In Reply to: "beta" of transistor posted by Alvonzo on 12/15/00 at 4:24 AM:
Hello, Alvonzo.
Imagine for a moment that you have a resistor of some given value. We characterize that resistor by the ratio of the voltage across it divided by the current flowing through it, but how do we arrive at that state of affairs?
If we place a voltage source across the resistor, the current flow is the consequence of doing that placement. Conversely, if we inject a current through that resistor, the resulting resistor voltage is the consequence of doing that injection. In either case however, the relationship between voltage and current simply is what it is! That ratio is the defining relationship of the resistor itself.
Now consider a transistor. Because a transistor is more complex than a resistor, the defining relationships are more complex. For example, "beta" is the ratio of collector current to base current while "alpha" is the ratio of collector current to emitter current. Between the base and emiiter leads, the base-to-emiiter voltage versus current relationship, aside from breakdown effects, is defined by a diode equation: Ib = Is * ( ( exp ( q V / (k t) ) - 1 ).
There are other relationships too, but the point is that our attention need only be directed to those relationships which help us analyze the situation(s) of our own particular circuit(s). Whatever our circuit(s) might be, the transistor(s) couldn't care less!!
Has this helped?
John Dunn - President
Ambertec, Inc.
ambertec@ieee.org