Calculation of input impedance (Common emitter amplifier)
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jazko
diyAudio Member

Join Date: Sep 2009
Location: Manchester
Calculation of input impedance (Common emitter amplifier)

Hello everyone, can someone give me a clue how to calculate the input impedance of the circuit shown? β=100.

Thanks
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 20th January 2011, 02:46 PM #2 sawreyrw   diyAudio Member   Join Date: Apr 2006 Location: Minnesota The input resistance of the transistor is Rpi=B/gm, where gm is the transconductance and is given by gm=40*Ic. Ic is the DC collector current.
 20th January 2011, 03:18 PM #3 alexcp   diyAudio Member     Join Date: Jun 2009 Be careful with the units; it is 40 (1/V) in the formula, so if you take Ic in amps, you get Gm in 1/ohm and Rpi in ohm. Milliamps will give you kiloohms, etc. You need to add R1 and R2 in parallel to the input resistance of the transistor to get the input impedance of the circuit. Last edited by alexcp; 20th January 2011 at 03:26 PM.
 20th January 2011, 03:56 PM #4 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders combining those two replies Input impedance = R1//R2//[Rpi=B/gm] R1=10k seems ultra low in value cf. R2=82k Should there be a term to represent the effect of Re//Rc has on the input impedance? __________________ regards Andrew T. Last edited by AndrewT; 20th January 2011 at 04:00 PM.
 20th January 2011, 03:58 PM #5 DF96   diyAudio Member   Join Date: May 2007 Homework?
SY
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Quote:
 Originally Posted by DF96 Homework?
Looks that way. And fortunately, no one has given a fully correct answer.

hint: the answer is frequency dependent...
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AndrewT
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Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by AndrewT Should there be a term to represent the effect of Re//Rc has on the input impedance?
Quote:
 Originally Posted by SY hint: the answer is frequency dependent...
Rc should give that frequency dependence.

Unless you mean the effect of the DC blocking capacitor.
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regards Andrew T.

 20th January 2011, 04:28 PM #8 DF96   diyAudio Member   Join Date: May 2007 What about Early effect? That will change the effective value of Rc. Should we ask whether the OP is looking for a first-year or second-year answer?
 20th January 2011, 04:36 PM #9 audiohead   diyAudio Member   Join Date: Mar 2005 Location: Los Angeles, California Zin = R1||R2|| hie. Art
wakibaki
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Join Date: Jan 2008
Quote:
 Originally Posted by SY hint: the answer is frequency dependent...
We're only interested in the range where the amplifier is going to be operated, so other than calculating and noting the lower cut-off frequency for all practical purposes the value can be taken as R1||R2||hfe*re.

Quote:
 Originally Posted by DF96 Should we ask whether the OP is looking for a first-year or second-year answer?
No. We should expect that if the OP knew the first-year answer he would have couched the question to show that.

w

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