shunt current source ramp

Yep. A perfectly good "poor man's current source".

If you have headroom add a zener in series with the emitter, so the voltage drop across the R is less variable with diverted current. ...Jim Thompson

On Tuesday, July 29, 2014 12:35:32 PM UTC-4, John Larkin wrote:

It's pretty linear. I added a reference ramp, then plotted

Cheers, James Arthur

Version 4 SHEET 1 968 680 WIRE 96 -192 -176 -192 WIRE 96 -112 96 -192 WIRE -176 -96 -176 -192 WIRE 96 16 96 -32 WIRE 256 16 96 16 WIRE 368 16 256 16 WIRE 432 16 368 16 WIRE 96 64 96 16 WIRE -176 96 -176 -16 WIRE 256 160 256 16 WIRE 96 208 96 144 WIRE 192 208 96 208 WIRE -80 272 -176 272 WIRE 96 272 96 208 WIRE 96 272 -16 272 WIRE 560 288 496 288 WIRE 640 288 560 288 WIRE 256 320 256 256 WIRE 640 320 640 288 WIRE -176 368 -176 272 WIRE 96 368 96 272 WIRE 496 368 496 288 WIRE 640 432 640 400 WIRE -176 512 -176 448 WIRE 96 512 96 432 WIRE 496 512 496 448 FLAG 256 320 0 FLAG -176 96 0 FLAG -176 512 0 FLAG 96 512 0 FLAG 368 16 OUT FLAG 496 512 0 FLAG 640 432 0 FLAG 560 288 ref_ramp SYMBOL res 80 -128 R0 WINDOW 0 -67 39 Left 2 WINDOW 3 -64 76 Left 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL res 80 48 R0 WINDOW 0 -60 37 Left 2 WINDOW 3 -62 73 Left 2 SYMATTR InstName R2 SYMATTR Value 6K SYMBOL pnp 192 256 M180 WINDOW 0 147 68 Left 2 WINDOW 3 123 32 Left 2 SYMATTR InstName Q1 SYMATTR Value 2N4403 SYMBOL voltage -176 -112 R0 WINDOW 0 61 42 Left 2 WINDOW 3 67 81 Left 2 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL voltage -176 352 R0 WINDOW 0 22 115 Left 2 WINDOW 3 -26 216 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(0 3.3 0 10n 10n 8m 10m 10) SYMBOL cap 80 368 R0 WINDOW 0 74 19 Left 2 WINDOW 3 74 50 Left 2 SYMATTR InstName C1

SYMBOL schottky -16 256 R90 WINDOW 0 -58 26 VBottom 2 WINDOW 3 -44 29 VTop 2 SYMATTR InstName D1 SYMATTR Value MBR0540 SYMATTR Description Diode SYMATTR Type diode SYMBOL voltage 496 352 R0 WINDOW 0 22 115 Left 2 WINDOW 3 -26 216 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value PULSE(0.84 2.05 0 8m 10n 10n 10m 10) SYMBOL res 624 304 R0 SYMATTR InstName R3 SYMATTR Value 10k TEXT 288 -96 Left 2 !.tran 50m TEXT -144 -304 Left 2 ;Linear ramp using PNP shunt current source/buffer TEXT -24 -248 Left 2 ;J Larkin July 29, 2014

I picked a 5 volt supply and 3.3 volt logic, which compromises linearity.

TC will be bad, but it might be useful for some sort of PWMish thing where mediocre ramp accuracy is ok. Or maybe it's not useful at all.

Interesting how many different circuits can be made from a few parts.