Hi, Im adding a bias generator to improve the sensitivity/linearity of a simple schotky diode multiplier RF vpp sniffer probe, using 2 aaaa battery so it can last a few years with no off switch. its only a bias generator rather than an amplifier so it can still measure input voltages of say 5vpp or more with only 2 batteries, (idealy it would just use 1)
using a micropower op amp I can easily supply a bias voltage tracked to the current through the detector, wich makes it quite linear down to well below
100mv, however the sensitivity falls off progresivly below 40 mv, presumably becuase the detector diodes arn't so nonlinear for very low voltage swings.I have a spare op amp I could use to try and do slope compensation but is there an easier way to compensate for this ?
Ive looked at several circuits and ICs on the web and they go down to about
100mv but are more than just a simple bias generator and so wont work with just 2 batteries, and there are of course profesional units wich go down to at least 10mv but can hardly be put inside a pen size case.It might not be worthwhile trying to make it sensitve below 40mv anyway but the challange is there !
Colin =^.^=
swcad file of current design ... (just for simulation uses lt1462 with +/-
3v instead of target ts942) :-Version 4 SHEET 1 1448 1056 WIRE -224 832 -224 816 WIRE -224 864 -224 832 WIRE -224 992 -224 944 WIRE -160 816 -224 816 WIRE 80 416 80 368 WIRE 80 592 80 560 WIRE 80 704 80 656 WIRE 80 720 80 704 WIRE 80 816 -96 816 WIRE 80 816 80 784 WIRE 80 864 80 816 WIRE 80 992 -224 992 WIRE 80 992 80 928 WIRE 224 368 80 368 WIRE 224 560 224 368 WIRE 224 704 80 704 WIRE 224 704 224 624 WIRE 224 752 224 704 WIRE 224 992 80 992 WIRE 224 992 224 816 WIRE 400 704 224 704 WIRE 400 704 400 672 WIRE 400 816 400 704 WIRE 480 784 480 672 WIRE 480 800 480 784 WIRE 480 992 224 992 WIRE 480 992 480 880 WIRE 496 368 224 368 WIRE 496 704 496 368 WIRE 528 784 480 784 WIRE 576 672 480 672 WIRE 576 704 496 704 WIRE 864 368 496 368 WIRE 864 480 864 368 WIRE 864 688 640 688 WIRE 864 688 864 560 WIRE 864 784 608 784 WIRE 864 784 864 688 WIRE 864 992 480 992 WIRE 912 368 864 368 WIRE 976 368 912 368 WIRE 976 720 976 368 WIRE 976 992 864 992 WIRE 976 992 976 800 WIRE 1168 704 1168 672 FLAG -224 832 sigin FLAG 400 592 v1.5 FLAG 608 656 v1.5 FLAG 912 368 vdmm+ FLAG 400 896 v-1.5 FLAG 608 720 v-1.5 FLAG -464 832 vin FLAG 400 704 0 FLAG -464 912 0 FLAG 864 992 vdmm- FLAG 1168 784 0 FLAG 1168 672 vout SYMBOL schottky 96 784 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D2 SYMATTR Value hsms2820 SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 208 560 R0 SYMATTR InstName C1 SYMATTR Value 50p SYMBOL voltage 400 576 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 3 SYMBOL schottky 64 496 R0 SYMATTR InstName D4 SYMATTR Value hsms2820 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 96 928 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D5 SYMATTR Value hsms2820 SYMATTR Description Diode SYMATTR Type diode SYMBOL cap -96 800 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value 100p SYMBOL cap 208 752 R0 SYMATTR InstName C3 SYMATTR Value 50p SYMBOL schottky 64 592 R0 SYMATTR InstName D1 SYMATTR Value hsms2820 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 464 784 R0 SYMATTR InstName R2 SYMATTR Value 50e6 SYMBOL res 848 464 R0 SYMATTR InstName R4 SYMATTR Value 1.3e6 SYMBOL res 624 768 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 50e6 SYMBOL voltage 400 800 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 3 SYMBOL bv -224 848 R0 SYMATTR InstName B1 SYMATTR Value V=sin(time*2*pi*F1)*v(Vin)/2 SYMBOL res 960 704 R0 SYMATTR InstName R3 SYMATTR Value 10e6 SYMBOL bv -464 816 R0 SYMATTR InstName B2 SYMATTR Value V=V1*time/tend SYMBOL res 64 400 R0 SYMATTR InstName R6 SYMATTR Value 120k SYMBOL bv 1168 688 R0 SYMATTR InstName B3 SYMATTR Value V=v(vdmm+,vdmm-) SYMBOL Opamps\\\\LT1462 608 624 R0 SYMATTR InstName U1 TEXT -370 1040 Left 0 !.tran 100ms TEXT 592 848 Left 0 !.parameter F1 1e5\\n.parameter V1 1\\n.parameter tend .1