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Questions about Q point-transistor

  1. Jan 27, 2015 #1
    So as the title says i need to learn some things about the function point Q of a transistor/amplifier.
    It is written in books that we need it steady,we need it not affected by β etc,it says how to find it etc

    But i don't know the meaning of this.What does the Q point represent?It refers to the funtion of the transistor(judging from its name-function point-),but how?What is the practical meaning of Q?Why we need it steady etc etc.Whatever information you can give me is more that welcome.Thanks in advance.
     
  2. jcsd
  3. Jan 27, 2015 #2

    rude man

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    You want a stable operating {"Q"} point so your signals are not distorted, the gain is optimized, etc. etc. There are many reasons the transistor current should vary only if the input signal(s) varies. Same goes for collector and emitter dc voltages.
    Beta of a transistor varies widely from transisitor to transistor and with temperature, to cite two prominent examples. Your circuit should not vary transistor c or e current as the base current varies. This means beta should be high as possible. This is like an op amp where the open-loop gain of the amplifier should not matter.
     
  4. Jan 28, 2015 #3

    NascentOxygen

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    The AC gain depends on circuit & transistor parameters, such as β, VBE and IE. Circuit parameters need to kept fixed and stable if AC gain is to be kept stable at its designed value. Transistor parameters can change with temperature and with drifts in VCC (as occurs when a battery starts to go flat). So we trade off some of the possible gain we potentially could have, for better stability at a lower gain. The result: a circuit with lower gain than it could be made to have, but at least it is stable and dependable.

    Moreover, by making it a relatively low-gain circuit, when any transistor burns out and needs to be replaced, we can plug in one of a similar type and expect the circuit to operate the same as it did before, despite the replacement transistor having a gain possibly many times different from the one it replaced (e.g., β for some types can be anywhere from 80 to 600).
     
    Last edited: Jan 28, 2015
  5. Feb 2, 2015 #4

    LvW

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    I`ve got the impression that this question was not yet answered properly.
    So - let me try:

    The most important input-output characteristic of the transistor is the "transfer curve" Ic=f(Vbe).
    This is (similar to a simple pn-diode) an exponential characteristic. Now - we wish that a small change at the input causes a corresponding output change in Ic.
    "Corresponding" means: Without modifying the form of the signal, hence: As linear as possible.
    From this, we must provide the transistor with a "good" starting point.
    That is a point (the Q-point !) within a part of the transfer curve which has a good (as good as possible) linearity.
    In many cases, such a point is selected with a current Ic of some milliamps.
    More than that, each working transistor needs a voltage Vce between C and E. Because this voltage (as long as it is not too small) has a minor influence on the whole operation only, it can be selected and fixed due to some external constraints (supply voltages, external resistors).
    As a result this pair (Ic; Vce) constitutes the Q-point of the transistor.
    As a consequence, after exciting the input with a signal to be amplified, the output current will swing around this Q-point.

    After this point is selected (on the paper), you must design your amplifier stage accordingly. This step, however, requires some more knowledge.
    To realize the desired Q-point (Ic, Vce) you need a certain base-emitter voltage Vbe=(0.6...0.7) volts.
    Because it is not possible to determine the exact value (tolerance, temperature effects) it is common practice to use an approximate value (0.65V) and a sort of dc feedback, which makes the collector current much less sensible to these Vbe uncertainties. However, this item (feedback) deserves a separate discussion.
     
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