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Current sources 101

  • Thread starter martin griffith
  • Start date
M

martin griffith

For the simple LED or 2-diode type sources, a major error is the
voltage divider formed by the bias resistor and the dynamic impedance
of the led or diodes. A resistor from the transistor emitter to ground
can cancel this error. Or drive the led/diodes with a very simple,
ground-referenced current source. Or bypass the hell out of them.

He doesn't show using an opamp to close the loop around a transistor
or fet, which gives better dynamic and temperature performance; maybe
in part 2?

John
I must admit I use the simple R and LED, and hadn't thought about it
that much. But I've never seen the emiitter to ground R before (I'm
assuming a pnp).

and, yes, maybe a part 2....


martin
 
J

John Larkin


For the simple LED or 2-diode type sources, a major error is the
voltage divider formed by the bias resistor and the dynamic impedance
of the led or diodes. A resistor from the transistor emitter to ground
can cancel this error. Or drive the led/diodes with a very simple,
ground-referenced current source. Or bypass the hell out of them.

He doesn't show using an opamp to close the loop around a transistor
or fet, which gives better dynamic and temperature performance; maybe
in part 2?

John
 
J

John Larkin

I must admit I use the simple R and LED, and hadn't thought about it
that much. But I've never seen the emiitter to ground R before (I'm
assuming a pnp).

Well, I just invented it!

John
 
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