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That is the Hybrid-Pi model of the voltage amplifier i have. In our lectures we haven't been taught about anything regarding the Hybrid-Pi or transistors input impedance up till today. I figured out that i need to find (equivalent) resistance, something like (R1 || R2 || Rpi).

But i don't know how to proceed! A little help would be really appreciated!

Hilton Khadka
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  • at (b) , as I recall to the base Rin = Re*hFE for Re being base emitter resistance which changes with bias. and if the emitter has a series R to ground looking into the emitter Rout=Rb/hFE so hFE increase base input impedance along with any emitter resistor added , and any shunt impedance added to base such as a cap, it lowers emitter output impedance looking back. Plus any external parts are added for series or parallel to the net. Note here Vin sine has 0 impedance. – Tony Stewart EE75 Oct 02 '16 at 14:28

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...not "something like (R1||R2||Rpi)". This is, of ocurse, already the correct expression for the dynamic input resistance (as can be derived from the diagram). Note that it would be more correct to write rpi instead of Rpi in order to clearly disinguish between dynamic and static resistances.

The dynamic resistance rpi is given by the inverse slope of the input chracteristic IB=f(VBE). Hence, we have rpi=d(VBE)/d(IB)=d(VBE)*B/d(IC).

Because d(VBE)/d(IC)=1/gm we can write rpi=B/gm=B/(IC/VT)=(B*VT)/IC.

B=DC current gain, gm=transconductance, VT=temperature voltage, IC=DC collector current.

Example: For B=200, IC=2mA and VT=26mV we get rpi=2.6 kOhm.

LvW
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  • I think, it would be important for the questioner to learn, which parts of my answer are wrong. I have tried to give a substantial answer to the question - anonymous downvoting without any explanation does not help at all. It does not match the quality requirements for a fair technical discussion. – LvW Oct 02 '16 at 16:36
  • Maybe someone wanted you to use \$\beta+1\$ instead of \$\beta\$ for the little-re multiplier? (Not me doing that. But that's a slight error I saw.) Also, just as a sidebar note. The OP's example is the linearized small-signal hybrid-\$\pi\$. There is also the fuller non-linear hybrid-\$\pi\$, which I document here: http://electronics.stackexchange.com/questions/252197/why-is-vbc-absent-from-bjt-equations/252199#252199 – jonk Oct 02 '16 at 18:24
  • @jonk, thank you for your comment. But - as you know - in electronics NOTHING is correct by 100%. Hence, every formula contains "slight errors". For instance, intentionaly, I have not included the parameter y12 in my answer. This would pretend an acuuracy that does not exist in reality. And - for my opinion - the same applies to (β+1) vs. β. Thank you. – LvW Oct 02 '16 at 20:15
  • I wasn't complaining. Just hoping to provide you with a reason you seemed to be seeking. I honestly can't read peoples' minds, though. So it was just an offering of sorts. – jonk Oct 02 '16 at 20:24
  • Yes - that was my understanding also. Thank you for your effort. – LvW Oct 03 '16 at 08:17