Electronic Circuit Analysis

Multistage Amplifiers 69
2.7.4 Output Resistance
The general expression for Ro ofa transistor in Common Collector Configuration in terms of Common
Emitter h-parameters is,
ROJ ==
Rs + hie
1 + h fe
Now for the transistor Q2' Rs is R OJ
'
Rs + hie
--=--=- +h·
1 + hfe Ie
Therefore, R02 is the output resistance of the Darlington Circuit.
R02 ==
Rs + hie hIe
(1 + hfeY + 1 + h fe
This is a small value, since, 1 + h fe
is » 1.
Therefore, the characteristic of Darlington Circuit are
1. Very High Input Resistance ( of the order of M.o).
2. Very Large Current Gain (of the order of 10,000).
3. Very Low Output Resistance (of the order offew .0).
4. Voltage Gain, Av < 1.
Darlington Pairs are available in a single package with just three leads, like one transistor in
integrated form.
2.7.5 Disadvantages
We have assumed that the h-parameters of both the transistors are identical. But in practice it is
difficult to make h-parameters depend upon the operating point of QJ and Q2' Since the emitter
current of transistor QJ is the base current for transistor Q2' the value of I C2 » ICJ
1. The quiescent or operating conditions of both the transistors will be different. hje value will
be small for the transistor QI. .: hje = (f/fJ.lb2 is less
CDfL make CIL997 is a transistor of Darlington Pair Configuration with hje == 1000.
2. The second drawback is leakage current of the first transistor Q I which is amplified by the
second transistor Q 2 (-: leI == fbi