Odp: [sdiy] OpAmp Output Current Question

[Scott Gravenhorst]

Thanks for that.  I can see that my circuit is not entirely linear since the
resistor will measure the sum of the base current and the LED current. 
Moving the LEDS out of the collector circuit and into the emitter circuit
would improve linearity because the LED current would also be the sum of the
base current and the collector current.  I shall make that modification.  But
I'm not terribly concerned about linearity, I doubt that the transfer
function of LED current to FET resistance is linear in the first place.  So
even if my current source were perfectly linear, I probably would not get
linear cutoff variation.  Since a Sallen-Key won't self oscillate (that I'm
aware of), I don't see this as an issue. 

"TIm Daugard" <daugard at sprintmail.com> wrote:
>> I'm assuming the transistor can be something unspecial like a 2N3904.
>
>
>St 2N3904 data sheet says: 200mA collector current. I would probably put the
>leds between ground and the emitter for an emitter follower circuit. You can
>then feed the emitter voltage through a resistor to the opamp to improve
>linearity.
>
>Tim Daugard
>
>

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