                            Amplifier Teaching Aid (GTZ, DED; 86 pages)  Preface  Introduction  Lesson 1 - Semiconductor Review  Lesson 2 - Bipolar Transistor  Bipolar Transistor II  First Evaluation  Lesson 4 - Transistor Fundamentals  Lesson 5 - Transistor Biasing  Lesson 6 - Transistor Biasing II  Second Evaluation  Lesson 7 - Small Signal Amplifier  Lesson 8 - Small Signal Amplifier II   Lesson Plan    Signals in a CE amplifier    Input impedance    Impedance matching   Handout No. 1   Worksheet No. 8  Lesson 9 - Small Signal Amplifier III  Lesson 10 - Large Signal Amplifier  Third 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   