* SPICE netlist
Q1 IN N005 N008 0 NP
Q2 IN N002 N005 0 NP
R1 N008 OUT .3
R2 N005 N008 2K
R3 OUT N010 {Rx}
R4 N010 0 2.6K
Q3 N016 N018 0 0 NP
Q4 N001 N001 IN 0 PN
R5 N013 N016 4K
R6 N002 N013 1K
Q5 N021 N021 0 0 NP
R7 N018 N021 4K
Q6 N016 N015 N018 0 NP
C1 N016 N015 20p
Q7 N015 N020 N022 0 NP
R8 N022 0 1K
R9 N011 N015 17K
Q8 OUT N010 N011 0 NP
Q9 N002 N007 OUT 0 NP
Q10 N002 N009 0 0 NP
Q11 N011 N017 N020 0 NP
R10 N020 0 12.1K
R11 N009 0 200
R12 N006 N009 3.4K
Q12 N017 N019 0 0 NP
R13 N014 N017 26
R14 N012 N014 1.9K
Q13 N012 N014 N019 0 NP
R15 N019 0 1.2K
Q14 N001 N010 N012 0 NP
R16 N001 N012 30K
Q15 IN N004 N006 0 NP
D1 0 N004 6.3V
R17 N008 N007 230
D2 N007 N003 DZ
R18 IN N003 10K
Q16 N004 N001 IN 0 PN
Q17 N002 N001 IN 0 PN
C2 OUT 0 1
Rload OUT 0 8
V1 IN 0 PWL(0 0 10m 35)
Q18 0 N016 N013 0 PN
.tran 10m
;.param Rx 905 ; 5V
;.param Rx 5.78K ; 12V
;.param Rx 7.87K ; 15V
.step param Rx list 905 5.78K 7.87K

* LTspice schematic (.asc)
Version 4
SHEET 1 1808 1136
WIRE -424 -568 -904 -568
WIRE -232 -568 -424 -568
WIRE 152 -568 -232 -568
WIRE 472 -568 152 -568
WIRE 584 -568 472 -568
WIRE -904 -552 -904 -568
WIRE -424 -552 -424 -568
WIRE -424 -552 -536 -552
WIRE -312 -552 -424 -552
WIRE -536 -536 -536 -552
WIRE -424 -536 -424 -552
WIRE -312 -536 -312 -552
WIRE 152 -536 152 -568
WIRE -600 -488 -616 -488
WIRE -488 -488 -600 -488
WIRE -376 -488 -488 -488
WIRE 472 -472 472 -568
WIRE -904 -456 -904 -472
WIRE -312 -424 -312 -440
WIRE -72 -424 -312 -424
WIRE 344 -424 -72 -424
WIRE 408 -424 344 -424
WIRE -616 -392 -616 -488
WIRE -536 -392 -536 -440
WIRE -536 -392 -616 -392
WIRE -232 -392 -232 -568
WIRE 344 -392 344 -424
WIRE 584 -392 584 -568
WIRE 152 -376 152 -456
WIRE -424 -344 -424 -440
WIRE -376 -344 -424 -344
WIRE -296 -344 -376 -344
WIRE 472 -344 472 -376
WIRE 520 -344 472 -344
WIRE 472 -328 472 -344
WIRE -72 -296 -72 -424
WIRE 40 -296 -72 -296
WIRE -232 -280 -232 -296
WIRE 40 -280 40 -296
WIRE -376 -232 -376 -344
WIRE -72 -232 -72 -296
WIRE 152 -232 152 -312
WIRE 152 -232 104 -232
WIRE 232 -232 152 -232
WIRE 472 -232 472 -248
WIRE 472 -232 312 -232
WIRE 584 -232 584 -296
WIRE 584 -232 472 -232
WIRE 584 -216 584 -232
WIRE -232 -184 -232 -200
WIRE -136 -184 -232 -184
WIRE -232 -168 -232 -184
WIRE 40 -120 40 -184
WIRE 584 -120 584 -136
WIRE 584 -120 40 -120
WIRE 792 -120 584 -120
WIRE 904 -120 792 -120
WIRE 584 -104 584 -120
WIRE 792 -104 792 -120
WIRE 904 -104 904 -120
WIRE -536 -88 -536 -392
WIRE -536 -88 -664 -88
WIRE -376 -72 -376 -168
WIRE -232 -72 -232 -88
WIRE -232 -72 -376 -72
WIRE -72 -72 -72 -136
WIRE -72 -72 -232 -72
WIRE -664 -56 -664 -88
WIRE -536 -56 -536 -88
WIRE 40 -56 40 -120
WIRE 104 -8 -472 -8
WIRE 584 -8 584 -24
WIRE 584 -8 104 -8
WIRE 792 -8 792 -40
WIRE 904 -8 904 -24
WIRE 40 56 40 40
WIRE 40 56 -104 56
WIRE -664 72 -664 24
WIRE -536 72 -536 40
WIRE -536 72 -664 72
WIRE -392 72 -536 72
WIRE 40 72 40 56
WIRE 344 72 344 -312
WIRE 472 72 344 72
WIRE -392 88 -392 72
WIRE 344 88 344 72
WIRE -536 136 -536 72
WIRE 472 136 472 72
WIRE -392 184 -392 168
WIRE -392 184 -472 184
WIRE 40 184 40 152
WIRE 72 184 40 184
WIRE 200 184 136 184
WIRE 344 184 344 168
WIRE 344 184 200 184
WIRE 408 184 344 184
WIRE 200 200 200 184
WIRE -392 216 -392 184
WIRE 40 248 40 184
WIRE 136 248 40 248
WIRE -104 264 -104 56
WIRE 344 264 344 184
WIRE -392 312 -392 296
WIRE -168 312 -392 312
WIRE 200 312 200 296
WIRE 280 312 200 312
WIRE -392 328 -392 312
