🚀 go-pugleaf

In article <1992Dec29.214231.1135@inmet.camb.inmet.com> bwhite@cobra.camb.inmet.com (Bill White) writes:
>Hello.  I am reading Horowitz and Hill's book on Electronics, but 
>I am having trouble understanding the grounded emitter circuit.  In
>particular, I am having trouble understanding why the grounded emitter
>is not thermally stable.  I believe it, I just don't understand why

>The axioms are:
>    a The Ebers-Moll model of transistor behavior states that under
>      certain conditions conditions which establish a well-conditioned
>      environment the following two equations hold.
>
>	I_{C} = I_{S}[exp(V_{BE}/V_{T})+1]	(*)
>	I_{B} = I_{C}/h_{FE}			(**)
>
>      Where V_{T} depends on temperature, and is kT/q =
>      25mV@20degreesC, and h_{FE} is a constant.  

	Only for a small-signal model is h_{FE} constant.
And, when dealing with power generated in the transistor, the
leakage current of the collector/base junction is an
important correction to the external base current.

	The current gain, h_{FE}, is a fairly strong function 
of temperature; my 2N2222 data sheet shows this current gain
at 100 C is roughly 1.6 times the value at room temperature
(25 C).  The leakage current doubles every 10C (rule-of-thumb),
and the external I_{B} is diminished by the amount of
the leakage current ('cuz that current comes in over a
different wire...).

	So, two of the important temperature effects are
simply omitted from this model: they are usually included
only when a power transistor is used, or when some unstable
sorts of biasing (like simple common-emitter bias) are employed.

	John Whitmore