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• if we go back to our circuit over here, you know, we have this current coming in here.

• And if I look at the current, I mean, we just switch over to my Miami eater really quickly here.

• And I see the current is either zero when it's off, But then when it's on its 00.3 million amps, so 0.3 million amps And so you can imagine I could I could use if I could send 0.30 point three million amps from my base to my to my emitter.

• Then this little guy inside the transistor, he's going to say, 0.3 million amps.

• I want to multiply that by 100 and I want to make sure that, you know the collector to admit her current is gonna be 100 times that.

• So he's gonna try and make that 30,000,030 million, 100 times, 1000.3 and 30 million amps is enough to turn on an led.

• And so by using a transistor, we can take that 0.3 million current and essentially amplify it to this 30 million pes.

• Now, in reality and led is not gonna draw 30 millions particularly if we have a resister in line and everything.

• You know, the Led wants to draw 20 million amps, but this guy's gonna try and get it as close as he can.

• So he's gonna turn this resistance down.

• He's gonna he's gonna basically kind of move.

• He's gonna move this this little, uh, this little variable resistor up to reduce the resistance to try to get as close to 30 millions as possible.

• But if he led only wants to draw 20 million amps, Um, then he's gonna have this turned up all the way.

• And so he's actually not gonna be able to get it 2 30 millions.

• He's only gonna get it toe maybe 20 to 20 million pes.

• And when that happens, there's There's kind of a word for this that you might see.

• It's called the transit transistor is is saturated or or sometimes we'll say it's in saturation.

• And so in saturation, um, is when Basically there's a short between the collector in the mirror.

• So as much current as possible will flow through there because the current that is flowing through in this case it may be its only 20 is, is less than what this guy is trying to make it.

• He's trying to get 100 times 1000.3, so he's trying to get 30 millions to flow.

• But just because of what's hooked up to this, if it's just in led and a resistor, it's only gonna draw 20 million pes.

• So he's gonna He's gonna turn this resistor down as much as possible and basically make this a short and we call that saturation.

• Um, Now, on the other hand, if this current, if the base current shuts off, which it's doing, remember, remember, in our circuit, if we look back on our circuit, it's either 0.3 or it's it's basically zero because this signal is turning on and off, on and off.

• So when it's 00.3 like this, then it's on.

• When it zero, it's off.

• So if this base current coming back here, if the base current is zero, then this guy's gonna want to make the the collector current over here zero as well.

• And so he's gonna end up turning this down.

• He's gonna end up moving this down and trying to create a much resistance as possible.

• And when that happens, the transistor.

• We have it.

• We have a word for that as well that you might see it.

• And that's called the transistor being cut off, not caught off, cut off to the transistors and cut off.

• So down here, the trend he's the transistor is basically not passing any current from the collector to the emitter.

• So cut off basically means that the collector of the current of the collector is equal to zero.

• And in saturation, it means that the current from the collector is as much as as much as it can be.

• It's basically maximum.

• It's passing.

• It's passing as much current as the circuit wants in that case.

• And so in this sense, this is why you see transistors sometimes, or you hear transistors sometimes referred to a switches because, um, with with the current situation like this, and actually this is going to be the case in our circuit, the transistor will either be saturated or cut off, and when it's saturated, it's just passing current through, and when it's cut off, it's not passing current through it all.

• And so this is why you sometimes hear transistors referred to not only as current amplifiers but also as switches.

• So in the next video, we'll we'll take everything we've.

• We've sort of looked at here and see if we can build the circuit.

if we go back to our circuit over here, you know, we have this current coming in here.

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# 半導體作為電流控制開關｜數字電子技術 (7 of 10) (The transistor as a current controlled switch | Digital electronics (7 of 10))

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林宜悉 發佈於 2021 年 01 月 14 日