Vi=input, Vo=output signal. The capacitors and resistors of the output terminal of the above circuit is supposed to act as low pass filter (i.e. attenuate all frequencies of the input signal greater than a particular frequency). The only problem is that I can not understand how a low pass filter is obtained at the output in the above circuit. The problem is that (as in my textbook) 'CU' is calculated as if it is in series with 'RC' and acting as that low pass filter. But 'CU' is parallel to 'RC'! Also 'CO' is calculated as if 'CO' is in series with 'RC' and acting as high pass filter when 'Vo' is considered as input. And thus it becomes very confusing. Please help me understand it.