you want across the collector and emitter resistors versus the collector-emitter of the transistor, then to remember that the base emitter voltage is always 0.5 to 0.7V. Once you get there, you're 90% done.

don't mean to be rude, but if you would write them once and put them on your own website, people would find them more easily.
Like most of the people here I think, I really devour your comments, and hate to see them go to waste.
But this is what we were looking for I think.

Here's the entire secret to biasing bipolar transistors: in the linear region, the base emitter **can't** be anything other than one diode drop apart.

So - you want an emitter follower. That means that the signal comes out the emitter.

That means that for biggest signal swing up and down, the emitter needs to sit at half the power supply voltage (assuming you aren't trying to make the emitter follower clip for some reason; that's not usually why people use EF's).

That means that the voltage across the emitter resistor is half the power supply.

That means that the base HAS to be one diode drop above (NPN) the emitter. Well, actually, it means that to get the emitter to half the power supply, you have to pull the base up to half-supply plus a diode with some biasing voltage. The emitter... follows 8-)

Ohm's law now tells you what your resistor is. You decide what current you want flowing through the transistor (make this 10x any load current) and then Re = (1/2Vcc)/current. If you pick currents too big, you will need mondo biasing currents to make that come true, and the transistor will dissipate lots of heat. If your biasing network can't supply the base with current/hfe, then the base voltage will sag, and so will the emitter voltage.

What about collector resistors? The collector resistor eats voltage too. It eats current*Rc volts. For biggest signal voltage swing (if that's what you're after) Rc = 0.

And that's how to bias bipolars. Period. Even for gain stages, you just reapportion how much voltage is dropped over the collector and emitter resistors and set the base to make the emitter be where you want it.

As to source followers:

FETs are more complicated than bipolars because although the source does follow the base, the offset voltage is not as tidy as always one voltage drop. MOSFETs have a positive offset voltage of gate to source to get them to conduct. This is in the range of 0.1 to 5.0V. You have to know your MOSFET. Read "Designing MOSFET Boosters" at GEO for how to do this.

For JFETs, the offset of gate to source is negative and big. The offset must be in the range of voltages to reverse bias the gate-source diode. A reverse voltage of 0V is fully ON for JFETs and a reverse bias of Vgsoff is fully off. In the middle is the linear region. The range of Vgsoff is from as little as 0.1V to as much as 12V. This is what makes JFETs hard to work with.

In all cases, you decide what voltage you want your source to sit at and how much current you want to flow in the source resistor. Then you put the gate at the voltage that makes that come true. It's just harder to figure out what that voltage is.

[Dragonfly]

don't mean to be rude, but if you would write them once and put them on your own website, people would find them more easily.
Like most of the people here I think, I really devour your comments, and hate to see them go to waste.
But this is what we were looking for I think.

[

Or, Ziokka...you could just save them to your own disk when you see them....web space isnt necessarily cheap...and I think the R.G. has done "plenty" for us...asking for "more" seems a bit "needy".

Just a thought. But I agree 100% ..I have learned TONS from R.G., Mark, Doug, etc....

AC

[The Tone God]

Like most of the people here I think, I really devour your comments, and hate to see them go to waste.

You maybe misunderstand how the forum software works here. Threads and content are not destroyed here but instead archived so any comments including near textbook length posts by members including R.G. are not lost. In fact they become easier to find via the search engine then some technical web sites. So I don't see how R.G.'s and any other's comments can go to waste.

Andrew

[mdh]

From the "Useful lLinks" thread...

http://www.101science.com/transistormenu.htm

this is a great PDF on it...http://www.ibiblio.org/obp/books/socratic/output/bjtbias.pdf
enjoy !

AC

Another thing to note is that the same guy who did that BJT biasing worksheet also has a whole series of free electronics texts: http://www.ibiblio.org/kuphaldt/electricCircuits/. The third volume is on semiconductors, and although some chapters are marked "incomplete" or "pending," there's a whole lot of info there. I've only read the first and a bit of the second volume, but they seem pretty good so far. As far as I can tell, the complete set of "Socratic Electronics" worksheets (http://www.ibiblio.org/kuphaldt/socratic/) could be used to provide exercises for self-study while reading through the texts. I didn't see any answer keys on the site, though.

[rockgardenlove]

So...
What I'm seeing here is that you just want to have it set up so the base is .7 volts (for most transistors) above the emitter...?

[zjokka]

I don't see how R.G.'s and any other's comments can go to waste.

If people keep asking the same questions over and over again, something must be going to waste.

I understand you learned a lot, Dragonfly, but with a post titled like this other people aren't learning along.

http://www.diystompboxes.com/smfforum/index.php?topic=55175.0

If you read my post once more, you see I'm not asking for more work, on the contrary, saying it could work with less.
that's all
zj

[mac]

One question,
Let's say that you have two voltage divider biasing schemes for the same transistor with different vce. Assuming that vc=vcc/2, what's the difference. I mean, which is going to clip harder or sooner, etc, etc.

mac

[R.G.]

Let's say that you have two voltage divider biasing schemes for the same transistor with different vce. Assuming that vc=vcc/2, what's the difference. I mean, which is going to clip harder or sooner, etc, etc.

That's what that business about deciding what voltages you want across the collector and emitter resistor was about.

First - different voltage divider schemes that provide the same Vb and Ib on the input have no effect on the output. They may load incoming signal more or less, or have an effect on noise, but no effect on clipping harder or sooner.

What does decide clipping is the voltage division. If you pick a bias point at anything other than the midpoint of the available voltage, then one end or the other of a symmetrical signal will bang into cutoff or saturation before the other does. If you pick a bias point that is not in the middle, then signals will hit either cutoff or saturation first. You may LIKE saturation better than cutoff, as in the Rangemaster. So you just get to pick distortion by where you bias the thing. The input network has little to say about it, other than setting the bias point.

Different input bias networks DO affect input impedance, thermal stability, and noise.

[rockgardenlove]

"What I'm seeing here is that you just want to have it set up so the base is .7 volts (for most transistors) above the emitter...?"

Is this deduction correct?

[Noplasticrobots]

Yikes, I just start plugging in resistors until I get what I think sounds nice.