Transistor's equivalent circuit

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Discussion Overview

The discussion revolves around the equivalent circuit of transistors, specifically comparing npn and pnp types in the context of small-signal analysis. Participants explore the implications of current direction and the validity of simplified equivalent diagrams in midfrequency applications.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the simplified equivalent circuit for transistors is valid for small signals, with the same structure for both npn and pnp types.
  • One participant notes that the equivalent circuit does not include DC values, as it focuses on small changes in DC voltages and currents.
  • Another participant challenges the notion that the current direction remains the same for both transistor types, citing conflicting information from different professors.
  • Some argue that the small-signal parameters are determined by transistor physics, which is identical for both npn and pnp transistors, suggesting comparable small-signal diagrams.
  • Concerns are expressed about the clarity of instruction when professors provide differing views on the topic.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the direction of current in the equivalent circuit for npn and pnp transistors, with conflicting opinions from different instructors contributing to the confusion. No consensus is reached on whether the equivalent circuits are identical or not.

Contextual Notes

Participants highlight potential limitations in understanding due to the lack of clarity in instructional materials and the dependence on the definitions of small-signal analysis. The discussion reflects a mix of theoretical and practical considerations without resolving the discrepancies in viewpoints.

Who May Find This Useful

This discussion may be of interest to students studying electronics, particularly those grappling with the concepts of transistor operation and equivalent circuits in small-signal analysis.

potatowhisperer
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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 ?
 
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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.
 
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.
 
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.
 
...and what is your opinion?
 
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 .
 
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).
 
thank you very very very much . honestly.
 

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