Input impedance of a HPF

Unless the 100nF cpacitor were connected directly to ground, it is unlikely that it would give you a problem. In most filter circuits, the input impedance is dominated by the resistive components on the other side of the capacitor.

Run a simulation, and look at the current flowig through the capacitor as a function of signal frequency.

-- Bill Sloman, Nijmegen

VCVC high pass filters mostly have a pronounced dip in input impedance near the cutoff frequency. They don't have the "near infinite" input resistance that the amplifier alone would have. But as BIll suggested, running some simulations or doing the mathematical analysis would provide a better clue. This form also tends to have relatively poor sensitivities to component variation.

Well, without knowing what your driving circuit is it's hard to say. Usually one gets _more_ concerned when one's input impedance goes down rather than less, as one is usually making voltage-amplification circuits.

Analyze the circuit -- what answer do you get?

"Analyze"? What do you think this is, a DESIGN group or something ?:-)

Pretty clear that the OP is describing a Sallen-Key (fancy wording for crap) Filter. ...Jim Thompson

A large C isn=92t necessarily bad. Opamps have no trouble driving an RC highpass filter. (in my experience)

George H.

I had to look up "VCVS" filter -- I ended up on Wikipedia's Sallen-Key filter page. Sallen-Key is a "degenerate" form of VCVS filter. Presumably that means it hangs out in the wrong sorts of bars, and propositions underage components to come to its apartment for "a little fun".

Bwahahahahahaha! Good one! ...Jim Thompson