Author Topic: Help Understanding PNP transistors  (Read 1610 times)

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Offline AngraMeloTopic starter

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Help Understanding PNP transistors
« on: October 01, 2019, 08:12:24 pm »
Hello guys,
I have been studying the theory and the practical side of electronics for a while. As Im doing everything on my own, I have large knowledge gaps spread out on different levels.
When I see explanations about transistor, they almost always use the NPN type to explain saturation, Vce and general calculations.
So I ended up having a lot of PNP theory to study and understand (the negative voltages and everything makes it harder for me)

When Im calculating the power dissipation of a transistor, I assume the formula is: P= (Vce)x(Ie)
So I know that if I put 10V at the base of a NPN, and its collector is at 20V, ill get around 11Vce.

How does that work for a PNP? is it exactly the same?
So if I have the emitter at 20V and I apply 10V to the base I will get Vec = 11V?
 

Offline eev_carl

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Re: Help Understanding PNP transistors
« Reply #1 on: October 01, 2019, 08:23:40 pm »
If you're talking about a common emitter, you may want to check out this recent thread.  It's on NPN, but the forum warned me about putting 1V at the base.  I imagine they'd have a problem with 10V as you'd need even more resistance (100k?) to produce the microamps of Ib.

https://www.eevblog.com/forum/beginners/question-on-npn-logic-inverter-from-textbook/

« Last Edit: October 01, 2019, 08:49:28 pm by eev_carl »
 

Offline AngraMeloTopic starter

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Re: Help Understanding PNP transistors
« Reply #2 on: October 01, 2019, 09:02:19 pm »
My case is only theoretical, but just for reference I was thinking about big power transistors.

Thank you for your input!
 

Offline Audioguru again

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Re: Help Understanding PNP transistors
« Reply #3 on: October 01, 2019, 11:43:14 pm »
The forward Vbe bias voltage of a transistor is 0.6V for low currents and is maybe 1.5V for high currents. If the emitter of a PNP transistor is at +20V then its emitter will also be +20V when the transistor is turned off and the base will be at +18.5V to +19.4V when it is turned on.
 

Offline langwadt

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Re: Help Understanding PNP transistors
« Reply #4 on: October 01, 2019, 11:47:17 pm »
just flip everything up side down
 

Offline Kirill V.

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Re: Help Understanding PNP transistors
« Reply #5 on: October 02, 2019, 12:52:17 am »
Simple currents directions replaces to opposite and voltage directions replaces to opposite (if we compare it with NPN directions).
Is purely conventional theoretical propositions which have been accepted for a very long time. But general principles of the circuit theory do not changes in this case.
 

Offline viperidae

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Re: Help Understanding PNP transistors
« Reply #6 on: October 02, 2019, 05:45:53 am »
I would recommend you read The Art of Electronics. I didn't understand how to use transistors properly until I read that book.
 

Offline AngraMeloTopic starter

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Re: Help Understanding PNP transistors
« Reply #7 on: October 02, 2019, 02:11:45 pm »
I would recommend you read The Art of Electronics. I didn't understand how to use transistors properly until I read that book.

I do have this book around somewhere. Ill find it and do some studying!
 

Online ledtester

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Re: Help Understanding PNP transistors
« Reply #8 on: October 02, 2019, 02:49:48 pm »
Another book I recommend, which you might be able to find online, is "Transistor Circuit Approximations" by Malvino. It has a lot of worked examples which AoE lacks.
 
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Offline GerryR

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Re: Help Understanding PNP transistors
« Reply #9 on: October 02, 2019, 04:49:52 pm »
Another book I recommend, which you might be able to find online, is "Transistor Circuit Approximations" by Malvino. It has a lot of worked examples which AoE lacks.

I 2nd the Malvino book;  excellent reference!
Still learning; good judgment comes from experience, which comes from bad judgment!!
 
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Offline AngraMeloTopic starter

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Re: Help Understanding PNP transistors
« Reply #10 on: October 04, 2019, 10:44:44 pm »
Another book I recommend, which you might be able to find online, is "Transistor Circuit Approximations" by Malvino. It has a lot of worked examples which AoE lacks.

Just bought for a couple bucks on amazon! awesome!
 

Offline Zero999

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Re: Help Understanding PNP transistors
« Reply #11 on: October 05, 2019, 03:45:57 pm »
How much NPN transistor theory do you understand? PNP transistors work in exactly the same way, but with the sign reversed. Indeed, if you build the same circuit but with PNP transistors and reverse the power supply voltage, it will work the same. You can even do all of the calculations with positive signs first and write negative signs next to all of the numbers later.

EDIT:
I've simulated a common emitter amplifier, both with an NPN and PNP transistor. The polarity of the power supply and electrolytic has been reversed in the PNP circuit and the voltages are the same magnitude, but the opposite sign.
« Last Edit: October 05, 2019, 04:19:42 pm by Zero999 »
 


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