For combinations of transfer functions simple rules for determining the resulting transfer function can be derived. The combinations are of the type that transfer function blocks are connected. In making any transfer function block connection it is assumed that the connection does not load the block to which the connection is being made.
From the diagram in Figure 3.4 it follows that
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For the output of both functions it follows from Figure 3.5 that
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From Figure 3.6 the output is
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As the output of 
is fed back via 
to the
input, this is called feedback. One has to
distinguish between a positive feedback
for positive adding of
and a negative feedback
for negative adding of
.
being a pure
amplifier with a high gain
, one obtains for
negative
feedback
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is an amplifier
with
then an element can be used to
realise any transfer function within certain limits.
