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In Reply to: RE: Ess9018, Diff. Pair and your CCS...will this work ? posted by multiblitz on February 11, 2017 at 08:31:55
Looking at the schematic you would need to build a negative PS (about -20V to -30V) to provide enough voltage margin to operate a cascode MOSFET current source. Although these CCSs will operate at 5mA current they perform better at currents greater than 10mA. In my experience a BJT cascode using BC549C transistors will give greater performance at currents less than 10mA. This would still entail building a negative PS.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Follow Ups:
I know that I need a negative supply of approc. -25V...my issue was if this negative supply changes the grid-voltage ? Or will still the Dac and the tube automatically sum up the offset of the dac with the negative cathode-grid voltage needs to allow for the bias the ccs has defined ? Maybe my thinking is twisted...
...I try to explain:
- The CCS has only two connections: Positive and negative. On th Negative we apply -25V. On the positive we have the connected cathodes of both tubes, right ?
- the tube will now develop a difference of voltage between cathode and grid which is equivalent to the current, let's say grid will become now -4V if cathode would be 0V.
-Now the Dac is connected with its +1,6V positive voltage offset to the grid.
- So which voltage will we now have at the grid ? Are the -25V completely consumed by the ccs ? Or is the negative supply influencing the absolute voltage on the grid referenced to ground ? The lm334 works currently, but has as well no negative supply...
The CCS will set the grids to wherever they need to be so the tube conducts the current set by the current source. The positive side of the negative supply will need to be connected to ground.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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