Exam studying.

Discussion in 'Homework Help' started by pfelectronicstech, Jul 12, 2012.

  1. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
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    I'm stuck on a couple of study questions I just can't get. The first one is this.

    Which one of the following actions will you take to decrease the rise time of pulses through an amplifier?

    Is it :Increase low frequency response
    :decrease low frequency response
    :increase high frequency response
    :decrease high frequency response

    I think its Increase low frequency response?

    Am I correct, am I on the right track? Thanks for the help guys
     
  2. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    No. "decrease rise time" = increase rise rate = faster
    Got it?
     
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  3. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    Ok got it, makes sense now. I was over thinking it before. Got it now, thanks.
     
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  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    NOPE.

    Here's a hint: look up the fourier frequency content of a square wave, figure out which parts have the most LF content and which have the HF content. It will answer the question.
     
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  5. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    Ok I am seriously stuck on another study question.

    To change the relationship between a waveform and the zero-volt axis, and at the same time keep the shape of the waveform, you would use which of these?

    1:Toggle flip flop
    2:d-c restorer
    3:limiter
    4:clipper

    I just can't find the answer in my book, I've looked and looked and usually I'm good at picking up the answer but I'm having trouble here. I'm thinking a "clipper" but again I'm stuck, I just can't find it. Thanks again for the help.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The key is in the question:

    It's badly phrased, but picture a sine wave on an oscilloscope. You want the waveform to remain the same size and shape, but you want to shift it up and down so the zero volt line passing through the sine wave moves with respect to the waveform center line. In other words, you want to change the DC level reference of the sine wave.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    I'll agree with bounty hunter in that this is very poorly phrased. I think the person writing the question was trying to find a way to describe the situation without using words that would give away the answer.

    However, without knowing what the exact intend of the question is, I think you can settle on answer 2.

    The key is that they specify that you want to keep the shape of the waveform. Both a limiter and a clipper (and what's the difference?) would either change the shape or do nothing. So those can't be the answer. A toggle FF would completely change the shape, even if it were a digital signal. So, by process of elimination, a d-c restorer is the only possible choice remaining. The name implies that it will add (restore) a DC level to a signal, which will shift the signal up/down by not change its basic shape.
     
  8. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    yeah i got it, its a clamping circuit which a D-C restorer is call. I found the answer, and yes its a bit of a awkward question. They sure try and trip you up with these questions.

    I have another question, the last one I think I am gonna have trouble with.

    To transmit only portions of an input wave lying on its side of an amplitude boundary, you would use what,

    1:limiter
    2:clipper
    3:toggle flip flop
    4:d-c restorer

    I'm looking for the answer but having a tough time. A hint would help? Thanks again.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    I would tend to say a clipper, but I don't understand the distinction they are making between a limiter and a clipper. I suspect it is a question of how that author has grown acustommed to using the words.

    Can you look in your material and see how they describe the two and what is different between them.
     
  10. bretm

    Member

    Feb 6, 2012
    152
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    I've heard these terms used with the distinction that a clipper is a one-ended limiter, i.e. a limiter restricts the output to a voltage range and a clipper just limits one end of the range (either the top or the bottom). But there's no way to tell without more context.
     
  11. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    It says this is an IMPORTANT fact, clipping and limiting circuits change the input waveform, but clamping circuits retain the original waveform.
    Then it says on another page: a d-c restorer goes by other names such as line stabalizer,clamping circuit or clamping network. Thanks again for the help.
     
  12. WBahn

    Moderator

    Mar 31, 2012
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    Wikipedia says they are the same thing. Doesn't surprise me that, at least some people, use one to mean that it clips only one end and the other to mean that it clips both ends. Wouldn't surprise me if another group of people meant it the same way but swapped the terms.

    Assuming the writer of the question is using the terms the way you describe, then I would say that the answer would be a clipper.
     
  13. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    WBahn, are you saying don't go with d-c restorer? The book seems to indicate this is the right answer?
     
  14. WBahn

    Moderator

    Mar 31, 2012
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    I should have quoted the material I was responding to, since your post wasn't there when I started writing mine. I was talking about a "clipper" and a "limiter" and that Wikipedia says they are the same thing.

    I think the best answer for the first question is a DC restorer but I think the best answer for the second one, the one about transmitting only the portion lieing on one side of a boundary, is a clipper. I do NOT see a DC restorer being the answer to that question.
     
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  15. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    Thanks we were right on both questions.

    I'm having trouble with Duty cycle and determining the duty cycle. Pain the butt if you ask me, but I gotta learn it. Any hints for learning duty cycle?
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    Duty cycle of what kind of waveform. In general, duty cycle is simply the fraction of time that a waveform is "on". Depending on the specifics of the problem, you sometimes have to get a bit creative about defining it in terms of the waveform being used. In that case, you can usually resort to equating it to what fraction of the average energy would be dumped into a resistive load compared to if the waveform was not being altered (i.e., 100% duty cycle).
     
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  17. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
    4
    Yeah a percentage question threw me off. "Determine the duty cycle for the pulse down". I can't post the graph but they give you 6V vertical and 22.5 usec horizontal. I don't know why but this lesson confused me some. Pretty stupid huh? Thanks again for all the help, I just want to be as good an ET as possible. Stuff like this shakes my confidence but I get through it.
     
  18. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
    4
    Well thanks for the help, I only got an 80 on my exam which I am not happy about at all. Passed but not good enough in my book, but thanks for all the help. See you in a few weeks when I am studying for another exam.
     
  19. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    My hint is that you do some internet research and learn this subject thoroughly. Duty cycle is the basic building block to understanding how switching converters work, which applies to about 99% of the power supplies built in modern times.

    If you don't learn this well, you will be like a blind man going forward in electronics.

    http://en.wikipedia.org/wiki/Duty_cycle

    http://en.wikipedia.org/wiki/Pulse-width_modulation
     
    Last edited: Jul 17, 2012
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  20. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
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    Thanks I read the wikipedia page already. I understand the concept, I just had some issues with calculations, and the formulas.
     
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