Pro's and con's of CCS circuit.

I'm looking for opinions of the constant current source in the schematic below. Is it good? Is it bad. Are there better ones? Your opinions (good or bad) are welcome. Thanks.
 

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mirlo

Member
Paid Member
2002-07-12 9:55 am
San Diego
I posted this opinion in another thread:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=6291

"I don't quite get the benefit of cascoding the Q3 current source with Q2; the output conductance of the current source shouldn't really matter because you have a 1K resistor and the load effectively shunting it anyway, and the cascode eats into the available voltage headroom before clipping."

Basically I think it is overkill, with the disadvantage of reduced headroom.

I would also wonder where are you going to get 3.5 V LEDs. They would have to be blue or white ones I'd think. Might it not be better to design for a green LED and reduce the 12 Ohm resistor accordingly?

-- mirlo
 

mirlo

Member
Paid Member
2002-07-12 9:55 am
San Diego
ordinary dual transistor CCS

By this I assume you mean: use one transistor to sense the current in a resistor between its base and emitter and use its collector to drive the base of the second transistor [that actually passes the current.]

That is probably a better regulator, and also has more headroom than thie cascode.

But there is a possible advantage to the LED biasing. It may be possible to select an LED such that, its forward voltage will have similar temperature behavior to the Vbe of the BJT. Both junctions will have negative temperature coefficients of about 2 mV/K. The two transistor current source suffers from the negative tempco.
 
I have standardized on using very large emitter (source) resistor to increase the compliance of a current source. I would think that this approach migh also benefit you in your application. So instead of dropping 3.5V or so on each leg, drop 7 volts or so over the sense resistor. I have at times dropped 20V and this is very effective ...

Petter