[sdiy] Current sink schematic/calculations help.

[Justin Owen]

Slowly working through the OTA/VCA stuff...

I breadboarded a current sink to test some ideas. I need a sink rather than a source because I'm driving (sinking?) a 4 transistor long tail pair as shown here: http://en.wikipedia.org/wiki/File:Long-tailed-pair.gif

I set up this schematic: http://www.sdiy.org/juz/current_sink.pdf based on Aaron Lanterman's excellent OTA lecture/video: http://users.ece.gatech.edu/~lanterma/ems/ - thanks Aaron!

I wanted to test the sink by itself so I ran the output into a Current to Voltage converter (IC1B) - I assumed that I would need to send the load 'somewhere' to measure across it.

My aim is to take a 0 to 5V CV and turn it into a 0 to 0.5mA current source. The CtoV amp should gain it back to 0 to 5V.

For sink R values I'm using the equations here: http://www.ecircuitcenter.com/circuits/curr_src1/curr_src1.htm
For CtoV R values I'm using the equations here: http://www.ecircuitcenter.com/circuits/opitov/opitov.htm
 
When measuring Iabc between the collector of Q1 and the INV input of IC1B I'm not getting anything that matches these calculations - they're not really making sense at all and changing R values isn't giving me anything I can get my bearings from. I'm getting values from a few hundred uA to a few hundred mA.

There's also a strange 'switch' thing going under certain values where there only seems to be two states - high and low.

Where have I gone!? Thanks.

J