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Topic: PNP and NPN beginner question (Read 1 time) previous topic - next topic

nish1013

I am confused with this PNP and NPN thing in transistors  :smiley-roll-blue:

Quote
1.Is there any easy way to remember this difference?
2.Is there any metaphor ?
3.What is the practical different in usage of two?

strykeroz

Hi,

NPN transistors switch the circuit on when their base current is high, PNP switch on when their base current is low.  For the Arduino that means they work opposite to each other.  There's heaps of info online, like this youtube video here that shows the difference or this simple one-page explanation.

Hope this helps,
Geoff
"There is no problem so bad you can't make it worse"
- retired astronaut Chris Hadfield

nish1013

Hi Geoff,

Many thanks for your input.

I have read that some needs current and voltage , what that means?


strykeroz

Hi

Your common PNP or NPN (bipolar) transistors are controlled by a current on the base pin.  A type of transistor called a FET or MOSFET are controlled by a voltage on their gate pin so do not need a current to flow through them to activate.

Cheers !
Geoff
"There is no problem so bad you can't make it worse"
- retired astronaut Chris Hadfield

bigred1212

This is how I remember it so don't give me grief about it. :.

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1.Is there any easy way to remember this difference?


You write low/negative to turn on a p-N-p and write high/positive to turn on an n-P-n

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3.What is the practical different in usage of two?


Often you see a N-p-N on the negative/low side of what is being switched and an P-n-P on the positive/high side of what is being switched.


pwillard

Another thing to consider.  NPN's are much more popular than PNP's.  The only place I have personally used PNP transistors in the past year was with Common Anode Multi-plexed 7 segment LED displays.  (where I used the PNP as a DIGIT selection high side switch for the digit anode driver)

So, when buying transistors for your parts bin... buy more NPN than PNP and you will be all set (and save a few pennies).

KeithRB

That is because positive voltages are more popular than negative ones. That is Ben Franklin's fault.

nish1013

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That is because positive voltages are more popular than negative ones. That is Ben Franklin's fault.


what is the diff between positive voltages and negative voltages??  :smiley-roll:

KeithRB

Just the way current flows.

Electrons flow from the most negative terminal into the most positive. So for a positive voltage, electrons flow from ground into the positive terminal. For a negative voltage, the electrons flow from the negative terminal into ground.

To confuse things, we typically think of current flowing from positive to negative "Conventional Current Flow", which makes no practical difference unless you are designing semiconductor devices.

michael_x

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what is the diff between positive voltages and negative voltages??

Negatives are only used for audio amplifiers ;)

For us digitals, GND is the most negative we deal with.

If you tied all Positive supplies together and call them GND, you have everything else more or less negative.
And had problems using NPN transistors the usual way ;)

KeithRB

"For us digitals, GND is the most negative we deal with."
Obviously, you have never used ECL.

http://en.wikipedia.org/wiki/Emitter-coupled_logic

MarkT


That is because positive voltages are more popular than negative ones. That is Ben Franklin's fault.


Doubt it.  Original electronics was using thermionic valve circuitry (valves behave a bit like n-channel MOSFETs -
the charge carriers are negative electrons) - the control electrodes were near ground potential for
simplicity and the HT supply positive to the anode load.

However it turns out NPN transistors are about 3 times faster than PNP (same for n-channel over
p-channel FETs) in silicon due to the ~3 times greater mobility of electrons over holes.  So NPN and
n-channel are preferred when there's a choice.  Often you need both (CMOS circuitry for example
uses n-FETs and p-FETs equally).
[ I won't respond to messages, use the forum please ]

KeithRB

My 3rd edition of "Reference Data for Radio Engineers" shows the cathode grounded and the control grids at *negative* voltage.

michael_x

#13
Jul 18, 2013, 08:41 pm Last Edit: Jul 18, 2013, 09:02 pm by michael_x Reason: 1
Quote
Obviously, you have never used ECL.


Yes, obviously.  ;)

Thanks. I'm learning a lot here.

[Added:]
Rather find new questions:
What's the levels of A, B and (A+B) , relative to Vcc and Vee ?
Why doesn't it  say Vee=0V Vcc=+5V

KeithRB

The Vcc/Vee question is answered in the wiki:
"The ECL circuits usually operate with negative power supplies (positive end of the supply is connected to ground) in contrast to other logic families in which negative end of the supply is grounded. This is done mainly to minimize the influence of the power supply variations on the logic levels as ECL is more sensitive to noise on the VCC and relatively immune to noise on VEE.[20] Because ground should be the most stable voltage in a system, ECL is specified with a positive ground. In this connection, when the supply voltage varies, the voltage drops across the collector resistors change slightly (in the case of emitter constant current source, they do not change at all). As the collector resistors are firmly "tied up" to ground, the output voltages "move" slightly (or not at all). If the negative end of the power supply was grounded, the collector resistors would be attached to the positive rail. As the constant voltage drops across the collector resistors change slightly (or not at all), the output voltages follow the supply voltage variations and the two circuit parts act as constant current level shifters. In this case, the voltage divider R1-R2 compensates the voltage variations to some extent. The positive power supply has another disadvantage - the output voltages will vary slightly (±0.4 V) against the background of high constant voltage (+3.9 V). Another reason for using a negative power supply is protection of the output transistors from an accidental short circuit developing between output and ground[21] (but the outputs are not protected from a short circuit with the negative rail)."

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