Connect with us

thevenin

Discussion in 'Electronic Basics' started by jason, May 11, 2005.

Scroll to continue with content
  1. jason

    jason Guest

    Hi All

    Anyone can explain more about deriving a small signal model of any
    mosfet or bipolar circuit configuration using two port amplifer and
    thevenin theorem?

    I do not understand this theory. ANyone can help by given a few
    examples?

    Any links to share?
    Thank you

    Jason
     
  2. Tom Biasi

    Tom Biasi Guest

    Hi Jason,
    Where have you looked so far for info?
     
  3. Matt Flyer

    Matt Flyer Guest

    I would recommend that you start with a book on micro-electronic
    circuits that covers dependant sources. It has been a few years, but if
    my memory serves, the small signal models typically model the transistor
    as a dependant source conencted with models of the input and output
    impedance. The models generally assume that the device is biased so
    that it is operating in the linear region.

    Sorry if this doesn't help much.

    One book that I found to be very helpfull in explaining transistors in
    "real world" terms is "The Art of Electronics". Some books I have
    encountered try to teach you to rely on paramaters that vary all over
    the place like a bjt's Beta, but this one doesn't.
     
  4. jason

    jason Guest

    Hi Tom and Matt

    Thanks a lot. I am looking around books such as The art of electronics
    and also some microelectronics book. I understand some of the
    explanation given in book but there are times I cannot understand which
    node I should short , or open circuit in order to get the correct
    voltage node or resistance?
    I am still reading and try to understand.
    I post here to see if anyone know the concept well and can share the
    way how they catch the concept.
    Thank you all

    rgds and thanks
    Jason
     
  5. jason

    jason Guest

    Hi Tom and Matt

    Thanks a lot. I am looking around books such as The art of electronics
    and also some microelectronics book. I understand some of the
    explanation given in book but there are times I cannot understand which
    node I should short , or open circuit in order to get the correct
    voltage node or resistance?
    I am still reading and try to understand.
    I post here to see if anyone know the concept well and can share the
    way how they catch the concept.
    Thank you all

    rgds and thanks
    Jason
     
  6. Matt Flyer

    Matt Flyer Guest

    Jason,

    I was just thinking about your question again. It occured to me
    generally one tries to design a transistor circuit so that the operation
    is not dependant on the properties of the transistor, but rather on the
    circuitry connected to it. For example, consider a simple small signal
    BJT. If I remember correcltly an example of such a transistor would be
    a 2n3904. To use this transistor in the small signal 'mode' it is first
    necessary to bias the transistor into the linear region by providing
    sufficient DC drive to activate the device. Once the device is properly
    biased, you could connect external resistors to the emitter and
    collector to provide a small signal gain. Since aproximately the same
    current would flow through the collector as the emitter the ratio of the
    two resistances (Rc / Re) determines the small signal gain (as long as
    one doesn't over drive into saturation). A similar model holds true for
    FETs.

    The point I am trying to make is, that it is likely not wise to model a
    circuit after the performance or parameters of a particular transistor
    as these paramaters vary alot from one device to another. Consequently,
    it is better to step back and design the circui so that it is relatively
    impervious to these paramaters and you can then develop a small signal
    model from that circuit.
     
  7. Don Kelly

    Don Kelly Guest

    --------------
    I think that you are trying to mix up two concepts- the modelling of a
    transistor circuit and the concept of a Thevenin or Norton model. The
    latter pair can be applied to a transistor device only in the small signal
    case where the circuit can be modelled as a linear one. Thevenin (and
    Norton), as with most circuit theorems, depend on "superposition" which is a
    mathematical concept associated with linearity {e.g (I1+I2)R =I1R +I2R}
    It can be shown that a source, however complex, can, if and only if, it is
    linear, can be represented by the open circuit voltage in series with the
    impedance seen looking back into the source when the voltage is set to 0.
    The Norton model is the short circuit current source in parallel with the
    impedance. In effect the Thevenin impedance becomes the open circuit voltage
    divided by the short circuit impedance, or; in the case of a transistor
    circuit -the slope of the I vs E curve at and near the operating point.
    Norton and Thevenin have been around much longer than transistors and are
    applicable in a much wider area of electrical devices and systems than
    electronic circuits.
     
  8. jason

    jason Guest

    Thanks All. I will read more. If there is anything to consult, I will
    write back.
    Thank you so much

    Jason
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-