# thevenin

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

1. ### jasonGuest

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?

Thank you

Jason

2. ### Tom BiasiGuest

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

3. ### Matt FlyerGuest

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. ### jasonGuest

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. ### jasonGuest

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 FlyerGuest

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 KellyGuest

--------------
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. ### jasonGuest

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

Jason  