Common Emitter BJT Calculations Driving Me Crazy!!!

[alanlan]

thanks, alanlan.

as i understand it, when current gain is low base current is no longer negligible and these approximations don't apply.  how low would you go with current gain (hfe) before considering these approximations inaccurate for biasing and other calcs?  i am thinking of the popularity of low gain transistors in fuzz face circuits, where below 50 is common.

I think ge transistors are another story, but assuming low gain and low leakage, the approximations still stand (perhaps more approximate).  I mean if you've got a hfe of 50 then the difference between the collector and emitter currents is only 2% in theory.  I think I said earlier, but once you start adding bypass caps and/or reducing the emitter (or source) resistor to lower values, a lot of the calculations become highly dependent on device characteristics due to the reduced negative feedback.

[demonstar]

I put the thing on breadboard and all it does is attenuate the signal!!!!

"http://www.aronnelson.com/gallery/main.php/v/Demonstars-Gallery/Circuit+4.jpg.html"

That's the circuit I made. I'm hoping I made a silly mistake putting it together. Otherwise the thing is pathetic!

I guess I could be loading the output down!

Any ideas?

[demonstar]

Done! 30sec debug complete.

I checked the voltages at the pins and I'd put the transistor in upside down... Doh!