zachtheterrible said:
So its basically just to stabilize from one transistor to the next then? and temperature as ron added. I can see how variations in the transistor's gain would affect the circuit, but I don't see how placing the resistor from base to ground will keep the transistor's variation from changing the circuit too much.
Draw the circuit and do the maths!, it's really VERY simple.
1) The base voltage is simply calculated from the potential divider values.
2) The emitter voltage is 0.7V lower than the base voltage.
3) The emitter current can be calculated from the emitter voltage and the value of the emitter resistor.
4) The collector current is the emitter current MINUS the base current, but the base current is only very small and can generally be ignored. So for this simple example, collector current equals emitter current.
5) The collector voltage can be calculated from the collector current and the collector resistor.
Notice that the transistors gain appears nowhere (I ignored it in part 4).
If you would now like to try introducing it?, the emitter current is the sum of base current and collector current - try doing the sums for different gain values?. Notice that if the gain of the transistor is only 100, the base current is only 1% of the emitter current, and for a gain of 200 only 0.5% of the emitter current - these sorts of figures can safely be ignored if you are using 5% resistors!.