transistors
The picture shows a detail of schematic diagram of the LM555.
I presume the horizontal line is the base, but I'm not sure I understand how the transistor works. What is the effect of connecting base with collector, and what's with the second collector? Is it a current mirror of the other collector?
What happens is:
As the base voltage rises, the transistor begins to turn on and it's collector voltage drops (assuming it has a collector resistor or similar current limiting element)
Normally a typical bipolar transistors saturation voltage is around 200mV or less. When the collector voltage, Vce drops below Vbe - Vschottky though, the schottky starts to conduct (now being forward biased) and the base current starts to flow through it into the collector. This "steals" current from the base, preventing the transistor turning on more and the collector reaching it's saturation voltage.
The system will reach a state of equilibrium, since the transistor can't turn on any more without it's base current dropping (you could see it as a form of negative feedback) and will settle just around Vbe-Vschotkky (e.g.~700mv-450mV as opposed to ~200mV)
So, to clarify things, the formula for Vce is:
Vce = Vbe - Vschottky
If we have this circuit and apply a ramped voltage from 0-2V:
We get simulation results like this:
Note that when Vcollector drops below ~700mV, the Schottky begins to conduct and the collector voltage levels out at around 650mV.
If we remove the Schottky, then:
We can see the collector drops all the way to 89mV (I used the cursor as it's hard to see from the graph)
An analogy may help to visualize this:
Think of the transistor as a valve or faucet. The base is the knob, the water tends to flow from the positive side (storage tank) to the ground (drain), if you follow the normal "current flow" directions.
The LED is like a little transparent glass section in the pipe, with a small ball loosely held in that section.
When the faucet is opened, water will be allowed to flow, and the little ball will jump around due to the water's flow.
This will happen whether the LED is above or below the faucet section.
Now for the case of electron flow, as opposed to conventional current flow direction.
Consider the same pipe and faucet, but with the ground being a source for some gas, say natural gas at high pressure underground.
The Vcc is the open air, normal barometric pressure.
Again, as the faucet is opened up, the gas will flow up the pipe, the little ball will bobble around. Again, the glass pipe section (LED) could be before or after the faucet, it won't matter.
I hope this analogy helped.
Best Answer
It's a multiple collector transistor, and in this application it's being used (as it often is in ICs) as a current mirror. The Vbe drops of transistors Q20 and Q21 and resistor R6 set the current through the upper collector of Q22, and this current is then mirrored back down into Q20 by the lower collector. It also sets the current through the long tail current source transistor Q11.