According to Pete: Transistor Biasing Configurations


It’s November, and time for another installment of According to Pete, where everyone’s favorite mustachioed engineer talks us through whatever concepts are on his mind. This month, Pete has transistor biasing configurations on the brain, and covers two of the three major configurations - common base and emitter follower. He’s saving the third major configuration - common emitter - as a special treat for a later date. Enjoy!

As always, if you have any questions for Pete, or maybe some poems you wrote about transistor configurations, we welcome them in the comments. Happy Tuesday!


Comments 31 comments

  • Where’s the love for these transistor configurations? Don’t forget that opamps are merely multiple instantiations and variations of these transistor configurations (you’d need to add current mirrors, too).

    Common base? Useless? Look up cascode, a wicked great use of the common base.

    Emitter follower? Useless? What do you think is the transistor stage chosen for the output of many non rail-rail output stages? If you look at the size of a discrete transistor versus the size of a whole opamp, when you need more current output than the opamp can source, who you gonna call? Emitter follower, that’s who.

    The utility of these configurations is often seen by looking at the input and output impedances of the configuration:
    Low input impedances are typically driven with current sources, high input impedances are typically driven with voltage sources. Low output impedances make great voltage sources, high output impedances make great current sources. Then you can utilize feedback to accentuate good characteristics and suppress bad characteristics.

    • Easy there, turbo. I didn’t say useless (wait, did I? Don’t really mean that), but I’d be lying if I said I had a lot of use for these configurations outside of the knowledge of where they are used within embedded electronics. And see my reply to ER. I love this stuff! But I worry that much of the audience may not, and I’d hate to only bring subject mater that I like to this forum while neglecting the greater needs of the community.

      • It may be becoming more and more esoteric, but this stuff is still definitely useful. In the last design I did, I used a PNP common base configuration to translate a 0-2.5V logic signal to a -5 to 0V-ish signal. Had to do a little brushing up on my transistor configurations, but a SPICE simulator does wonders for that! :-)

  • Can we buy Pete some markets please :)

  • Pete, your markers are giving you trouble because they’re so often pointed up and gravity pulls the ink away from the tip. If you can change your habit and point with the back of your marker instead of the tip, they’ll be pointed down and always ready to write.

  • So many huge thanks! This video is so much what I need in terms of speed learning, friendliness, not too math, etc. I’m thrilled to look at your other videos!

    Thanks again and long life to “According to Pete” - wow!

  • Cool mustache. That’s all I got since I haven’t watched it yet, but got it queued for later review.

  • I’m not sure what the fascination with video is these days is. These are great, but 23 minutes of video ~= 5 minutes of reading.

    • I think the exact contrary. This video helped me more than hours of reading I did before. Thanks Pete, don’t let criticisms stop you shoot great videos for the rest of us.

  • A little tip I learned about NPN vs PNP transistor schematic symbols. NPN stands for “Not Pointing iN”. PNP stands for “Pointing iN Pointer”. Not sure if you ever heard of those or not, but thought I’d comment anyway.

  • As far as MOS biasing, it’s virtually the same as BJT. Main differences are square law vs exponential relationship between current and voltage, and you get infinite input impedance at the gate (MOS equivalent of base) so ‘ib’ = 0 for MOS. All of the main ideas (and schematics) are the same.

    As for the circuits shown here, resistor biasing is OK, but if you need any control over the actual current values then you’ll want to go for something a bit more sophisticated like using a ‘diode connected load’ or a current mirror. Perhaps those would be good videos for the future as well?

  • I enjoyed the video and found it interesting. I think I need to watch it a few more times before I completely grasp the utility of these circuits, but that is only due to my own ignorance. I think what I really need is a good (and not overly technical) tutorial about impedance. Just when I think I understand it, I find something like this and realize that it still hasn’t clicked 100% for me. Thanks for another great tutorial, Pete!

  • Pete, may I suggest using a simulation program next time like LTspice. Who knows maybe Linear Tech will sponsor your ‘show’. Nice tutorial though…

    • I normally shy away from simulations, preferring to build a circuit and really show what it’s really doing. But maybe both for common emitter? I haven’t played with spice for a while, so it might be fun. Prolly could do a whole episode just on the spice simulation. Hmm… I’ll see what I can do.

