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