[sdiy] Current sink schematic/calculations help.

[Justin Owen]

http://www.sdiy.org/juz/current_sink_02.pdf

This is working better now - thanks to everyone that chipped in. With R1 at 10K, measuring between the Collector of Q1 and VCC I get 0 to 0.5mA as intended.

When the output of IC1A gets down to less than approx. 0.6V - which I assume is Vbe? - it suddenly swings down to approx. -8V.

With a CV of 0 to 5V (that, at some point, could be coming from some external source) - what's the best way to ensure that the Base of Q1 doesn't swing negative or is biased at 0.6V (considering it's in a feedback loop)?

If I can get that nailed I can look at hooking it up to a current mirror and then on to the output.

Thanks again,

Justin