Please explain this simple circuit to me

[Richardcavell]

Hi, everyone.

1. According to my multimeter, there's about a 0.09V drop from E to C in this circuit. Is that normal? Does that voltage drop have a name?

2. The E-B voltage drop is 0.7 Volts. Given that's the standard voltage drop of a silicon diode, does it have some sort of special name?

3. What does that 100k resistor do?

4. According to my multimeter, the current through E-B is 0.1 milliAmps and through E-C is 9 milliAmps. That means I'm getting a gain of about 90, right?

5. I know that electronic parts are always described as though the current goes from +ve to -ve. But in fact the electrons go the other way. Is this ever relevant for semiconductors? That is, would I ever need to know which way the electrons go?

6. Is it possible to extend this circuit so that the output of this transistor feeds into an identical transistor, and so on, creating a ladder of transistors? I'd need some sort of resistor where the LED is, right? What value should I use?

Richard

 

[MrAl]

Hi,

1. That voltage drop is called the saturation voltage. It can vary quite widely depending on transistor type and operating conditions.
2. That's the base emitter forward voltage drop.
3. The 100k keeps the transistor fully turned off when not connected to an external circuit.
4. Right, that's the current gain.
5. Any time electronics meets up with the real phyiscal world it's good to know this, such as in electromagnetics. In control theory it's good to forget this
6. You would do that if you need more current gain. You dont seem to need more current gain here, but if you did you would connect the base of the next transistor to the emitter of the previous transistor. There's a few more details such as a base resistor.