WIRE 200 328 200 312
WIRE 40 344 40 248
WIRE -536 376 -536 232
WIRE -456 376 -536 376
WIRE 584 376 584 -8
WIRE -104 392 -104 360
WIRE -24 392 -104 392
WIRE -104 424 -104 392
WIRE 200 424 200 408
WIRE 280 424 200 424
WIRE -536 440 -536 376
WIRE 200 440 200 424
WIRE 40 456 40 440
WIRE 280 488 280 424
WIRE 280 488 264 488
WIRE -536 552 -536 520
WIRE -392 552 -392 424
WIRE -392 552 -536 552
WIRE -232 552 -232 -72
WIRE -232 552 -392 552
WIRE -104 552 -104 504
WIRE -104 552 -232 552
WIRE 40 552 40 536
WIRE 40 552 -104 552
WIRE 200 552 200 536
WIRE 200 552 40 552
WIRE 344 552 344 360
WIRE 344 552 200 552
WIRE 472 552 472 232
WIRE 472 552 344 552
WIRE 584 552 584 456
WIRE 584 552 472 552
WIRE 584 568 584 552
FLAG 584 568 0
FLAG 904 -120 OUT
FLAG -904 -568 IN
FLAG -904 -456 0
FLAG 904 -8 0
FLAG 792 -8 0
SYMBOL NPN 520 -392 R0
SYMATTR InstName Q1
SYMATTR Value NP
SYMBOL npn 408 -472 R0
SYMATTR InstName Q2
SYMATTR Value NP
SYMBOL RES 568 -232 R0
SYMATTR InstName R1
SYMATTR Value .3
SYMBOL res 456 -344 R0
SYMATTR InstName R2
SYMATTR Value 2K
SYMBOL res 568 -120 R0
SYMATTR InstName R3
SYMATTR Value {Rx}
SYMBOL res 568 360 R0
SYMATTR InstName R4
SYMATTR Value 2.6K
SYMBOL npn 280 264 R0
SYMATTR InstName Q3
SYMATTR Value NP
SYMBOL PNP -600 -440 M180
SYMATTR InstName Q4
SYMATTR Value PN
SYMBOL res 328 72 R0
SYMATTR InstName R5
SYMATTR Value 4K
SYMBOL res 328 -408 R0
SYMATTR InstName R6
SYMATTR Value 1K
SYMBOL npn 264 440 M0
SYMATTR InstName Q5
SYMATTR Value NP
SYMBOL res 184 312 R0
SYMATTR InstName R7
SYMATTR Value 4K
SYMBOL npn 136 200 R0
SYMATTR InstName Q6
SYMATTR Value NP
SYMBOL cap 136 168 R90
SYMATTR InstName C1
SYMATTR Value 20p
SYMBOL npn -24 344 R0
SYMATTR InstName Q7
SYMATTR Value NP
SYMBOL res 24 440 R0
SYMATTR InstName R8
SYMATTR Value 1K
SYMBOL res 24 56 R0
SYMATTR InstName R9
SYMATTR Value 17K
SYMBOL npn 104 -56 M0
SYMATTR InstName Q8
SYMATTR Value NP
SYMBOL npn 104 -280 M0
SYMATTR InstName Q9
SYMATTR Value NP
SYMBOL npn -136 -232 R0
SYMATTR InstName Q10
SYMATTR Value NP
SYMBOL npn -168 264 R0
SYMATTR InstName Q11
SYMATTR Value NP
SYMBOL res -120 408 R0
SYMATTR InstName R10
SYMATTR Value 12.1K
SYMBOL res -248 -184 R0
SYMATTR InstName R11
SYMATTR Value 200
SYMBOL res -248 -296 R0
SYMATTR InstName R12
SYMATTR Value 3.4K
SYMBOL npn -456 328 R0
SYMATTR InstName Q12
SYMATTR Value NP
SYMBOL res -408 200 R0
SYMATTR InstName R13
SYMATTR Value 26
SYMBOL res -408 72 R0
SYMATTR InstName R14
SYMATTR Value 1.9K
SYMBOL npn -472 136 M0
SYMATTR InstName Q13
SYMATTR Value NP
SYMBOL res -552 424 R0
SYMATTR InstName R15
SYMATTR Value 1.2K
SYMBOL npn -472 -56 M0
SYMATTR InstName Q14
SYMATTR Value NP
SYMBOL res -680 -72 R0
SYMATTR InstName R16
SYMATTR Value 30K
SYMBOL npn -296 -392 R0
SYMATTR InstName Q15
SYMATTR Value NP
SYMBOL zener -392 -168 M180
SYMATTR InstName D1
SYMATTR Value 6.3V
SYMBOL res 328 -248 R90
SYMATTR InstName R17
SYMATTR Value 230
SYMBOL zener 136 -312 M180
SYMATTR InstName D2
SYMATTR Value DZ
SYMBOL res 136 -552 R0
SYMATTR InstName R18
SYMATTR Value 10K
SYMBOL pnp -488 -440 M180
SYMATTR InstName Q16
SYMATTR Value PN
SYMBOL pnp -376 -440 M180
SYMATTR InstName Q17
SYMATTR Value PN
SYMBOL cap 776 -104 R0
SYMATTR InstName C2
SYMATTR Value 1
SYMBOL res 888 -120 R0
SYMATTR InstName Rload
SYMATTR Value 8
SYMBOL VOLTAGE -904 -568 R0
SYMATTR InstName V1
SYMATTR Value PWL(0 0 10m 35)
SYMBOL pnp 408 232 M180
SYMATTR InstName Q18
SYMATTR Value PN