Designing transistor circuits, what about the parameters

Discussion in 'General Electronics Chat' started by matrixofdynamism, Jan 9, 2012.

  1. matrixofdynamism

    Thread Starter Member

    Feb 8, 2009
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    I am somewhat confused here. When we wish to create a biased transistor circuit, what are the parameters that WE decide and what are the ones that are already fixed or become dependant on our other choices.
    For example, we have beta (current gain), Ic, Ib, Ie, Vc, Vbe, Vce among some others.

    I have seen a circuit where Vce is chosen (and thus Vc for the given supply voltage), and Ic is chosen. But the problem is we can't just get that Ic since we need to get enough Ib as well, and use beta here. So at the end we Choose Rc, Rb. Fair enough,

    Vbe and beta are fixed almost always for a transistor. Other things are chosen by us right? But how can we choose a Collector current value when in fact the value of beta is never constant as the one used by us in calcuations. Thus when we make the acutal circuit we will NOT get the same value of Ic (that we wanted and used to Rc to get) and Ib (which we found by using the required Ic) since even though Rc and Rb is same, the beta is different in the real circuit.

    What I am confused about is whats the point of all the calculations when the real transistor is not going to have the beta value that we have used in calculation and therefore not going to produce the required Vce and Ic in practice? Why not take a transistor and use trial and error instead to get the required Ic and Vce??
     
  2. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    You've asked a very good question, obviously you've spent some time thinking about this. Look at your transistor book and you'll find biasing circuits called "beta independent" or something similar. An example is the typical voltage divider base bias. See the link below.

    http://webtools.delmarlearning.com/sample_chapters/05.pdf
     
    matrixofdynamism likes this.
  3. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Normally you use a "beta independent" bias scheme as JMac3108 mentioned. The usual practice is to design circuits that are relatively insensitive to beta or work at minimum beta, since beta varies over a wide range from unit to unit. It's bad practice to design a circuit that needs a particular value of beta for proper operation.
     
  4. matrixofdynamism

    Thread Starter Member

    Feb 8, 2009
    11
    0
    <Image supposed to go here>

    We first calculate the base current and than the collector current. Of course there are some assumtions like the transistor is not in the active region. My question is since VRc = Ic * Rc, thus if I get a bigger value of Rc does that mean that VRc>Vcc eventually? This equation does not make sense. Besides if I increase the value of Ib and this Ic becomes bigger as a result than does that mean that VRc>Vcc eventually? Of course this does not make sense.
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    If we start analysis of this circuit

    [​IMG]

    At the beginning we don't know in which "state" transistor is.
    So we start by assuming that BJT is in active region.
    And now we can use well known procedure. Additional we must assume the Beta and Vbe (but we also can use the iteration to find Vbe and Beta).
    Next we use Ib and beta to find the collector current and VRc voltage.
    And If VRc < Vcc ( Ib*β < Vcc/Rc) the BJT is in active region.
    If not, the BJT is in saturation region. And our calculation for Ic current is wrong. In saturation Ic ≈ Vcc/Rc.

    But in real life the beta value is never constant so we never use this circuit as a linear amplifier.

    In real life we use this version

    [​IMG]

    We add voltage divider and Re.
    And now we can design the circuit so that we can ignore Vbe and Beta variations.
    If we choose Ve > 1V and voltage divider current much larger than the base current (10 * Ib typical ) we can design the circuit using only beta_min from the data sheet.
     
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