# The emitter has a higher potential than base ! How?

#### ILBS

Jun 27, 2022
15
Hi,

in this video, the author is explaining the cut off mode of a NPN transistor.

24:07

he says:
The emitter has a higher potential than the base
How could this be?

how?

if the emitter is grounded and the base has a resistor to connected to a positive voltage source. It seems to me it’s the opposite!

when Vb is measured from base to ground there will be a very small voltage where as when you measure from Emitter to ground it’s a short !!! I don’t understand why the VE shouldn’t be less than Vb????

Last edited by a moderator:

#### CircutScoper

Mar 29, 2022
300
Hi,

in this video, the author is explaining the cut off mode of a NPN transistor.

24:07

he says:
The emitter has a higher potential then base
How could this be?

how?

if the emitter is grounded and the base has a resistor to connected to a positive voltage source. It seems to me it’s the opposite!

when Vb is measured from base to ground there will be a very small voltage where as when you measure from Emitter to ground it’s a short !!! I don’t understand why the VE shouldn’t be less than Vb????

At what time mark is that said? I'm not watching the whole stupid thing.

#### John Canon

Jun 1, 2022
92
When VidGuy says "emitter has a higher potential than base" he is demonstrating a theoretical cutoff mode at the junction level. He is not applying the transistor to a wired circuit. So he is correct at the junction level of discussion, but you cannot put that transisor into a conventional circuit and say "can you show me that again?"

Don't forget, a transistor conducts depending on base current, not voltage.

And don't get me going about 'conventional current' flow versus 'electron current' flow. Electricity is electrons, not holes.

#### ILBS

Jun 27, 2022
15
Hi John,

Oh so He’s talking at the atomic structure level !!!

ok thanks for thé clarification !

#### ILBS

Jun 27, 2022
15
24:07 CircuitScoper!

#### Audioguru

Sep 24, 2016
3,653
Many You Tube videos have errors.
The video WRONGLY says, "In the cutoff region, the emitter of the NPN transistor has a higher potential the base".

In most circuits, a transistor is in the cutoff region when the base and emitter have the same potential.
Most transistors have a maximum allowed emitter-base reverse voltage of only 5V or 6V.

#### Attachments

• transistors potentials.png
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Feb 19, 2021
407
Bipolar transistor is a V controlled device -

But we find using a current based model easier to calc bias and operating point
values.

Regards, Dana.

#### Martaine2005

May 12, 2015
4,558
Bipolar transistor is a V controlled device
Some would say current controlled!
But let’s not go there..

Martin

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,826
Some would say current controlled!
But let’s not go there..

Martin
We have already BEEN THERE hundreds, if not thousands, of times. But, you are right: let's not go there again.
Besides, as any seasoned experimenter knows, all semiconductors operate on the FM principal, and if you carelessly let the magic smoke out they quit working: so, leave the smoke and the magic in and go about your business as best you can.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,826
Electricity is electrons, not holes.
Well, except in well-known cases such as electroplating of metals and electrophoresis of organic molecules, electricity is all about energy transfer and that takes place very rapidly through the means of electromagnetic fields. Electrons, holes, and ions respond rather slowly to electromagnetic fields, although they all do respond. But don't get me going on biophysics... that's where the REAL magic is.

#### Ratch

Mar 10, 2013
1,098
Hi,

in this video, the author is explaining the cut off mode of a NPN transistor.

24:07

he says:
The emitter has a higher potential then base
How could this be?

how?

if the emitter is grounded and the base has a resistor to connected to a positive voltage source. It seems to me it’s the opposite!

when Vb is measured from base to ground there will be a very small voltage where as when you measure from Emitter to ground it’s a short !!! I don’t understand why the VE shouldn’t be less than Vb????
The video is correct. If the emitter is grounded, and has a higher potential than the base, that means the base has a negative voltagde with resplect to ground. Putting a negative voltage with respect the emitter on the base of a NPN will cut off the collector current. Even equal voltages will reuce the collector current to a minuscule level.

