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eMOSFET and common emitter amplifier

Discussion in 'General Electronics Discussion' started by Daniel Hammond, Jan 18, 2016.

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  1. Daniel Hammond

    Daniel Hammond

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    Jan 18, 2016
    Please see attached file.

    Thanks.
     

    Attached Files:

  2. LvW

    LvW

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    Apr 12, 2014
    Are you familiar with the properties and consequences of negative feedback?
    In both circuits the resistor in the source resp. emitter path provides negative feedback.
    More than that, it would me much easier for us to help you if you would spend some time for typing your questions instead of providing us with a pdf file and handwritten sentences.
     
    duke37 likes this.
  3. duke37

    duke37

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    Jan 9, 2011
    The gain in your npn circuit could be limited by the collector voltage. Have you worked out the DC conditions and the maximum output voltage swing that you can get?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    In general, the formulas for the gain of both circuits is very similar.

    Is this homework?
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Okay, i hate to say this... but this is one instance where an analog simulation program, such as LTSpice (free), will shed some light on the subject. And you can use it to draw and upload decent schematics too.
     
  6. LvW

    LvW

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    Apr 12, 2014
    Hevans1944 - may I ask you: Why do you hate to say this?
    For my opinion, circuit simulation programs are a very helpful and versatile tool for improving the understanding of electronic circuits - provided the user is familiar with the properties and limitations of the various available analyses. (That means: Not the program has important limitations, but the various analyses!).
    For example, one must know what the AC analysis can do and what it cannot do.
    As another example: An opamp with resistive positive feedback. The results of AC or DC analyses show that the circuit is stable and has gain. And the program did not make any error!
     
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  7. hevans1944

    hevans1944 Hop - AC8NS

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    Well, maybe hate is too strong a word. My first encounter with simulation was in a college class in the 1970s where we were required to solve differential equations representing circuits using the Runge-Kutta method of approximation... which algorithms we had to program on Hollerith punched cards. For transient analysis it was always a real PITA trying to find the iteration interval that would provide "reasonable" results for fast transitions while not taking inordinate amounts of expensive mainframe CPU time for slower transitions... like slowly damped ringing oscillations. Things have improved since then with the invention of SPICE programs that were later ported to personal computers and finally made available for free. But the whole classroom experience soured me on simulation, especially simulation of simple circuits, when I could just pop upstairs to my electronics laboratory and "simulate" a circuit with real components and real test equipment.

    I suspect that today many newbies start out with SPICE simulations without understanding any of the underlying assumptions or computational limitations. SPICE is always an approximation to the reality provided by real components connected in real circuits, but it takes some experience working with those real components and real circuits to appreciate the limitations of the approximation. Coming from a hardware background, first as an electronics technician and much later as an electrical engineer, I am a firm believer in "hands on" construction as the first approach to learning electronics. Learn the maths required for circuit analysis along the way, then use computer software tools such as SPICE to speed up design iteration and act as "sanity checks" for proposed designs. Ultimately, however, the real sanity check is a design that actually works with real components, not a computer model.

    So, yeah, I agree with you. Circuit simulation programs are very helpful and a versatile tool for improving the understanding of electronic circuits... with all the caveats you mentioned. I just hate to see newbies dive into SPICE and think it represents the "real world" of circuit design and analysis. Sure, it helped back in the day to design integrated circuits (the reason it was invented) but it isn't the be-all and end-all of electronics design.

    In the present case, for such simple circuits as @Daniel Hammond offered, SPICE can be a good "what if" tool to aid in his understanding. It allows easy substitution of different component values to see what effect this has on circuit operation... within limits that are probably not apparent without using actual components wired into actual circuits, but certainly faster and cheaper. So, in this simple case: GO SPICE! Oohrah.

    Hop
     
  8. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    No-one has explained WHY? or what is wrong.
    Every-one is too intent on using simulation software because they don't know how to design a circuit.
    Here is the major flaw.
    In the first transistor example the base bias resistors put about 2v2 on the base and this will put 2v2 - 0.6v = 1v6 on the emitter.
    If 1v6 is dropped across 1k, 5v8 will be dropped across 3k9 and the collector voltage will be 5v8.
    If you replace the collector load with 10k, the voltage drop across 10k will be 16v. But the supply is only 12v, so the collector will use up all the 12v and pull the voltage down to 1v6 plus about 0v2 (the minimum collector-emitter voltage when the transistor is fully turned ON) = 1v8.
    This means the collector has nowhere to go in one direction. The transistor is fully saturated.
    You will need to lower the base voltage by a small amount before the transistor will come out of saturation and this is one of the factors that prevents a gain of 10.
    This is how to look at a circuit before you wildly fit it into a program that will simply confuse you more than before.
    It like looking at a CRO, seeing all the background hash and wondering why the circuit is not working.
    You have to know how to use these crutches.
     
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  9. hevans1944

    hevans1944 Hop - AC8NS

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    My point exactly! Except I would call them tools instead of the more pejorative term crutches.

    Gee, Colin, did you really download that PDF and squint at the schematic and comments?! I am impressed. Pretty good explanation, too, and not one mention of beta or alpha or anything else irrelevant to understanding the problem.
     
  10. duke37

    duke37

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    Jan 9, 2011
    Not quite, in #3 I suggested that he should look at the DC conditions, he did not do so and Colin has done this for him.
    Get the DC conditions correct to start and then add AC variations.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I will also note the last question in post number 4, the single post by Daniel, and the observation that it has been seen in the past where questions of this nature are asked on a plethora of web sites hoping for a quick answer without any additional effort...

    However I complement Daniel for asking the question in a way that is very difficult to spot duplicates.
     
  12. LvW

    LvW

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    Apr 12, 2014
    Hevans1944, I completely agree to everything you wrote.
    However, just a small comment:
    Because you have mentioned the limitations and the necessary approximations of the simulation program I think we should realize that all hand made calculations (based on formulas) will contain much more approximations and simplifications if compared with circuit simulation.
    (Example: In Colin Mitchells contribution (a) the base current was neglected and (b) an estimated value of Vbe=0.6 V was assumed).

    More than that, I think the error introduced by the mentioned limitations/approximations will be much smaller than errors caused by parts tolerances.
     
  13. LvW

    LvW

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    Apr 12, 2014
    Colin, of course, I know what you mean. However, I doubt if the questioner (a newcomer?) will be able to follow you.
    At first, you should mention the fact that the base current was neglected in your calculation.
    More than that, it is not the base voltage that "puts" 1.6V on the emitter. It is the emitter current which produces this voltage across Re - thereby allowing the estimated standard value of Vbe=(0.6...0.7) V. This is the result of DC feedback provided by Re.
    No - across the 3k9 resistor the voltage will be 3.9 times the voltage across Re: 3.9*1.6V=6.24V.
     
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