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20th January 2011, 03:27 PM  #1 
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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 
20th January 2011, 03:46 PM  #2 
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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, 04:18 PM  #3 
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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 04:26 PM. 
20th January 2011, 04:56 PM  #4 
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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?
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regards Andrew T. Last edited by AndrewT; 20th January 2011 at 05:00 PM. 
20th January 2011, 04:58 PM  #5 
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Homework?

20th January 2011, 05:06 PM  #6 
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Looks that way. And fortunately, no one has given a fully correct answer.
hint: the answer is frequency dependent...
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20th January 2011, 05:11 PM  #7  
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Join Date: Jul 2004
Location: Scottish Borders

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

20th January 2011, 05:28 PM  #8 
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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 firstyear or secondyear answer? 
20th January 2011, 05:36 PM  #9 
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Join Date: Mar 2005
Location: Los Angeles, California

Zin = R1R2 hie.
Art 
20th January 2011, 06:05 PM  #10  
Banned

We're only interested in the range where the amplifier is going to be operated, so other than calculating and noting the lower cutoff frequency for all practical purposes the value can be taken as R1R2hfe*re.
Quote:
w 

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