[sdiy] Current sink schematic/calculations help.

[Phil Macphail]

Hi Justin,
Your current source Q1 will only work as a current source if the collector
voltage is at least a Vsat above the emitter voltage. The emitter voltage
rises from 0 to 5Volts, but the op-amp is holding the collector at 0, so
the transistor won't work as expected. I suspect that the abrupt change in
current is avalanche breakdown of the base-collector junction (reverse
biased).
If you use an ammeter from the collector to the positive supply it should
work as you expect (if you disconnect the op-amp load). To connect the
current source as drawn, to the op-amp as drawn, you would need to use a
pnp current mirror referenced to the positive supply in between.
You're getting close, good luck with the last step,

Phil.

On 16/12/2011 17:35, "Justin Owen" <juzowen at gmail.com> wrote:

>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 
>
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