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Biasing Transistor

J

Jack// ani

Jan 1, 1970
0
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

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
 
R

Rheilly Phoull

Jan 1, 1970
0
Jack// ani said:
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

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

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

Jonathan Kirwan

Jan 1, 1970
0
More like the finger has a resistance of a few K's. For real saturation just
moisten the digit.

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

Jon
 
B

Ban

Jan 1, 1970
0
Jack// ani said:
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??
..
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.
 
J

Jack// ani

Jan 1, 1970
0
Ban said:
In the first place you will need more than +1.2V, because the darlington has
2 junctions in series.

whoops, I forget that fact.
This is mainly because of 60Hz mains field radiating all over
the place.

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
 
B

Ban

Jan 1, 1970
0
Jack// ani said:
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

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.
 
J

Jack// ani

Jan 1, 1970
0
Ban said:
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.

Thanks
 
R

Rich Grise

Jan 1, 1970
0
Oh yeah, exactly.

If it's that simple to make a touch sensor, then howcome all
those touch sensor circuits are so complicated?

Thanks,
Rich
 
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