From: shumaker@eisner.decus.org
Newsgroups: sci.electronics
Subject: Re: Negative Resistors

In article , peake@dstos3.dsto.gov.au (Alan Peake) writes:

> Does anyone have a circuit of a Frequency Dependant Negative Resistor (FDNR) 
> using 2 op-amps?
                                        ^
                                        |            |\
                                        |------------|+\
                                        |            |  \
                                   C  -----          |  / ---|
                                      -----     |----| /     |
                                        |       |    |/      |
                    |-------------------|       |            |
                    |                   |       |            |
                    |                   \       |            |
                    |              R2   /       |            |
                    |                   \       |            |
                    |     /|            |       |            |
                    |    / |------------*-------|            |
                    |---/  |            |                    |
                        \  |          -----  C               |
                         \+|----|     -----                  |
                          \|    |       |                    |
                                |       |--------------------|
                                |       |
                                |       \
                                |       /  R4
                                |       \
                                |       |
                                |-------|
                                        |
                                        \
                                        /
Z11 = R5 / (s^2 * C^2 * R2 * R4)        \  R5
                                        |
Substituting s = j omega                |
                                      -----
Z11 = -R5 / (omega^2 * C^2 * R2 * R4)  ---
                                        -

Williams, A. B., "Electronic Filter Design Handbook", McGraw-Hill, 1981
has a little introduction and some good cookbook implementations using
FDNRs, and some other bibliography.