In chapter 2 when discussing early effect, it is explained that ro=(VA+Vce)/Ic. Later explains that when emitter degeneration is used, Rout=ro*degeneration factor. Using these formulas, when I do the math for his common emitter stage example (figure 2.5b), I get the correct results that match those in the book.

Here is where the trouble starts. Later in the chapter when current sources are discussed, my output impedance results are way off from the book's.

Here is an example:

In figure 2.10c, a 5mA current source is biased with a green LED running at .5mA. The load is 5k, and the emitter resistor is 214 ohms. The positive rail is 50v and the negative rail is at ground. The resulting voltages are 25v at the collector, and 1.08v across the emitter resistor. Bob seems to be giving output impedance results that ignore the load, and my math comes out wrong either way.

Here is how I am doing it.

As stated in the book, I am using a VA of 100 for a 2n5551. Vce is ~24v. Ic is 5mA. (100+24)/.005 = 24,800.

Ro = 24,800

re' = 26/5

re' = 5.2

emitter resistor + re' = 5.2 + 214 which is 219.2

degeneration factor = 219.2/5.2 = 42.1

Rout= ro*degeneration factor = 24,800*42.1

Rout =~1 meg

The book says the Rout for this current source should only be 750k. This is actually closer to my answer than I got with most of the other sources, and it seems that the the higher the stated output resistance of the example, the more off my calculations are. I'm always way too high with my answer.

I suspect my problem has something to do with the finite transconductance of the transistor, but I have no clue how to incorporate that into my figures.

Any help?