From: firstname.lastname@example.org (Carlos)
Subject: Problem with Spice simulation of Linkwitz-Riley filter
Date: 18 Sep 2002 03:49:21 -0700
NNTP-Posting-Date: 18 Sep 2002 10:49:21 GMT
I'm having a strange problem with a Spice simulation of a Lowpass 2nd
order Linkwitz-Riley filter (that's two 1st order Butterworth in
series). I have the evaluation version of Spice, which should work ok
with two opamps and such a simple circuit.
If I simulate the following (it's very simple so I'll just describe
Vin -> R in series -> C to ground -> unity gain non-inverting opamp ->
R in series -> C to ground -> unity gain non-inverting opamp -> Load,
I do get the expected result and cutoff frequency (With R = 110 k and
C = 18 nF, F6 = about 80 Hz).
The problem is that if I make the gain of the first opamp > 1 with the
usual couple of resistors in the feedback loop, the cutoff frequency
goes down a lot (in some cases to 18Hz). I've done the maths and also
double checked in Lancaster's Active Filter Cookbook, and apparently
there should be no problem doing this. In fact, funnily enough, if I
remove the second opamp (so it's just R/C -> opamp with gain>1 ->
R/C), it works fine.
Things I have tried:
- Changing the order of magnitude of the feedback resistors in the
- Putting a resistor (several values) between the second R/C section
and the second opamp.
- Putting a resistor (several values) in the feedback loop of the
- Both at the same time.
- Making the first opamp unity gain and the second one gain > 1.
- Using different opamp models (LF411, LM324, uA741).
In all cases, as long as one of the opamps has gain > 1, the cutoff
frequency goes down noticeably.
What's going on?
Many thanks in advance,
Carlos (I'm really at clorenzogNO@SPAMnavegalia.com, you know what to