# Transistor's equivalent circuit

1. Nov 7, 2015

### potatowhisperer

well in midfrequency the transistor has a simplified equivalent system . our professor told us that even though pnp and npn transistors have different current directions they both have the same circuit, and that the direction of the currents in the equivalent circuit doesn t change whether it s pnp or npn .
the currents come from B and C and go towards E.
the reason he said was because we are working with alternating current.
is this true ?

2. Nov 8, 2015

### LvW

Yes - it is true. The "simplified" equivalent diagram you have mentioned is valid for small signals only .
With other words: For small changes of the dc voltages/currents only
For this reason, the dc values do NOT appear in this equivalent diagram - but the values of the small-signal parameters depend on the dc currents.

3. Nov 8, 2015

### meBigGuy

Good answer LvW.

Your professor's statement that you are working with alternating current is technically imprecise. The current is not alternating except in the sense that you could think of the small signal variations as a small AC signal imposed on a larger DC bias.

4. Nov 9, 2015

### potatowhisperer

ah ok . but is the equivalent circuit exactly the same for both npn and pnp transistors ?
cause i am in a loss now , as another professor that works exercises with us , told us that the current in the simplified diagram changes direction depending if it s npn or pnp . and the other says that they do not .
thank you very much for your help and assistance.

5. Nov 9, 2015

### LvW

...and what is your opinion?

6. Nov 9, 2015

### potatowhisperer

i am sorry , i am just a bit dumb with electronics , and professors not agreeing is just not a good sign .
i think they are the same , for pnp or npn , courses say that this is the equiv circuit of a transistor not an npn , but then go and show examples only using npn transistors.
i m sorry again .

7. Nov 9, 2015

### LvW

The small-signal parameters (as shown in the equivalent circuit diagram) are the slopes of the corresponding characteristiv curces I=f(V).
The form and the slope of these curves is determined by transistor physics only. And the physics is identical for npn and pnp.
Hence, the small-signal diagrams of both types are comparable (and the structure is identical).

8. Nov 9, 2015

### potatowhisperer

thank you very very very much . honestly.

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