aaron_do
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Hi,
First let me say that i'm not really an expert in the area of filter design, but I have done some design in complex bpf before so I am aware of some of the issues.
If you only simulate AC performance, you will not be able to see the linearity of the circuit. Also, you only need a very moderate loop gain in order to get a nice looking filter curve around the passband so don't be deceived. From the AC analysis, you can generally see how high your loop gain UGB is by observing the frequency at which the transfer function of the filter starts to deviate from a "nice" looking curve. Generally speaking, op-amps can be approximated as single pole up to the unity gain bandwidth, UGB. Therefore, the higher the UGB, the higher the loop gain of the op-amp at the frequency of interest. If you don't already know, you can characterize the loop gain by performing a stb analysis in spectre.
1. It is well known that the op-amps linearity is dependent on the loop gain. The linearity can be classified by IIP3, IIP2, P1dB and THD among other parameters. Please see,
A. A. Abidi, "General Relations Between IP2, IP3, and Offsets in Differential Circuits and the Effects of Feedback" IEEE Microwave theory and techniques, vol. 51, no. 5, May 2003.
OR
W. Sansen, "Distortion in Elementary Transistor Circuits", IEEE TCAS-I, vol. 46, no. 3, March 1999.
2. I'm not sure what application you are using the ppf for, but in communications, the received signal is subject to out-of-channel interference. These interferers can intermodulate with each other due to the non-linearity of the circuit. Since they are outside of the passband of the filter, you must ensure that the linearity outside of the passband meets requirements. The linearity out of the passband can possibly be degraded by the reduced loop gain at high frequencies. On the other hand, the linearity might actually improve due to the increasing contribution of direct forward transmission of the signal through the feedback network at high frequencies. My advice is simulate the linearity for as many cases as possible.
3. As for the interstage buffers, it depends on whether the driving capability of your op-amps and the loads being driven. It will come down to loop-gain in my opinion. If the loading of the next stage severly affects the loop gain then a buffer might be necessary.
cheers, Aaron
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