      • Couple thoughts: 1. I so way value the approach you’ve taken so far, it’s what I end up needing to do on the fly on my bench quite often. 2. ICircuit (http://icircuitapp.com/) which I use on my Surface, but can run on iPads, droid devices, etc is a nice and simple simulator that helps a bit in initially sketching out these types of circuits, seeing if my math is on track, thinking isn’t wrong, etc.

        That said, I like the small byte sized According to Pete segments and one or two on how to use LTSpice would be great :)

  • For those of you thinking of getting into an EE major…listen up…because you will take a class (more than likely two semesters worth) on this subject alone of transistor biasing. For me they were called Active Networks I and II. Like Pete said, this is not a precises thing. If you are not very interested in this kind of thing, it can be very hard to get practical examples of where this is used in the classroom (thank God for the internet).

    If you do take a class on this, I hope you have a teacher like Pete vs. a tenured professor who teaches this from their tenured lesson plan that is grossly out of date so that they can spend more time on research and less time adding value to what you learn (I’m not bitter). Thanks for lesson and bringing back memories of late night study sessions Pete, it reminds me of why I do IT now ;-)

  • We literally just went over this stuff in my Devices I class. We pretty much skipped over common-base and emitter- following. Looking forward to seeing the common-emitter bias (the one! They throw a bunch of formulas at us but never really teach why it’s useful and when you would use it.

  • Is it useful, is it interesting? Oh Pete! Or better yet, for Pete’s sakes! It’s awesome! Honestly, I struggle with this stuff on my hobby time alot. What am I getting out your mini-vid lesson on these simple transistor circuits? How to do a bit of transistor design and analysis. Yeah, your showing common base and common collector, but both are how to work with these really cool and for me at least incredibly slippery little devils that as you rightly pointed out can do all kinds of cool things. I’m sitting here just dying to get home to dig in and power up a simple circuit and see ifn' I can duplicate what you described. Please, o pretty please, don’t stop there! Common emitter is cool and the more examples I chew through and bread board up and play with teh better I get at showin them electronics who’s boss:)

  • Can you recommend some kind of reference guide to the different configurations, base ideas on how to set it up, and some common uses for each? I feel like every time I need to use a transistor, I have to google and try and find someone who’s done something similar to know how to wire it up. It would be nice having a reference so I don’t have to do that.

    • SparkFun sells a couple of the Forrest Mimms books which I rather like and Electrical Engineering 101 which is another one that’s a good read as well. Also, the Encyclopedia of Electronic components Volume 1 is pretty good as it’ll help you with coming up with common circuits used for various components,etc.

      I’m also fond of The Art of Electronics (and especially the student manual) and Practical Electronics for Inventors.

      I’m sure Pete and most the other folks that actually know what they are doing have even better references, but these are the ones I’m forever reaching for.

  • Just curious, What happened with the Spectra Symbol contest. Are you guys still working on it, or was it a wash?

  • This brings back some memories

    • I know, right? I love this sort of very base manipulation of electrons. It’s very low-level stuff, and it’s arguably an art form. But it may also be becoming too esoteric, and I’m not sure if even offering this information is interesting to you all. Realistically, when was the last time anybody did this? (Radio guys, chime in now) Is this interesting but useless? Any feedback you’ve got will be good.

      • Hi Pete. I like to leave the Arduino IRC channels running in the background, and questions relating directly to this stuff come up all the time. Even with 450 people on, someone willing to answer or help is rare other than to offer an awkward “work around” because they don’t know how to use transistors. So, I would say your video is extremely useful and relevant.

        If you guys don’t already have a tutorial on using MOSFETs with Arduinish devices (any GPIO from embedded processors) to drive motors and steppers, that would be a great one for a lot of people.

        P.S. 1uA times 100 = ?

        • I know! I puckered when I watched the video and heard that (and a couple of others). I’m amazed that I wasn’t called out sooner. Points for paying attention.

          • You will appreciate this. I found a cool bug and put it on the home page at www.andahammer.com (Not fishing for traffic. I don’t think we overlap much with SF).


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