Below is the schematic of the Wenzel circulator which was the winner of the 1991 RF design award contest. It's a very useful circuit, and with modern amplifiers is useful from dc to a few hundred MHz.
I first saw this circuit in the mid 80's, having seen it in a colleagues college notes which he would have been given sometime in the early 70's at the latest.
Has anyone seen it in any published aricle or book that far back (or even the single active bridge section)?
Version 4 SHEET 1 1756 680 WIRE 336 -128 272 -128 WIRE 448 -128 416 -128 WIRE 784 -128 720 -128 WIRE 896 -128 864 -128 WIRE 1232 -128 1168 -128 WIRE 1344 -128 1312 -128 WIRE 1344 -96 1344 -128 WIRE 272 0 272 -128 WIRE 336 0 272 0 WIRE 464 0 416 0 WIRE 528 0 464 0 WIRE 640 0 608 0 WIRE 720 0 720 -128 WIRE 784 0 720 0 WIRE 912 0 864 0 WIRE 976 0 912 0 WIRE 1088 0 1056 0 WIRE 1168 0 1168 -128 WIRE 1232 0 1168 0 WIRE 1360 0 1312 0 WIRE 1424 0 1360 0 WIRE 1536 0 1504 0 WIRE 1168 32 1168 0 WIRE 272 48 272 0 WIRE 720 48 720 0 WIRE 1360 128 1360 0 WIRE 1440 128 1360 128 WIRE 912 144 912 0 WIRE 992 144 912 144 WIRE 1536 144 1536 0 WIRE 1536 144 1504 144 WIRE 464 160 464 0 WIRE 544 160 464 160 WIRE 1088 160 1088 0 WIRE 1088 160 1056 160 WIRE 1168 160 1168 112 WIRE 1168 160 1088 160 WIRE 1232 160 1168 160 WIRE 1360 160 1312 160 WIRE 1440 160 1360 160 WIRE 640 176 640 0 WIRE 640 176 608 176 WIRE 720 176 720 128 WIRE 720 176 640 176 WIRE 784 176 720 176 WIRE 912 176 864 176 WIRE 992 176 912 176 WIRE 272 192 272 128 WIRE 336 192 272 192 WIRE 464 192 416 192 WIRE 544 192 464 192 WIRE 1360 192 1360 160 WIRE 912 208 912 176 WIRE 464 224 464 192 WIRE 464 352 464 304 WIRE 912 352 912 288 WIRE 1360 352 1360 272 WIRE 272 416 272 192 WIRE 1536 416 1536 144 WIRE 1536 416 272 416 FLAG 1360 352 0 FLAG 1344 -96 0 FLAG 848 -256 0 FLAG 848 -336 vy FLAG 560 -256 0 FLAG 560 -336 vx FLAG 912 352 0 FLAG 464 352 0 FLAG 448 -128 vx FLAG 896 -128 vy FLAG 272 -128 p1 FLAG 720 -128 p2 FLAG 1168 -128 p3 SYMBOL res 1344 176 R0 SYMATTR InstName R13 SYMATTR Value 100 SYMBOL res 1216 176 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R14 SYMATTR Value 323.6 SYMBOL res 1216 16 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R15 SYMATTR Value 100 SYMBOL res 1520 -16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R16 SYMATTR Value 323.6 SYMBOL res 1328 -144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R17 SYMATTR Value 50 SYMBOL res 1152 16 R0 SYMATTR InstName R18 SYMATTR Value 100 SYMBOL voltage 848 -352 R0 WINDOW 123 47 52 Left 2 WINDOW 39 0 0 Left 0 WINDOW 3 49 79 Left 2 SYMATTR Value2 AC 0 SYMATTR InstName V3 SYMATTR Value 0 SYMBOL voltage 560 -352 R0 WINDOW 123 46 47 Left 2 WINDOW 39 0 0 Left 0 WINDOW 3 43 79 Left 2 SYMATTR Value2 AC 1 SYMATTR InstName V4 SYMATTR Value SINE(0 2 1Meg) SYMBOL opamp 1472 80 R0 SYMATTR InstName U3 SYMATTR SpiceLine Aol=120K SYMATTR SpiceLine2 GBW=60Meg SYMBOL res 896 192 R0 SYMATTR InstName R1 SYMATTR Value 100 SYMBOL res 768 192 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R2 SYMATTR Value 323.6 SYMBOL res 768 16 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R3 SYMATTR Value 100 SYMBOL res 1072 -16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 323.6 SYMBOL res 880 -144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value {rp2} SYMBOL res 704 32 R0 SYMATTR InstName R7 SYMATTR Value 100 SYMBOL opamp 1024 96 R0 SYMATTR InstName U1 SYMATTR SpiceLine Aol=120K SYMATTR SpiceLine2 GBW=60Meg SYMBOL res 448 208 R0 SYMATTR InstName R8 SYMATTR Value 100 SYMBOL res 320 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R9 SYMATTR Value 323.6 SYMBOL res 320 16 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R10 SYMATTR Value 100 SYMBOL res 624 -16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R12 SYMATTR Value 323.6 SYMBOL res 432 -144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R19 SYMATTR Value 50 SYMBOL res 256 32 R0 SYMATTR InstName R21 SYMATTR Value 100 SYMBOL opamp 576 112 R0 SYMATTR InstName U2 SYMATTR SpiceLine Aol=120K SYMATTR SpiceLine2 GBW=60Meg TEXT -168 0 Left 2 !.lib opamp.sub TEXT -170 30 Left 2 !.tran 10u TEXT -168 64 Left 2 !.param rp2=50 TEXT -168 96 Left 2 !.step param rp2 list 1m 50 1G TEXT -128 -296 Left 3 ;Wenzel Circulator