#### Ratch

Mar 10, 2013
1,098
When VidGuy says "emitter has a higher potential than base" he is demonstrating a theoretical cutoff mode at the junction level. He is not applying the transistor to a wired circuit. So he is correct at the junction level of discussion, but you cannot put that transisor into a conventional circuit and say "can you show me that again?"

Don't forget, a transistor conducts depending on base current, not voltage.

And don't get me going about 'conventional current' flow versus 'electron current' flow. Electricity is electrons, not holes.
That is absolutely wrong! A BJT by itself is a transcondance amplifier (voltage controls current), just like a vacuum tube and FET. I have heard folks say to me many times, "You can look at it two ways, voltage or current." Then they try to show that when they drive the base with a current supply, the collector current shows a rather linear relationship within a reasonable range . "That proves a BJT is a current controlled device, right?" Wrong! They are ignoring the physicsof the BJT.

Take a NPN. The emitter is highly doped with N-material and has plenty electrons itching to diflfuse into the very thin P-base and fill up those holes. When they do, they leave behind positive ions in the emitter and create negative ions in the base. This creates an increasing back-voltage that stops the migration when a equilibrium is reached. By applying a positive voltage to the base, the back-voltage is lowered and the charge carriers (electrons in this case) can whiz through the very thin base (human hair size) and be sucked up by the higher voltage of the collector. Unfortunately, some of the electrons that meke up the collector current get diverted into the base circuit by the positive base voltage. This causes a waste current to exist in the base circuit that does absolutely nothing to control lthe collector current. The higher the transistor beta, the less waste current. Fortunately or not, this waste base current is proportional to the collector current and fools a lot of folks into thinking the base current is controlling the collector current when it is not. It is the voltage on the base terminal that is controlling the collector current by modulating the back-voltage of the emitter-base junction. The physics ofthe transistor determine what is controlling the collector.

Now, when you hook up a current source to a BJt, you are in effect insereting a high voltage source in series with a big fat resistor value into the base circuit. That is what a current source is. So you don't have a simple transistor anymore. Instead you have a transistor CIRCUIT. You can make just about anlything with voltage amplifiers (op-amps), true current amplifiers (magnetic amplifiers), BJTs, FETS, vacuum tubes (transconductance amplifiers) by incorporating them into a circuit. That is what folks are doing when they attach a current generator to the base of a BJT. They are making a current amplifier curcuit, but the transistor itself is still controlled by the voltage of the current source.

#### Ratch

Mar 10, 2013
1,098
When VidGuy says "emitter has a higher potential than base" he is demonstrating a theoretical cutoff mode at the junction level. He is not applying the transistor to a wired circuit. So he is correct at the junction level of discussion, but you cannot put that transisor into a conventional circuit and say "can you show me that again?"

Don't forget, a transistor conducts depending on base current, not voltage.

And don't get me going about 'conventional current' flow versus 'electron current' flow. Electricity is electrons, not holes.
I have seen folks really get wrapped around the axle by charge flow. First of all, when you say "current flow", you are Talking Technical Trash (TTT, T^3). Current flow literally means "charge flow flow" which is redundant and ridiculous. You should instead say "charge flow", "current exists", or "current present." Charge does not flow twice. So what is the current direction? It depends on the polarity of the charge carriers and the polarity of the voltage driving the charge carriers. Mathematical current (aka conventional current) assumes that positive voltage will repel a positive charge carrier. Therefore, if youhookup a battery to a resistor, the mathematical current direction will be from thepositive terminal through the resistor and into the negative terminal. All calculations can be made this way. Semiconductors and ammeters are marked and labeled this way. They all assume the charge carriers are positive and the charge flows from positive to negative. Now, if you really have to know the real current direction, you must know the polarity of the charge carriers. Thenit is simple. If you are dealing with negative charge carriers like current in a wire, change lthe sign of the answer. If calculating the hole current of a transistor, the mathematical current is the answer.

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