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The conditions under which an active device exhibits an input impedance at one terminal pair which is exactly the negative of the load impedance connected to the other terminal pair are discussed. There are an infinite number of ways by which such a device, which is called a negative-impedance converter (NIC), can be made. One of the limits of this set is represented by a transistor circuit which was discussed previously by Linvill; the other limit, by a new circuit introduced recently. Practical negative-impedance converters are not exact; however, they may be made so by the addition of a pair of compensating impedances. The parameters of the NIC which most affect circuit performance are indicated as a guide to NIC design. Two typical transistor NIC circuits are compared on this basis. Experimental results are given.