We had a rainy weekend here and I decided to spend some time simulating Battjes' Ft doubler and to compare it with a more ordinary differential cascode amplifier with similar gain and bias conditions.
I sort-of expected to find that the bandwidth of the Ft doubler would roughly be twice that of the simple differential stage, but I did not find that. In fact, the bandwidth is about the same. I conclude that I don't understand the point of the Ft doubler. What gives?
Included below the .asc file of the Ft doubler. To compare with a simple differential pair, just delete one pair and tweak the resistor between the emitters of the remaining pair to restore the gain. I get a
-3dB bandwidth of about 2.6GHz either way.
Jeroen Belleman
Version 4 SHEET 1 1256 680 WIRE 144 -416 -416 -416 WIRE 720 -416 144 -416 WIRE -416 -352 -416 -416 WIRE 144 -352 144 -416 WIRE 720 -352 720 -416 WIRE -416 -208 -416 -272 WIRE 720 -208 720 -272 WIRE 144 -160 144 -272 WIRE 720 -160 720 -208 WIRE 784 -112 208 -112 WIRE 816 -112 784 -112 WIRE 944 -112 896 -112 WIRE 1056 -112 1024 -112 WIRE 144 48 144 -64 WIRE 336 48 144 48 WIRE 720 48 720 -64 WIRE 720 48 512 48 WIRE 336 112 512 48 WIRE 528 112 336 48 WIRE 144 144 144 48 WIRE 336 144 336 112 WIRE 528 144 528 112 WIRE 720 144 720 48 WIRE -256 192 -288 192 WIRE -96 192 -176 192 WIRE 80 192 -16 192 WIRE 432 192 400 192 WIRE 464 192 432 192 WIRE 816 192 784 192 WIRE 944 192 896 192 WIRE 1056 192 1024 192 WIRE 144 272 144 240 WIRE 176 272 144 272 WIRE 336 272 336 240 WIRE 336 272 256 272 WIRE 528 272 528 240 WIRE 560 272 528 272 WIRE 720 272 720 240 WIRE 720 272 640 272 WIRE 432 304 432 192 WIRE 144 336 144 272 WIRE 240 336 144 336 WIRE 336 336 336 272 WIRE 336 336 304 336 WIRE 528 336 528 272 WIRE 624 336 528 336 WIRE 720 336 720 272 WIRE 720 336 688 336 WIRE 144 368 144 336 WIRE 336 368 336 336 WIRE 528 368 528 336 WIRE 720 368 720 336 WIRE 432 432 432 384 WIRE 144 480 144 448 WIRE 240 480 144 480 WIRE 336 480 336 448 WIRE 336 480 240 480 WIRE 528 480 528 448 WIRE 624 480 528 480 WIRE 720 480 720 448 WIRE 720 480 624 480 WIRE 240 512 240 480 WIRE 624 512 624 480 WIRE 240 624 240 592 WIRE 624 624 624 592 FLAG -288 192 0 FLAG -416 -208 0 FLAG 240 624 0 FLAG 720 -208 out1 FLAG 624 624 0 FLAG 1056 192 0 FLAG 1056 -112 0 FLAG 432 432 0 SYMBOL npn 80 144 R0 SYMATTR InstName Q1 SYMATTR Value BFR92A SYMBOL npn 400 144 M0 SYMATTR InstName Q2 SYMATTR Value BFR92A SYMBOL res 704 -368 R0 SYMATTR InstName R2 SYMATTR Value 500 SYMBOL res 128 352 R0 SYMATTR InstName R3 SYMATTR Value 1200 SYMBOL voltage -416 -368 R0 SYMATTR InstName V1 SYMATTR Value 10V SYMBOL voltage 240 608 R180 WINDOW 0 24 96 Left 2 WINDOW 3 24 16 Left 2 SYMATTR InstName V2 SYMATTR Value 5V SYMBOL voltage -160 192 R90 WINDOW 3 24 152 Left 2 WINDOW 123 24 124 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value PULSE(-1m 1m 0 100p 100p 1n 2n) SYMATTR Value2 AC 1 SYMATTR InstName V3 SYMBOL res 0 176 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 50 SYMBOL res 912 176 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 50 SYMBOL voltage 624 608 R180 WINDOW 0 24 96 Left 2 WINDOW 3 24 16 Left 2 SYMATTR InstName V4 SYMATTR Value 5V SYMBOL npn 784 -160 M0 SYMATTR InstName Q3 SYMATTR Value BFR92A SYMBOL voltage 928 -112 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 -32 56 VBottom 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V6 SYMATTR Value 3 SYMBOL npn 208 -160 M0 SYMATTR InstName Q6 SYMATTR Value BFR92A SYMBOL res 416 288 R0 SYMATTR InstName R8 SYMATTR Value {rb} SYMBOL res 912 -128 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 100 SYMBOL res 128 -368 R0 SYMATTR InstName R1 SYMATTR Value 500 SYMBOL npn 784 144 M0 SYMATTR InstName Q4 SYMATTR Value BFR92A SYMBOL npn 464 144 R0 SYMATTR InstName Q5 SYMATTR Value BFR92A SYMBOL res 320 352 R0 SYMATTR InstName R7 SYMATTR Value 1200 SYMBOL res 512 352 R0 SYMATTR InstName R6 SYMATTR Value 1200 SYMBOL res 704 352 R0 SYMATTR InstName R10 SYMATTR Value 1200 SYMBOL res 272 256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 240 SYMBOL res 656 256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R12 SYMATTR Value 240 SYMBOL cap 304 320 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 900f SYMBOL cap 688 320 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 900f SYMBOL voltage 1040 192 R90 WINDOW 3 24 152 Left 2 WINDOW 123 24 124 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value PULSE(-1m 1m 0 100p 100p 1n 2n) SYMATTR Value2 AC 1 SYMATTR InstName V5 TEXT 16 -520 Left 2 !;tran 5u TEXT 8 -464 Left 2 !.ac dec 100 1k 4G TEXT 336 -520 Left 2 !.param rb=50 TEXT 328 -464 Left 2 !;.step param rb list 5 50 500 5k