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Bipolar Transistor Circuit Design & Analysis, Part 6

Summary
There are many applications for one or two transistors. These can include voltage and current amplification useful for interfacing between other system blocks (perhaps ICs).

Described in this series was an easy to understand way of visualizing the operation of a bipolar transistor in its linear operating region. This technique can be applied to the design of simple as well as sophisticated circuits. We explored the Base, Emitter, and Collector impedances and an easy way to calculate the circuit’s ac voltage gain. Also included was a smattering of other related concepts to stimulate your creativity.

All circuit designs involve a series of tradeoffs that must be balanced to optimize performance. A few of the more important considerations have been suggested. A “good” design is one that does a reliable job with the least complexity, cost, power, size, etc.

As a general rule, high-performance circuits (e.g., high gain, wide bandwidth, low DC errors) are best approached with integrated circuit technology where using a large number of devices does not necessarily involve high cost or size. Of course, to justify the design of an IC solution requires sufficient volume, economic, or possibly reliability factors to make it practical.

Nevertheless, when interfacing existing IC components or sensors, it is sometimes advantageous to design simple “glue” circuits to make the transitions. This is where this tutorial should be most useful.

19 comments on “Bipolar Transistor Circuit Design & Analysis, Part 6

  1. otter30
    March 25, 2014

    Howard, congratulations on a very nicely written series. I will point it out to our younger guys here.

    One point I hope you'll touch on in your Ask the Experts session is the tendency for many BJT tutorials to attack every amplifier design with small signal equivalent circuits without regard for when small signal analysis techniques are applicable. 

    Along those lines perhaps you could suggest and discuss analytical techniques for large signal (AC) transistor amplifiers and oscillators.

    Thanks again for the series. It would be great if you could follow up with future installments on JFET and MOSFET amplifier basics as well.

  2. samicksha
    March 26, 2014

    Thank's from my end as well, i would like to take its one of the demerit, Exposure of the transistor to ionizing radiation which can cause radiation damage.

  3. HowardS6
    March 26, 2014

    Thank you for your comment.  You are correct.  In some cases, external factors such as radiation and RF susceptibly must be considered.  This can be particularly important in military and space applications.  Some IC processes are “naturally” radiation harder. For example, dielectric isolation, which separates one device from another with silicon dioxide (glass) rather than a large reverse biased junction, as in typical IC processes, is generally superior.

  4. HowardS6
    March 26, 2014

    Thank you for your excellent suggestions.  These are worthy topics for future discussion.

  5. chirshadblog
    March 27, 2014

    Indeed and I think this will help to make things more proactive.    

  6. chirshadblog
    March 27, 2014

    @samicksha: Well if that is the scenario then the damage has to be analysed. If it has a big impact then things will not be that easy if they really want to go ahead with it.     

  7. etnapowers
    March 27, 2014

    In presence of radiation an extra current in the base terminal due to ionizing radiation modifies the BETA of the transistor and hence its current gain and the polarization point so BJT are really sensitive to radiations

  8. etnapowers
    March 27, 2014

    Damages have also be prevented by mean of a proper design of the power BJT for radiation hard environment.

  9. fasmicro
    March 27, 2014

    >> Thanks again for the series. It would be great if you could follow up with future installments on JFET and MOSFET amplifier basics as well.

    Very great contents. As you noted, CMOS will be highly appreciated. In the industry, RF and power circuits still command the needs of BJT expertise but most other things are in the world of CMOS. Most amps are made with CMOS. The biasing of CMOS in the weak inversion takes us to the physics of BJT though

  10. fasmicro
    March 27, 2014

    I think you achieve such by using a different process and not just the transistor. Even if you use a BJT, for radiation-hardened design, you may need a new type of process. The problem is not just electrical, it is mechanical. A unique process helps.

  11. Netcrawl
    March 27, 2014

    @etnapowers I agree with you proper design is key, BJTs are quite sensitive to ionizing radiation, such radiation causes junction leakage and gain degradation. 

  12. chirshadblog
    March 27, 2014

    @Netcrawl: True but if the design patterns are matched at least 85-90%, the risks of degrading will be very low isn't it ?             

  13. chirshadblog
    March 27, 2014

    @etnapowers: Yes it can be reduced not eliminated isn't it ?              

  14. chirshadblog
    March 27, 2014

    @etnapowers: Well the radiation does have some serious impacts, so its vital to identify the damage and try to minimize it from the beginning itself. If not it won't be easy as such to move forward.  

  15. etnapowers
    May 2, 2014

    @fasmicro, I agree with you on the need of a robust process to ensure a good reliability, but it's important to remember that also the architecture of the system plays a role in the overall reliability, so the designer is a key figure of the production process flow as well as the product engineer in charge of the development of the technology.

  16. etnapowers
    May 2, 2014

    Netcrawl, that's absolutely correct, it's important to choose the appropriate device for the specific application , taking into account the electrical specifications requirements and the sensitivity to radiations of the ICs utilized.

  17. etnapowers
    May 2, 2014

    @chirshadblog: yes, you're correct, it can be only reduced by a proper design of the application circuit board and a good choice of the process, but unfortunately it cannot be eliminated.

  18. etnapowers
    May 2, 2014

    @chirshadblog: agreed, that's absolutely very important not only to identify the failure but moreover to prevent it and to have a good estimation of the possible type of failure, whatever will be destructive or temporary.

  19. etnapowers
    May 2, 2014

    A temporary failure is a failure affecting the functionality of the IC for a certain timeframe. It can generate the failure of the overall circuitry so it has to be accurately avoided because it is as dangerous as a destructive failure.

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