| Electronics > Beginners |
| NPN Transistor as a switch: what's the use of the emitter resistor? |
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| Moriambar:
Hi, as I stated elsewhere I'm a noob with BJT transistor, since I almost always used FETs (which I find way easier). Regardless, I've seen some design of npn transistors used as switches and they almost always feature a 100R resistor from the emitter to ground. What's the purpose of it? Thanks |
| bd139:
That's not a plain switch then. It's an emitter follower. |
| Moriambar:
--- Quote from: bd139 on March 19, 2019, 09:05:46 am ---That's not a plain switch then. It's an emitter follower. --- End quote --- I thought that in the emitter-follower configuration the load should be connected to the emitter. Mine is connected to the collector. The emitter has just 100R to ground. I provided an example to clarify what I'm saying |
| bd139:
The load should be connected between emitter and ground with the emitter follower. This is used for linear devices and current sources rather than switching though. It may switch but it'll be incredibly inefficient. To switch with a transistor, the emitter should be grounded, the load connected across the positive rail and the collector. The base current should be appropriate to switch the transistor on fully ((Ic / beta) * 2 approx to jam the transistor on hard) |
| Moriambar:
--- Quote from: bd139 on March 19, 2019, 09:24:47 am ---The load should be connected between emitter and ground with the emitter follower. This is used for linear devices and current sources rather than switching though. It may switch but it'll be incredibly inefficient. To switch with a transistor, the emitter should be grounded, the load connected across the positive rail and the collector. The base current should be appropriate to switch the transistor on fully ((Ic / beta) * 2 approx to jam the transistor on hard) --- End quote --- Ok so either I found wrong examples (I've not them at hand) or I did not understand the purpose of the circuit. 100% they were not emitter followers, though. |
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