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close this bookAmplifier Teaching Aid (GTZ, DED; 86 pages)
View the documentPreface
View the documentIntroduction
Open this folder and view contentsLesson 1 - Semiconductor Review
Open this folder and view contentsLesson 2 - Bipolar Transistor
Open this folder and view contentsBipolar Transistor II
View the documentFirst Evaluation
Open this folder and view contentsLesson 4 - Transistor Fundamentals
Open this folder and view contentsLesson 5 - Transistor Biasing
Open this folder and view contentsLesson 6 - Transistor Biasing II
View the documentSecond Evaluation
Open this folder and view contentsLesson 7 - Small Signal Amplifier
close this folderLesson 8 - Small Signal Amplifier II
close this folderLesson Plan
View the documentSignals in a CE amplifier
View the documentInput impedance
View the documentImpedance matching
View the documentHandout No. 1
View the documentWorksheet No. 8
Open this folder and view contentsLesson 9 - Small Signal Amplifier III
Open this folder and view contentsLesson 10 - Large Signal Amplifier
View the documentThird Evaluation
 

Input impedance

Another important value to analyze is the input impedance of an amplifier circuit.

Recall the ac equivalent circuit in Lesson 7:

(see Fig. 8-1 on the next page)


Fig. 8-1: AC equivalent circuit

RG is equal to the internal resistance of the signal source. Input impedance looking into the base:

ie/ib is approximately equal to β:

zb = β * re

The input impedance of a amplifier stage is the combination of base impedance and biasing resistance:

zin = R1//R2//β * re

Ex: What is the input impedance in Fig. 8-1?

re = 22.7 Ω, β = 100

zin = 1.8 KΩ//22.7 Ω * 100


= 202 Ω

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