dual-range TIA circuit

This is an improved version of the one in the X-chapters.

The dumb summer should be smarter to optimize s/n. If we digitize V1
and V2 and do some tweaks, that should work.

BFT25 is sadly EOL. Its c-b junction is about the best diode anywhere.
I'll have to find a comparable part somewhere.

Version 4
SHEET 1 1192 680
WIRE -16 -304 -64 -304
WIRE 32 -304 -16 -304
WIRE 288 -304 256 -304
WIRE 320 -304 288 -304
WIRE -64 -272 -64 -304
WIRE 144 -240 112 -240
WIRE 256 -240 256 -304
WIRE 256 -240 224 -240
WIRE 288 -240 256 -240
WIRE 384 -240 352 -240
WIRE 448 -240 384 -240
WIRE 560 -240 528 -240
WIRE 592 -240 560 -240
WIRE -64 -176 -64 -192
WIRE 384 -176 384 -240
WIRE 560 -144 512 -144
WIRE 592 -144 560 -144
WIRE 512 -96 512 -144
WIRE 112 -48 112 -240
WIRE 112 -48 -64 -48
WIRE 224 -48 112 -48
WIRE 384 -48 384 -96
WIRE 384 -48 288 -48
WIRE -64 0 -64 -48
WIRE 112 16 112 -48
WIRE 224 16 112 16
WIRE 384 16 384 -48
WIRE 384 16 304 16
WIRE 512 16 512 -16
WIRE -64 112 -64 80
WIRE 288 144 256 144
WIRE 320 144 288 144
WIRE 0 176 -64 176
WIRE 32 176 0 176
WIRE 256 192 256 144
WIRE -64 208 -64 176
WIRE 112 208 112 16
WIRE 224 208 112 208
WIRE 384 224 384 16
WIRE 384 224 288 224
WIRE 432 224 384 224
WIRE 464 224 432 224
WIRE 224 240 192 240
WIRE 192 272 192 240
WIRE -64 320 -64 288
WIRE 256 320 256 256
WIRE 304 320 256 320
WIRE 320 320 304 320
FLAG 192 272 0
FLAG -64 112 0
FLAG 432 224 V1
FLAG 304 320 V-
FLAG 560 -240 V-
FLAG -64 -176 0
FLAG -64 320 0
FLAG 288 144 V+
FLAG -16 -304 V+
FLAG 0 176 V-
FLAG 288 -304 V2
FLAG 512 16 0
FLAG 560 -144 SUM
SYMBOL Opamps\\UniversalOpamp2 256 224 R0
WINDOW 0 19 -45 Left 2
SYMATTR InstName U1
SYMATTR SpiceLine2 En=0 Enk=0 In=0 Ink=0 Rin=500Meg Ro=100 Ccm=1p
Cdiff=2p
SYMBOL res 320 0 R90
WINDOW 0 64 61 VBottom 2
WINDOW 3 70 62 VTop 2
SYMATTR InstName R1
SYMATTR Value 1e7
SYMBOL diode 352 -256 R90
WINDOW 0 71 46 VBottom 2
WINDOW 3 80 47 VTop 2
SYMATTR InstName D1
SYMATTR Value BFT25
SYMBOL res 240 -256 R90
WINDOW 0 70 54 VBottom 2
WINDOW 3 78 54 VTop 2
SYMATTR InstName R2
SYMATTR Value 400K
SYMBOL cap 288 -64 R90
WINDOW 0 -41 31 VBottom 2
WINDOW 3 -35 29 VTop 2
SYMATTR InstName C1
SYMATTR Value 1p
SYMBOL current -64 0 R0
WINDOW 0 19 -32 Left 2
WINDOW 3 -71 -74 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value PULSE(0 2u 0 1 0 10)
SYMBOL res 368 -192 R0
WINDOW 0 52 37 Left 2
WINDOW 3 52 71 Left 2
SYMATTR InstName R3
SYMATTR Value 5K
SYMBOL res 544 -256 R90
WINDOW 0 -39 55 VBottom 2
WINDOW 3 -31 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 5K
SYMBOL voltage -64 -288 R0
WINDOW 0 55 41 Left 2
WINDOW 3 57 67 Left 2
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL voltage -64 192 R0
WINDOW 0 53 39 Left 2
WINDOW 3 52 69 Left 2
SYMATTR InstName V2
SYMATTR Value -5
SYMBOL bv 512 -112 R0
WINDOW 0 62 64 Left 2
WINDOW 3 15 103 Left 2
SYMATTR InstName B1
SYMATTR Value V=V(V1) + 25 * V(V2)
TEXT 528 72 Left 2 ;Dual-segment TIA
TEXT 536 184 Left 2 ;JL  Oct 18  2021
TEXT 568 320 Left 2 !.tran 2
TEXT 136 48 Left 2 ;0V
TEXT 488 272 Left 2 !.model BFT25 D(Is=20f)
TEXT 488 120 Left 2 ;Input range 20 pA to 2 uA



-- 

Father Brown's figure remained quite dark and still; 
but in that instant he had lost his head. His head was
always most valuable when he had lost it.