TABLE OF CONTENTS The following optional exercises offer additional practice.
To provide high input impedance and reduce the input base current, one can cascade two transistors in series, i.e. buffer the input with an emitter-follower stage. This Darlington transistor pair acts like a single transistor whose current gain is the product of the two ? s and whose V BE drop is the sum of the two V BE s. Build the circuit of Fig. 4.10.
With the input grounded, what quiescent currents do you observe through R B and R E? What does this imply for the combined ? value of the Darlington pair?
Now apply an input signal what do you see at the output? What is the DC voltage drop from input to output?
The input impedance should be so big that you can t measure any decrease in signal amplitude across the 10 k resistor check this assumption. What minimum value does this imply for the input impedance? What input impedance do you expect, and why?
Darlington pairs are available encapsulated in three-lead packages, for example the 2N6426 with combined ? value of about 100 000. The Darlington connection is particularly useful for power transistors, to compensate for their low ? ( ? ? 20 is not uncommon). For example, the TIP110 50 W power Darlington has a minimum combined ? value of 500.
