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Old 20th January 2011, 03:27 PM   #1
jazko is offline jazko  United Kingdom
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Default 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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Old 20th January 2011, 03:46 PM   #2
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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.
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Old 20th January 2011, 04:18 PM   #3
alexcp is online now alexcp  United States
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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.

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Old 20th January 2011, 04:56 PM   #4
AndrewT is offline AndrewT  Scotland
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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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Last edited by AndrewT; 20th January 2011 at 05:00 PM.
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Old 20th January 2011, 04:58 PM   #5
DF96 is offline DF96  England
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Homework?
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Old 20th January 2011, 05:06 PM   #6
SY is offline SY  United States
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Quote:
Originally Posted by DF96 View Post
Homework?
Looks that way. And fortunately, no one has given a fully correct answer.

hint: the answer is frequency dependent...
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Old 20th January 2011, 05:11 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
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Should there be a term to represent the effect of Re//Rc has on the input impedance?
Quote:
Originally Posted by SY View Post
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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Old 20th January 2011, 05:28 PM   #8
DF96 is offline DF96  England
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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?
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Old 20th January 2011, 05:36 PM   #9
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Zin = R1||R2|| hie.

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Old 20th January 2011, 06:05 PM   #10
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Quote:
Originally Posted by SY View Post
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.

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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.

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