thevenin

Hi Jason, Where have you looked so far for info?

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.

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

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

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.

-------------- 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.

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

Jason