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#185: Back to Basics: Bipolar Transistor bias circuits and Beta …

#185: Back to Basics: Bipolar Transistor bias circuits and Beta ...



This tutorial back-to-basics video discusses the operating point (or quiescent point, Q-point, bias point, etc.) of a bipolar transistor (BJT) circuit, and how the choice of the circuit design can affect how sensitive the bias point will be to the value of Beta (current gain) of the transistor. Ideally, you'd want to use a circuit which is completely independent of beta, so that a wide variety of transistors could potentially be used. Three different transistor bias circuits are demonstrated with three transistors, each with a different value of Beta. The current gain of the three transistors are measured and recorded, and then the resulting voltage ACROSS the collector resistors is measured for each transistor in each configuration. In other words the voltage between VCC and the collector is what is measured and recorded (not the collector voltage with respect to ground). A PDF of the drawings can be found here:
http://www.dorkage.com/youtube/BJT_bias_beta_dependence.pdf

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Tags: transistors

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44 Comments

  1. Date: December 2, 2014 at 21:08

    Thank you Alan for a great explanation. Better than most classes I had in university.

  2. Date: December 2, 2014 at 21:34
    Author: Kevin O'Brien

    In 2nd to last eqn at 15:51 if Rc is very large compared to Rb does it not then just reduce to Vrc = Vcc – Vbe , just using algebra I thought the (beta+1)*Rc would both completely cancel out top and bottom (In the limit with Rb being tiny compared to either Rc or beta). I got all of this except that bit. Not sure why you said Rc at bottom was still present. Help!

  3. Date: December 3, 2014 at 00:23
    Author: bob rock

    Voltage divider is you're best bet for bias for maximum swing ?

  4. Date: December 3, 2014 at 03:30
    Author: 321reh

    Thank You Alan For such a Great Video on one of my most Interesting Topics …Transistor Biasing Circuits!!!!

  5. Date: December 3, 2014 at 05:35
    Author: Applied Science

    Good stuff! I've never done the experiment to determine beta sensitivity with high/low transistor values. It varied a bit more than I was expecting even for the standard divider bias. Thanks

  6. Date: December 3, 2014 at 06:25
    Author: InXLsisDeo

    I love your notebooks. They look so clean and to the point. You should make a video on how to write great notes. I know this sounds stupid, but few people know how to write notes clearly like you.
    (oh and BTW, would it be interesting to simulate the circuits on LTSpice, after the notebook explanation ?)

  7. Date: December 3, 2014 at 22:00
    Author: SeAfasia

    Excellent video. …the third example with voltage dividers resistor is best way to work it.
    Next time teach us the way how power supply a transistor NPN or PNP.
    Thanks..
    Kostas

  8. Date: December 4, 2014 at 08:17
    Author: javitooo

    Thank you very much for your instructional videos, bring me good memories of me EE studies!
    Thank you from Sweden

  9. Date: December 5, 2014 at 15:44
    Author: Michael Katt

    Another great video, well done. If I may could I request that you continue on with this topic and discuss cascading BJT amplifies stages and the effects of input and output impedance specifically as it pertains to RF frequencies and Maximum Power transfer. I know that there is a lot of confusion out there when it comes to this topic, at least there is with me, and I have never really understood why in some applications you design for high input impedance and low output impedance and in other applications you match the input impedances to achieve maximum power transfer.

  10. Date: December 7, 2014 at 23:06
    Author: Jjames Anderson

    I love all your videos, I could not find where to send you a private email,so I hope you don't mind if I ask you a question here. These non contact voltage detectors work by capacitance.what I want to know is how they work in detail like you are so good at. They ring on the hot side but not on the neutral. If you use an amp clamp you can clamp on eather side and it will read current .it would be a great video . Thanks

  11. Date: December 10, 2014 at 01:23
    Author: Allan Anderson

    I watch all your videos very carefully. Well done! I'd vote for a step up in complexity, to, idk, start designing a pre-amp for 2m or HF. 73's de WB0WHD.

  12. Date: December 11, 2014 at 15:04
    Author: Jeffrey Bail

    I think you need a Back To Basics video for this Back to Basics video

  13. Date: December 11, 2014 at 16:47

    L.G. Cowles says:
    the collector feedback circuit needs to be Vcc > 8 volts, 

    and thanks again well done

  14. Date: December 12, 2014 at 11:25
    Author: ElectronFun.com

    Great explanation! I didn't realize the differences could be so significant.

  15. Date: December 18, 2014 at 14:27

    Thanks for your informative vids.

    does beta differ between one certain transistor? say 2N2222 from another 2N2222.

  16. Date: December 24, 2014 at 20:34
    Author: douro20

    The IT-18 is a very nice instrument, and extremely sensitive as well- I have found the beta adjust to be very touchy.

  17. Date: December 28, 2014 at 02:45
    Author: Jeff S

    Thank you for all the work you put into making these videos highly informative and understandable! 

  18. Date: January 4, 2015 at 23:16
    Author: Mikael Karlsson

    I lift my hat off and say: -"Thank you for educating us, professor"

    Keep up your good work.

  19. Date: March 23, 2015 at 02:14
    Author: Pablo Gonzalez

    Great video that really shows why circuits should be made independent of Beta.  You should have probably mentioned that the voltage divider bias you had in this particular example was not very good because of the very low input impedance resulting in higher current drawn from the power supply which in turn makes it a bad circuit to couple with another input signal.

  20. Date: July 6, 2015 at 09:59
    Author: senthil Kumar

    you are a great teacher .
    way of delivering informations are great.
    you make most of them understand
    great job

  21. Date: February 10, 2016 at 13:02
    Author: Chris Gonzales

    This would have been SO useful when I was wading thru the transistor section of the "Art of Electronics" the first time. As with many such books, I almost had to understand the device before I could figure out what the text was about. Of course they also offered a student guide, which was a book to help you understand the book. Here you dispense with all that…

  22. Date: August 6, 2016 at 07:42
    Author: ChueyMr11

    Great video, Can you do a video on emitter feedback bias and emitter bias with the 2 dc supplys.those are the two I don't understand.

  23. Date: August 6, 2016 at 08:51
    Author: ChueyMr11

    Sir at 15:10 where your talking about beta dependence vs the ratio of RC to RB.The IB equation IB= Vcc-Vbe/ Rb + [B+1]Rc  Doesn"t this imply to reduce dependence onBeta you would want to swamp  out [B+1]Rc by making Rb much  greater than  BRc?

  24. Date: January 9, 2017 at 05:59
    Author: Dathan Walters

    Darlington pair transistors have a faster rise time/transition time compared to a signal bipolar transistor? and Darlington pair transistor have an open collector output compared to a signal bipolar transistor?

  25. Date: May 28, 2017 at 04:30
    Author: Mark

    W2AEW DE KD5SMF
    Let's say you want to design a HF transceiver that had a true output of 5 Watts minimum & 10 Watts Max output at the Antenna. Most qrp is actually less than 5 watts & no one actually even gets a true full output of the actual designed circuit. Which transistors and MOSFETS would you use to build a Superhet transciever?

  26. Date: June 26, 2017 at 05:43
    Author: Hari harank

    Sir
    What is the product and model number of the transistor tester you are using? Will you please suggest any schematic for the same?

  27. Date: December 2, 2017 at 07:01
    Author: MrEkg98

    I missed this video earlier. Watching it now.

  28. Date: December 13, 2017 at 15:22
    Author: Philippe Colin

    Hi Alan,
    Is there a simple formula to calculate Ib in this circuit else than Ib=Ic/beta ?
    Thank you !

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