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finding the Vce and Ib for BC108

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gjpollitt

New Member
If there is no graph for the above on the data sheet then how do you determine the q point?

The examples I have seen on the net all refer to the graphs on the datasheets yet there isnt a graph for base current vs collecter voltage for the SST BC108

thanks
Graham
 

bloki

Member
Base current is allmost constant with collector-emitter voltage, except at large collector currents and part of characteristics below the knee voltage. Allthough, nobody has published Ib versus Vce for constant Ic curve yet. But you may derive such curve from Ic vs Vce curve familly.
 

audioguru

Well-Known Member
Most Helpful Member
Does anyone still make an old BC108? I used them more than 40 years ago. They even have an old metal case. I've used BC547 or 2N3904 transistors for about the last 30 years.

How can you bias a transistor with a voltage source? The gain and Vbe varies all over the map for different samples and with temperature change. It might even be influenced by the fullness of the moonlight. :lol:

Transistors need negative feedback so that all circuits work with any sample of transistor. The negative feedback can be created by an emitter resistor, or a resistor from the collector to the base.

Siemens rates the gain as from 180 to 460, and their Vbe from 0.55V to 0.7V. If you choose a Q-point without negative feedback for one in the middle, then ones on each side won't work!
If you want a graph showing a "typical" transistor as a starting point, Siemens have it in their datasheet:
 

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audioguru

Well-Known Member
Most Helpful Member
Russlk said:
What are you trying to do? In most cases, you need to linearize the transistor characteristic with resistors.
Yeah, he is trying to use only 1 transistor and amplify like mad, instead of using 2 or 3 stages with less individual gain and therefore with much less distortion and a wider bandwidth. :lol:
 
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