# Common Base BJT hybrid-pi vs schematic pnp or npn

#### helloeveryone

Joined Apr 8, 2011
64
Hi, just for deeper understanding I'm trying to figure out what the equivalent schematic of the given hybrid-pi model of a common base bjt looks like. Just looking at the emitter and collector currents it seems as the third picture would be correct. The emitter current goes towards the base in both and comes out of the base to the collector in both. The only difference that is confusing me is that V_CB is labelled plus minus but in the third picture the battery polarity is the opposite. My opinion is that the hybrid-pi model shown is for a pnp transistor. Am I correct? What if I_C had pointed in the opposite direction in the hybrid-pi model would that make it a npn or pnp transistor? Any tips for converting from one to the other?

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#### Jony130

Joined Feb 17, 2009
5,439

In the upper part of a diagram, you show an h-parameter model for Common Base configuration. And in the last two diagrams, I see a very simplified (unrealistic) DC flow in the BJT's supplied from two different batteries .

#### helloeveryone

Joined Apr 8, 2011
64
What kind of transistor does the first picture represent? An NPN transistor or PNP transistor? How can you tell which it represents? Does it represent both NPN and PNP at the same time? Does changing the direction of the collector current or the emitter current change what kind, NPN or PNP, of transistor it represents? Given the first picture draw a simplified schematic, is the arrow going to point in or out in the transistor? I've made up this question to try to understand what the first picture represents in a normal schematic.This what I know for sure:
1) It represents a transistor
2) The transistor has a common base configuration
Now I want to convert back to a normal schematic. How do I do this? This can be simplified and conceptual I don't need exact values.
On slide number 3:
http://www.ittc.ku.edu/~jstiles/412...Models/The Hybrid Pi and T Models lecture.pdf
You can see that the hybrid pi model for a common emitter configuration isn't the same for a npn transistor and a pnp transistor. I'm trying to figure out which one is shown in the first picture for a common base configuration npn or pnp.

#### freak101

Joined Aug 7, 2017
37
Hybrid pie model for npn and pnp transistors is same.
But what you have provided is not a hybrid pie model.

#### helloeveryone

Joined Apr 8, 2011
64
Hybrid pie model for npn and pnp transistors is same.
But what you have provided is not a hybrid pie model.
Well, slide 3 shows that hybrid pi model for npn and pnp is NOT the same. What is that which I have provided if it is not a hybrid pi model? Why is it not a hybrid model?

#### Jony130

Joined Feb 17, 2009
5,439
What kind of transistor does the first picture represent?
This small-signal h-parameter model (not hybrid-pi)

Represents NPN transistor in CB configuration. But it may sound strange but you can use the same model for PNP transistor.

But to avoid confusion some times use two "different" models.
As shown here

And this time we have a truly Hybrid-pi model.

#### helloeveryone

Joined Apr 8, 2011
64
What is the significance of the direction of the emitter and collector currents? Does changing the direction tell you if it is NPN or PNP or does the current drawn tell you nothing about if it is NPN or PNP?

#### Jony130

Joined Feb 17, 2009
5,439
Why did you not change the direction of current (hfb*IE) instead?

#### helloeveryone

Joined Apr 8, 2011
64
Why did you not change the direction of current (hfb*IE) instead?
Because I haven't seen hfb*IE pointing in the opposite direction, but I have seen the four cases I have drawn and labelled.

#### Jony130

Joined Feb 17, 2009
5,439
Your small signal diagram is using h-parameters. And in this case were treating the BJT's as a "black-box". And we are using Two-port network theory to analize it.
https://en.wikipedia.org/wiki/Two-port_network#Hybrid_parameters_(h-parameters)
Note that the definition of the h-parameter set has nothing to do with the physical behavior of the BJT. And this is why we do not distinguish between NPN vs PNP. Also from what I know almost no one use hybrid H parameters anymore (exempt h21e = hfe = β) when designing the amplifiers. Only EE students are forced by their mad professors to use it.

http://www.ittc.ku.edu/~jstiles/412...Small Signal Operation and Models lecture.pdf

#### anhnha

Joined Apr 19, 2012
901
Also from what I know almost no one use hybrid H parameters anymore (exempt h21e = hfe = β) when designing the amplifiers. Only EE students are forced by their mad professors to use it.
Is there any case where h parameters is preferred than others?

#### helloeveryone

Joined Apr 8, 2011
64
So,
1) Can be NPN or PNP
2) Can be NPN or PNP
3) Can be NPN or PNP
4) Can be NPN or PNP ?

#### Jony130

Joined Feb 17, 2009
5,439
So,
1) Can be NPN or PNP
2) Can be NPN or PNP
3) Can be NPN or PNP
4) Can be NPN or PNP ?
You must understand that here we are talking about the small-signal model. A linear representation of a BJT's circuit on the corresponding DC bias point in active region.

See the example

As you can see we have two stage CE amplifier. And the small-signal equivalent circuit. As you can see I use the same "symbol model" for NPN and PNP.
Because it doesn't matter in small-signal analysis.

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