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Biasing Transistor

Hi all,

Placing a finger on the base pin of this darlington configuration glows the LED (in fact reaches very close to saturation)!

I learned that you need a threshold voltage of about 0.7volts across the base-emitter junction in order to bring transistor into conduction.

So does this imply that my finger having a potential of 0.7volts??

Thanks

VCC + | | .-. | | | | '-' | | V LED - | -------| | | |/ | o--------o| | |> | | | | |/ -----| |>

| | | | | | === GND

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Reply to
Jack// ani
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Oh yeah, exactly.

Reply to
Jack// ani

I think the OP was saying that ONLY one finger was used. He's acting kind of like a radio antenna, I suspect.

Jon

Reply to
Jonathan Kirwan

More like the finger has a resistance of a few K's. For real saturation just moisten the digit.

-- Regards ........... Rheilly Phoull

Reply to
Rheilly Phoull

. In the first place you will need more than +1.2V, because the darlington has

2 junctions in series. And your finger will have an AC-potential, which you can measure with an oscilloscope. This is mainly because of 60Hz mains field radiating all over the place. The transistor conducts only half of the time, so the intensity will be less than half of the maximum. Since the current gain of a darlington is very high (maybe 10000 or even more) you will need only a uA to bias the base into conduction. 
--
ciao Ban
Bordighera, Italy
Reply to
Ban

whoops, I forget that fact.

Oh yes, I can clearly see a 50Hz ac/noise. But why there exist only

50Hz, what about my cell and cordless phones which emits MHz, KHz frequencies?

Any help is highly appreciated

Reply to
Jack// ani

Really? You remind me of my youngest daughter, one question answered, two new ones are popping up immediately. :-) Check out the input impedance of the darlington, we have seen already

1.2V/1.2uA = 1Megohm DC-resistance and now calculate the input capacitance of that darlington. Hint: it is high because of the Miller-capacitance. And then you can calculate the 3dB frequency of that lowpass, and you will see it is low. And BTW, what kind of antenna is your finger? Must have some loss at 800MHz? You need a tuned circuit for RF to establish some power transfer. 
--
ciao Ban
Bordighera, Italy
Reply to
Ban

Thanks

Reply to
Jack// ani

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