Biasing for Beginners

[ryanscissorhands]

Since we are ALL trying to make useful names with newb questions, I figured I'd try to start a series of "_______ for beginners" topics.

First, let me se if I understand biasing. Biasing a transistor is done with a "trim pot" whihc is usually not accessible from outside of the enclosure. It is meant to "tweak" the gain fo the transistor to the desired range, and to be left until it is accidentally moved or is no longer desireable. Am I right on this, or just not getting it?

Now a few other questions.

1) Why do we need to bias transistors? Is it because of a variance from part to part that needs to be adjusted for?

2) Why do some circuits need them and some do not?

If you are also a newb, please try to continue the "_____ for beginners" idea to make that kind of stuff easier to find.

[aron]

Letsee, from the FAQ:

How can I find out more about transistor biasing?

From R.G.Keen:

By far the most comprehensive and comprehensible book on transistor bias and gain that I've run into is "Practical Transistor Circuit Design and Analysis"by Gerald Williams; 1973 McGraw Hill, TK7871.9.W53, ISBN 0-07-070398-1.

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Also from FAQ:

http://www.geofex.com/fxdebug/bias_prob.htm

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For FETS

http://ourworld.compuserve.com/homepages/g_knott/elect247.htm

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For Bipolar Transistors:

http://www.interq.or.jp/japan/se-inoue/e_dance26.htm

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And finally, take a look at this pic:





OK, see the sine waveform???

See that line in the middle of the waveform? When you adjust the bias, you move that waveform "up" or "down" with respect to the outside dotted lines. See those dotted lines? Imagine that is the where your waveform might be clipped (i.e. chopped off) if you exceed those points.

OK, now the middle line is your bias, the outer bottom line is your negative "swing point" and the top line is the positive "swing point".  If you move the waveform around with respect to the outside lines, either the top or the bottom of the waveform might be chopped off.

An extreme condition is this picture of a class B amplifier. See that a whole half of the waveform is gone.



Check out this page from the FAQ:

http://ourworld.compuserve.com/homepages/g_knott/index1.htm

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From google.com

Here's more links:

http://www.tpub.com/neets/book7/25c.htm

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

Once you have read all of the above, let me know how I can improve the FAQ to make it easier.

Thanks!

Aron

[aron]

Check this out:

http://www.bcae1.com/transres.htm

[Noplasticrobots]

Thank you Aron! I've read that first article before and was completely lost without a calculator.

[Seljer]

Heres another nice article on transistors that really explained a lot of things to me: http://www.faqs.org/docs/electric/Semi/SEMI_4.html

[Jay Doyle]

See those dotted lines? Imagine that is the where your waveform might be clipped (i.e. chopped off) if you exceed those points.

Actually, the waveform will get clipped when it's limits (the dotted lines) intersect or exceed either axis. When the waveform hits the Ic axis, the trasistor is saturated, when it hits the Vce axis (or more importantly when Ic=0) the trasistor is all the way off.

In the Class B example you can see how one half of the waveform is below the Vce axis (or tries to make the transistor shut off more than it can because Ic=0).

Hope this helps,

Jay

[the_stig]

Great links ! I've read it all. I think I'm beginning to understand some of it  I've still got some question though. There is this half watt amplifier made by Lovepedal. There's a page about it here: http://www.lovepedal.com/lovesquarebass.htm .
http://video.google.com/videoplay?docid=3700083473289620474&q=fatback - it is said here that the volume and gain is controlled by the bias knob. Looking at the schematic available here: http://www.interq.or.jp/japan/se-inoue/gif/dance2611.gif is it the Rb or the Rc that is controlling the gain and volume output ?

Philip