There are a couple ways you could look at this circuit, but try this:
Assume diodes have a constant 0.6V drop across them, as long as they are forward biased. So the two diodes in series have a total of a 1.2V drop.
Since the base-emitter junction and 500 Ω resistor are in parallel with these two diodes, the voltage across them must be the same. And since the base-emitter junction of a transistor is also a diode, the voltage across that must also be 0.6V as long as it's forward biased, which it is here. So, the voltage across the 500 Ω resistor must be 0.6V.
By ohm's law then, the current through the 500 Ω resistor must be 0.6 V / 500Ω = 1.2 mA.
All the current through that resistor must go in the transistor's emitter. And an equal current must go out somewhere, and there are two options:
- out the base
- out the collector
Because a transistor has gain, ideally a lot of gain, the collector current will always be much higher than the base current as long as the transistor is appropriately biased. Let's just assume it's appropriately biased for now, and consider the conditions in which that isn't true later.
If we assume the transistor has infinite gain, then all the current entering the emitter of the transistor must exit the collector, because the base current is zero. Since the only path for current out the collector is through the yellow load, the voltage across that resistor must be whatever value required to make the current in the load equal to the current through the 500 Ω resistor.
Having grasped the basic operation of the circuit, consider:
The simulation shows 1.33 mA through the 500 Ω resistor, which is slightly different from the 1.2 mA calculated above. Why is this different? What's the voltage across the diodes in the simulation?
What happens if the load resistor is extremely large, say 100 MΩ? How much voltage would need to be across it to get the desired current, and can the circuit still work under those conditions?
Real transistors don't have infinite gain. How does that affect the circuit?
What can affect the accuracy of this circuit? Temperature? Device variation? How and why?