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The PNP transistor is
upside down with respect
the NPN transistor and
thus has the correct
operating voltages.
If the circuit is
designed so that the PNP
collector voltage is a
suitable value for the
NPN transistor base,
then there is no need
for bias resistors for
the NPN transistor.
The circuit will amplify
dc signals as well as ac
ones. This is because
there is no coupling
capacitor to block dc.
It will also amplify
very low ac frequencies
because there is no
capacitive reactance to
oppose them.
However, if temperature
changes cause dc drift
in the first transistor,
then these will be
amplified by the second
transistor and change
its normal operating
voltages.
This is usually
prevented by some form
of dc stabilization.
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