Help Understanding PNP transistors - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 01, 2019, 08:12:24 pm
content: 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?

author: eev_carl
author_rank: Regular Contributor
timestamp: October 01, 2019, 08:23:40 pm
content: 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/

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 01, 2019, 09:02:19 pm
content: My case is only theoretical, but just for reference I was thinking about big power transistors.Thank you for your input!

author: Audioguru again
author_rank: Regular Contributor
timestamp: October 01, 2019, 11:43:14 pm
content: 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.

author: langwadt
author_rank: Super Contributor
timestamp: October 01, 2019, 11:47:17 pm
content: just flip everything up side down

author: Kirill V.
author_rank: Regular Contributor
timestamp: October 02, 2019, 12:52:17 am
content: 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.

author: viperidae
author_rank: Frequent Contributor
timestamp: October 02, 2019, 05:45:53 am
content: I would recommend you read The Art of Electronics. I didn't understand how to use transistors properly until I read that book.

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 02, 2019, 02:11:45 pm
content: Quote from: viperidae on October 02, 2019, 05:45:53 amI do have this book around somewhere. Ill find it and do some studying!

author: ledtester
author_rank: Super Contributor
timestamp: October 02, 2019, 02:49:48 pm
content: 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.

author: GerryR
author_rank: Frequent Contributor
timestamp: October 02, 2019, 04:49:52 pm
content: Quote from: ledtester on October 02, 2019, 02:49:48 pmI 2nd the Malvino book; excellent reference!

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 04, 2019, 10:44:44 pm
content: Quote from: ledtester on October 02, 2019, 02:49:48 pmJust bought for a couple bucks on amazon! awesome!

author: Zero999
author_rank: Super Contributor
timestamp: October 05, 2019, 03:45:57 pm
content: 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.