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Design >> Analog Design >> stability of bandpass filter
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Message started by zhonghan on Apr 23^rd, 2007, 6:02am

Title: stability of bandpass filter
Post by zhonghan on Apr 23^rd, 2007, 6:02am

Figure below shows my bandpass filter's topology. Its Q factor can be adjusted by the two resistors R1 an R2. Resonant frequency is sqrt(Gm1*Gm2/C1C2). But after investigating the return ratio by breaking the loop at p, the phase margin is nearly 0 degree at moderate frequencies. Then i tried transient analysis with HSPICE, it seems stable from the results.

I am puzzled with this issue., and eager for any constructive suggestions .

Title: Re: stability of bandpass filter
Post by SATurn on May 5^th, 2007, 3:22am

Apparantl, there are two integrators and the phase would be -180. Actually, this is a BPF from phase point of view is similar to an oscillator and just the Q value is controlled by the feedback around the second OTA (Q is proportional to 1/sqrt(gm2)). As far as the Q value in AC simulations is not much higher than the expected value, the circuit should be stable.

SATurn