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)
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?
[Ukrainian] [English] [Russian]