Transistor Power Amplifiers Unit 6 – The Darlington Pair
Exercise 1 – Current Gain Characteristics
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine the current gain of a
Darlington pair by using measured values. You will verify your results with a multimeter.
DISCUSSION
- A Darlington pair has the emitter of the first stage transistor directly connected to the base of
the second stage transistor. - In a Darlington pair, the Q2 base current (IB2) equals the Q1 emitter current (IB1) or
essentially equals the collector current (IC1):
IB2 = IC1 - The Q1 and Q2 betas equal the ratio of the collector and base currents:
E 1 = IC1 / IB1
E 2 = IC2 / IB2 - The Darlington pair current gain (ED) equals the Q2 collector current (IC2) divided by the Q1
base current (IB1):
ED = IC2 / IB1 - The Darlington pair current gain (ED) equals the product of the betas of each stage:
ED = E 1 x E 2 - If the two transistors in a Darlington pair are very carefully matched to have equal betas, the
Darlington pair beta (ED) equals the square of the transistor beta:
ED = (E)^2