Need help with active filters ???

Discussion in 'Electronic Basics' started by [email protected], Jul 31, 2008.

1. Guest

I have been reading a book about filters both passive and active and
have a few questions about active filters that I need some help on. I
have a link to pages 109-115 containing the information that I have
questions over.

https://ilocker.bsu.edu/users/jalbers/WORLD_SHARED/Electronics/ActiveFilter.PDF

1. For non-inverting op amps the book says that "Typically gain
remains constant up to 10 Khz , then falls steadily to reach 1 at at 1
Mhz." Why is this so?

2. In the inverting configuration, why is the resistor connected to
non inverting terminal Rb connected to ground through a resistor
having the resistance value of Ra and Rf in parallel? The book makes
a statement that for less accuracy the non inverting terminal could be
connected to ground.

3. On the bottom of page 113 a first order active filter circuit is
described. It is basically a low pass RC filter connected to the
input of a non inverting op amp. As far as I can tell from the
formulas the gain of the op amp is 1+Rf/Ra which means that the gain
has to be larger than 1 which means that the op amp is amplifying the
signal instead of attenuating the signal? I am thinking that it would
be better to have a gain of less than 1 to attenuate the signal.

4. I have never been to clear as to what output impedance really
means. I can understand why op amps need to have a high input
impeadance so that they do not put too much of a drain on the circuit
that they are trying to measure. The book says that the output
impeadance of an op amp is low (around 75 ohms) for example. What
does the 75 ohms mean?

Any help would be greatly appreciated. Thanks

2. Michael BlackGuest

Check the copyright on the book. Likely it's old. And once upon a time,
op-amps had pretty limited specs, A 1MHz gain-bandwidth was pretty good,
yet once you try to get gain from it you don't get much actual bandwidth.
You could use them fine if you just connected them as voltage followers,
ie no gain, but start trying to get much gain from them and the bandwidth
goes away. It was misleading 35 years ago, for people first playing
with op-amps and seeing only that "1MHz" figure.

I bet that's the limitation, the book is old and the op-amps current
to the book had limited gain-bandwidth.

More recent op-amps have much better gain-bandwidth, and thus can
supply more gain up to a higher frequency.

Michael