jason said:
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.