Transistor Power Amplifiers Unit 6 – The Darlington Pair
Exercise 2 – Input and Output Impedances
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine the input impedance of a
typical Darlington pair circuit. You will verify your results with a multimeter.
DISCUSSION
- A Darlington pair in a common collector circuit is useful in matching a very high impedance
source to a very low impedance. - The input impedance of a Darlington pair is very high (in the range of 500k: to 10M: for a
small signal transistor).
Zi (D) = E 1 x E 2 x R5 = ED x R5 - The input impedance in a common emitter circuit is very low (in the range of 1: to 50: for
a small signal transistor).