Enough of filters. Lets now look at the simple amplifier model in figure 3.14
Figure 3.14: A simple amplifier model.
Notice that 
is with respect to ground while
is a voltage difference.
For a typical operational amplifier 
at 
.
As 
, 
due to internal
capacitance, ie. the amplifier behaves like a low-pass filter.
where 
is the transfer function of a low-pass
filter.