What does mean this?: The BJT doesn't work because the emitter output HAS to be lower than the base

crutschow

Joined Mar 14, 2008
34,281
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In fact that was just a question to me. I'm trying to know what's going on in a transistor.:)
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A bipolar transistor has two semiconductor junctions in a three layer sandwich.
This is N-doped (collector), P-doped (base), N-doped (emitter) semiconductor material for an NPN transistor.
Reverse that for a PNP.
If you measure each of these junctions with an ohmmeter, they will look like a normal junction diode with the base being the anode and the collector and emitter the cathodes (for an NPN).
During normal operation the base-emitter junction is forward biased and the collector-base junction is reverse biased.
To turn on the transistor the base-emitter junction must be forward biased to the point that base-emitter current flows. The collector current that then flows due to transistor action (complex solid-state physics stuff) is the base current multiplied by the current gain of the transistor (Beta or hFE).
Since the forward biased base-emitter junction looks like a diode, the typical base-emitter voltage (Vbe), when the transistor is on, is about 0.65-0.7V.
 

Thread Starter

booboo

Joined Apr 25, 2015
168
To open (turn it on) NPN transistor the voltage at base must by 0.5V...0.6V higher than the emitter voltage.
Also base current must flow into the base
Thanks. good point.
If the base and emitter were the same in your water analogy, when the base gate opened water would also flow up from the emitter, leaving nowhere for the collector water to flow
Well said. good.
Booboo:

When you understand this simulation, you will have a good understanding of NPN and PNP transistors and how they are commonly used:

Note that the drive voltage V(in) (?from a microprocessor port pin, perhaps?) is the same for all four cases. Compare the four outputs.

Why do two of the cases "follow" the input voltage? Why does V(Nf) stop following V(in) when it reaches 4.2V? Why does V(Pf) stop following V(in) when it reaches 0.8V?

Why do two of the cases "square up" (amplify?) the input voltage? What is the switching threshold for these two cases?

View attachment 89156
Hi and thanks for good example. now answer to questions:

Why does V(Nf) stop following V(in) when it reaches 4.2V? Because it's an NPN BJT and there is a diode between Base and Emiter that has a 0.7v forward voltage drop.

Why does V(Pf) stop following V(in) when it reaches 0.8V? Because it's a PNP BJT and there is a diode between Cellector and Base and it has a 0.7v forward voltage drop.

Why do two of the cases "follow" the input voltage? Because looks like to me that in those cases transistors haven't turned on yet completely.

Were my answers correct?

I like this/your technique to learn electronic. that's perfect. thanks a lot!:)

If you measure each of these junctions with an ohmmeter, they will look like a normal junction diode with the base being the anode and the collector and emitter the cathodes (for an NPN).
Thanks for reply
Can I measure that forward voltage drop when transistor is turned off? I have tried by my multimeter(its ohmmeter) to measure this resistance but I couldn't do it.(I have seen nothing on display)

What is the 5V MCU that you are using? If you are trying to save power, have you considered putting the 5V MCU to sleep, and wake on a change of the pin to which you have connected the level shifter?
I'm not going to do it because this isn't my circuit and I don't need this. I had some problem about BJTs then I asked them here. that's all.
 

crutschow

Joined Mar 14, 2008
34,281
If your multimeter has a diode tester setting, use that.
Yes. The normal resistance measurement modes usually have too low a voltage to forward bias a silicon junction but the diode test setting does.
In that mode the multimeter will typically display the diode forward voltage at the small current that it uses for the test. In the reverse direction it will show no change in the display.
This test should be done with the transistor not in the circuit or energized in anyway.
 
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