Biasing bipolar transistors

applied algebra.

V_be = constant I_e = -Ib * H_fe

The question must first be asked: optimum in what respect? There are several factors that may influence the choice of biasing circuit. E.g., -

- stability with variations in temperature

- stability with variations in power supply

- stability with variations in transistor characteristics from sample to sample

- for linear amplification, expected amplitude of input and output swing and there are others like noise, frequency response, impedance, gain, power dissipation, waveform distortion, dc and ac feedback, etc.

Simulation?

Why not use LTSpice? It's fantastic and free.

I'm playing with it right now, studying an old design for a discrete transistor DC amplifier.

I use Vbe = 0.6V (or so) and do it on paper for a first cut (if you want things to work over temperature remember that Vbe can be quite different at -40C and +55C). Then I use LTSpice to verify that I didn't have my head stuck deeply into my assumptions.

Then, since I have LTSpice open anyway, I use it to check the rest of the circuit.

Remember that there are a whole bunch of ways to bias a transistor circuit from amazingly simple to simply amazing; all have their place and what's 'optimum' for one circuit is often woefully inadequate or terrible overkill for another.

--
www.wescottdesign.com

Single BJT common emitter voltage amp? If so, I can walk you through some of the details.

Jon

Building it is the worst way to design the bias bits, unless you intend to only ever build one. Transistor betas can vary over a 10:1 range, and Vbe changes about 2.5 millivolts per degree C, and transistors often self-heat. A bias design should be reasonably insensitive to those effects.

If you can't do it analytically, simulate it with LT Spice. But vary beta and Vbe to make sure it will work over the full datasheet beta range and over temperature.

John

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Use a Spice simulation program that has automatic worst case analysis.

Kevin Aylward

SuperSpice

Hi, Kevin. Been a while.

I think the OP wrote "while it's still all on paper." Which is a skill worth working on, even if "optimum" is never fully achieved and even if the OP didn't say what the circumstances were. It means the OP needs to get a handle on BJTs, which is good.

For some specific, worked examples of designing a degenerative 1-BJT common emitter voltage amp stage, the better I've seen done by hand were in the Student Manual for the Art of Electronics, 2nd ed. Not the textbook, but the student manual, where it walks one through a cookbook ordering of steps with some whys. Together with the textbook, it's decent.

I remember your descriptions and your web links (which I appreciated) regarding these things, too. But one needs a little more background, I think, to appreciate the approaches better.

Also, the OP provides _no_ information about their own background, skills, what they feel they understand. The one thing we do know is that the OP does _not_ feel they understand "BJT biasing," for whatever purpose they may have in mind.

Two things that helped me quite a bit, book-learning wise and aside from AofA and its student manual, were:

Ian Getreu's "Modeling the Bipolar Transistor," 1979 printing of the 1976 book; and, Andrei Vladimirescu's "The SPICE Book," 1994

Ian's book has been digitized and made available from Lulu, about a month ago. (Tektronix was the prior publisher and they stopped doing that in 1979.) One of the things it does is to approach, for those interested, the BJT models from a physical understanding approach starting with Ebers & Moll's 1954 paper and moving forward with explanations and reasons along the way through Gummel-Poon (1970). These include excellent graphs and charts which illustrate in great detail many important elements in a way I haven't seen in print before. That's the first half of the book. The other half is about how to set up and measure the parameter values, which enhances the theory a great deal I think.

Clive Maxfield has done a recent review:

But without knowing much about the OP or what problem is being worked, it's really hard to know what to suggest. And the OP has remained silent on this. Oh, well.

Hope you are doing fine, Jon

Perhaps I scared him off with my earlier reply, saying that certain criteria should first be defined to make the term "optimum" meaningful, while citing several examples of such criteria. If that's the case, it was not my intention.

Hey pimpom,

No no, not scared, off, trying to get a handle on all that was mentioned for thorough understanding and application of such. Still working on it, as I only have a little time each day to work on this.

What I am trying to do: build a "hearing aid" for my shortwave radio, essentially an RF amplifier that I can tune, which will filter out most of the crap surrounding the station of interest, before feeding the output to my Sony ICF 7600 for further cleanup/amplification. Originally tried working with JFETs, but gave up on that when I found they don't really offer any greater amplification than bipolars. Finally settled on the NTE199 (and I don't know what that replaces) due to it's minimum 400Hfe. Am thinking three of these in series would probably suffice for what I am wanting. Found a biasing tutorial at

(think I have that right,)which looks helpful, and am fiddling with it at the moment. Also hitting the books again, so I can understand what it is I am trying to do (Electronic Principles-Malvino and Art of Electronics-Horowitz & Hill, 2nd ed).

My background: Taught myself the fundamentals of electronics in high school (30+ years ago) and went to a trade school to get a better understanding of such. Worked on peripheral computer equipment in a mainframe environment and than in a mini-system/small business environment for 12 years before auto-accident/head injury left me on disability. Am trying to get back into the loop, just to stay mentally limber if for no other reason. Also fix things for family and friends on occasion.

Current understanding: I am thinking that if I can plot the load line for the transistor in question and determine the Q-point, I will be halfway there. Am just wondering if there is anything else I should do before setting down at the workstation.

The reason for all of this is, I live in a house with steal siding that shields my whip antenna from all but the most powerful broadcasts under 30 MHz. Am wanting to be able to pick up Voice of Korea without an outside antenna, from my bedroom. Not easy. Have already built the basic project, but it doesn't work as well as I think it should. Trying to improve it.

That's the basic blurb. Didn't get scared away, just impressed with all there is to know and understand. Sorry it took me so long to reply. Holiday busyness, and my disability got in the way. Can upload the entire scematic to abse, if anyone really cares. It's still in transition though.

Thanks, all, for your input. I am busy chasing down ideas now.

Dave

PS: Merry Christmas, everybody.

Some of us have trouble accessing binaries at abse. Perhaps you could upload the schematic to a free image hosting site instead. Imageshack is just one example of such a site.

Ah. Noted. If anyone wants it, will put it there. Have to check that out.

Thank you.

Dave